freebsd-dev/lib/libc/sys/_umtx_op.2

.\" Copyright (c) 2016 The FreeBSD Foundation, Inc.
.\"
.\" This documentation was written by
.\" Konstantin Belousov <kib@FreeBSD.org> under sponsorship
.\" from the FreeBSD Foundation.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHORS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.Dd November 23, 2020
.Dt _UMTX_OP 2
.Os
.Sh NAME
.Nm _umtx_op
.Nd interface for implementation of userspace threading synchronization primitives
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/umtx.h
.Ft int
.Fn _umtx_op "void *obj" "int op" "u_long val" "void *uaddr" "void *uaddr2"
.Sh DESCRIPTION
The
.Fn _umtx_op
system call provides kernel support for userspace implementation of
the threading synchronization primitives.
The
.Lb libthr
uses the syscall to implement
.St -p1003.1-2001
pthread locks, like mutexes, condition variables and so on.
.Ss STRUCTURES
The operations, performed by the
.Fn _umtx_op
syscall, operate on userspace objects which are described
by the following structures.
Reserved fields and paddings are omitted.
All objects require ABI-mandated alignment, but this is not currently
enforced consistently on all architectures.
.Pp
The following flags are defined for flag fields of all structures:
.Bl -tag -width indent
.It Dv USYNC_PROCESS_SHARED
Allow selection of the process-shared sleep queue for the thread sleep
container, when the lock ownership cannot be granted immediately,
and the operation must sleep.
The process-shared or process-private sleep queue is selected based on
the attributes of the memory mapping which contains the first byte of
the structure, see
.Xr mmap 2 .
Otherwise, if the flag is not specified, the process-private sleep queue
is selected regardless of the memory mapping attributes, as an optimization.
.Pp
See the
.Sx SLEEP QUEUES
subsection below for more details on sleep queues.
.El
.Bl -hang -offset indent
.It Sy Mutex
.Bd -literal
struct umutex {
	volatile lwpid_t m_owner;
	uint32_t         m_flags;
	uint32_t         m_ceilings[2];
	uintptr_t        m_rb_lnk;
};
.Ed
.Pp
The
.Dv m_owner
field is the actual lock.
It contains either the thread identifier of the lock owner in the
locked state, or zero when the lock is unowned.
The highest bit set indicates that there is contention on the lock.
The constants are defined for special values:
.Bl -tag -width indent
.It Dv UMUTEX_UNOWNED
Zero, the value stored in the unowned lock.
.It Dv UMUTEX_CONTESTED
The contention indicator.
.It Dv UMUTEX_RB_OWNERDEAD
A thread owning the robust mutex terminated.
The mutex is in unlocked state.
.It Dv UMUTEX_RB_NOTRECOV
The robust mutex is in a non-recoverable state.
It cannot be locked until reinitialized.
.El
.Pp
The
.Dv m_flags
field may contain the following umutex-specific flags, in addition to
the common flags:
.Bl -tag -width indent
.It Dv UMUTEX_PRIO_INHERIT
Mutex implements
.Em Priority Inheritance
protocol.
.It Dv UMUTEX_PRIO_PROTECT
Mutex implements
.Em Priority Protection
protocol.
.It Dv UMUTEX_ROBUST
Mutex is robust, as described in the
.Sx ROBUST UMUTEXES
section below.
.It Dv UMUTEX_NONCONSISTENT
Robust mutex is in a transient non-consistent state.
Not used by kernel.
.El
.Pp
In the manual page, mutexes not having
.Dv UMUTEX_PRIO_INHERIT
and
.Dv UMUTEX_PRIO_PROTECT
flags set, are called normal mutexes.
Each type of mutex
.Pq normal, priority-inherited, and priority-protected
has a separate sleep queue associated
with the given key.
.Pp
For priority protected mutexes, the
.Dv m_ceilings
array contains priority ceiling values.
The
.Dv m_ceilings[0]
is the ceiling value for the mutex, as specified by
.St -p1003.1-2008
for the
.Em Priority Protected
mutex protocol.
The
.Dv m_ceilings[1]
is used only for the unlock of a priority protected mutex, when
unlock is done in an order other than the reversed lock order.
In this case,
.Dv m_ceilings[1]
must contain the ceiling value for the last locked priority protected
mutex, for proper priority reassignment.
If, instead, the unlocking mutex was the last priority propagated
mutex locked by the thread,
.Dv m_ceilings[1]
should contain \-1.
This is required because kernel does not maintain the ordered lock list.
.It Sy Condition variable
.Bd -literal
struct ucond {
	volatile uint32_t c_has_waiters;
	uint32_t          c_flags;
	uint32_t          c_clockid;
};
.Ed
.Pp
A non-zero
.Dv c_has_waiters
value indicates that there are in-kernel waiters for the condition,
executing the
.Dv UMTX_OP_CV_WAIT
request.
.Pp
The
.Dv c_flags
field contains flags.
Only the common flags
.Pq Dv USYNC_PROCESS_SHARED
are defined for ucond.
.Pp
The
.Dv c_clockid
member provides the clock identifier to use for timeout, when the
.Dv UMTX_OP_CV_WAIT
request has both the
.Dv CVWAIT_CLOCKID
flag and the timeout specified.
Valid clock identifiers are a subset of those for
.Xr clock_gettime 2 :
.Bl -bullet -compact
.It
.Dv CLOCK_MONOTONIC
.It
.Dv CLOCK_MONOTONIC_FAST
.It
.Dv CLOCK_MONOTONIC_PRECISE
.It
.Dv CLOCK_PROF
.It
.Dv CLOCK_REALTIME
.It
.Dv CLOCK_REALTIME_FAST
.It
.Dv CLOCK_REALTIME_PRECISE
.It
.Dv CLOCK_SECOND
.It
.Dv CLOCK_UPTIME
.It
.Dv CLOCK_UPTIME_FAST
.It
.Dv CLOCK_UPTIME_PRECISE
.It
.Dv CLOCK_VIRTUAL
.El
.It Sy Reader/writer lock
.Bd -literal
struct urwlock {
	volatile int32_t rw_state;
	uint32_t         rw_flags;
	uint32_t         rw_blocked_readers;
	uint32_t         rw_blocked_writers;
};
.Ed
.Pp
The
.Dv rw_state
field is the actual lock.
It contains both the flags and counter of the read locks which were
granted.
Names of the
.Dv rw_state
bits are following:
.Bl -tag -width indent
.It Dv URWLOCK_WRITE_OWNER
Write lock was granted.
.It Dv URWLOCK_WRITE_WAITERS
There are write lock waiters.
.It Dv URWLOCK_READ_WAITERS
There are read lock waiters.
.It Dv URWLOCK_READER_COUNT(c)
Returns the count of currently granted read locks.
.El
.Pp
At any given time there may be only one thread to which the writer lock
is granted on the
.Vt struct rwlock ,
and no threads are granted read lock.
Or, at the given time, up to
.Dv URWLOCK_MAX_READERS
threads may be granted the read lock simultaneously, but write lock is
not granted to any thread.
.Pp
The following flags for the
.Dv rw_flags
member of
.Vt struct urwlock
are defined, in addition to the common flags:
.Bl -tag -width indent
.It Dv URWLOCK_PREFER_READER
If specified, immediately grant read lock requests when
.Dv urwlock
is already read-locked, even in presence of unsatisfied write
lock requests.
By default, if there is a write lock waiter, further read requests are
not granted, to prevent unfair write lock waiter starvation.
.El
.Pp
The
.Dv rw_blocked_readers
and
.Dv rw_blocked_writers
members contain the count of threads which are sleeping in kernel,
waiting for the associated request type to be granted.
The fields are used by kernel to update the
.Dv URWLOCK_READ_WAITERS
and
.Dv URWLOCK_WRITE_WAITERS
flags of the
.Dv rw_state
lock after requesting thread was woken up.
.It Sy Semaphore
.Bd -literal
struct _usem2 {
	volatile uint32_t _count;
	uint32_t          _flags;
};
.Ed
.Pp
The
.Dv _count
word represents a counting semaphore.
A non-zero value indicates an unlocked (posted) semaphore, while zero
represents the locked state.
The maximal supported semaphore count is
.Dv USEM_MAX_COUNT .
.Pp
The
.Dv _count
word, besides the counter of posts (unlocks), also contains the
.Dv USEM_HAS_WAITERS
bit, which indicates that locked semaphore has waiting threads.
.Pp
The
.Dv USEM_COUNT()
macro, applied to the
.Dv _count
word, returns the current semaphore counter, which is the number of posts
issued on the semaphore.
.Pp
The following bits for the
.Dv _flags
member of
.Vt struct _usem2
are defined, in addition to the common flags:
.Bl -tag -width indent
.It Dv USEM_NAMED
Flag is ignored by kernel.
.El
.It Sy Timeout parameter
.Bd -literal
struct _umtx_time {
	struct timespec _timeout;
	uint32_t        _flags;
	uint32_t        _clockid;
};
.Ed
.Pp
Several
.Fn _umtx_op
operations allow the blocking time to be limited, failing the request
if it cannot be satisfied in the specified time period.
The timeout is specified by passing either the address of
.Vt struct timespec ,
or its extended variant,
.Vt struct _umtx_time ,
as the
.Fa uaddr2
argument of
.Fn _umtx_op .
They are distinguished by the
.Fa uaddr
value, which must be equal to the size of the structure pointed to by
.Fa uaddr2 ,
casted to
.Vt uintptr_t .
.Pp
The
.Dv _timeout
member specifies the time when the timeout should occur.
Legal values for clock identifier
.Dv _clockid
are shared with the
.Fa clock_id
argument to the
.Xr clock_gettime 2
function,
and use the same underlying clocks.
The specified clock is used to obtain the current time value.
Interval counting is always performed by the monotonic wall clock.
.Pp
The
.Dv _flags
argument allows the following flags to further define the timeout behaviour:
.Bl -tag -width indent
.It Dv UMTX_ABSTIME
The
.Dv _timeout
value is the absolute time.
The thread will be unblocked and the request failed when specified
clock value is equal or exceeds the
.Dv _timeout.
.Pp
If the flag is absent, the timeout value is relative, that is the amount
of time, measured by the monotonic wall clock from the moment of the request
start.
.El
.El
.Ss SLEEP QUEUES
When a locking request cannot be immediately satisfied, the thread is
typically put to
.Em sleep ,
which is a non-runnable state terminated by the
.Em wake
operation.
Lock operations include a
.Em try
variant which returns an error rather than sleeping if the lock cannot
be obtained.
Also,
.Fn _umtx_op
provides requests which explicitly put the thread to sleep.
.Pp
Wakes need to know which threads to make runnable, so sleeping threads
are grouped into containers called
.Em sleep queues .
A sleep queue is identified by a key, which for
.Fn _umtx_op
is defined as the physical address of some variable.
Note that the
.Em physical
address is used, which means that same variable mapped multiple
times will give one key value.
This mechanism enables the construction of
.Em process-shared
locks.
.Pp
A related attribute of the key is shareability.
Some requests always interpret keys as private for the current process,
creating sleep queues with the scope of the current process even if
the memory is shared.
Others either select the shareability automatically from the
mapping attributes, or take additional input as the
.Dv USYNC_PROCESS_SHARED
common flag.
This is done as optimization, allowing the lock scope to be limited
regardless of the kind of backing memory.
.Pp
Only the address of the start byte of the variable specified as key is
important for determining corresponding sleep queue.
The size of the variable does not matter, so, for example, sleep on the same
address interpeted as
.Vt uint32_t
and
.Vt long
on a little-endian 64-bit platform would collide.
.Pp
The last attribute of the key is the object type.
The sleep queue to which a sleeping thread is assigned is an individual
one for simple wait requests, mutexes, rwlocks, condvars and other
primitives, even when the physical address of the key is same.
.Pp
When waking up a limited number of threads from a given sleep queue,
the highest priority threads that have been blocked for the longest on
the queue are selected.
.Ss ROBUST UMUTEXES
The
.Em robust umutexes
are provided as a substrate for a userspace library to implement
.Tn POSIX
robust mutexes.
A robust umutex must have the
.Dv UMUTEX_ROBUST
flag set.
.Pp
On thread termination, the kernel walks two lists of mutexes.
The two lists head addresses must be provided by a prior call to
.Dv UMTX_OP_ROBUST_LISTS
request.
The lists are singly-linked.
The link to next element is provided by the
.Dv m_rb_lnk
member of the
.Vt struct umutex .
.Pp
Robust list processing is aborted if the kernel finds a mutex
with any of the following conditions:
.Bl -dash -offset indent -compact
.It
the
.Dv UMUTEX_ROBUST
flag is not set
.It
not owned by the current thread, except when the mutex is pointed to
by the
.Dv robust_inactive
member of the
.Vt struct umtx_robust_lists_params ,
registered for the current thread
.It
the combination of mutex flags is invalid
.It
read of the umutex memory faults
.It
the list length limit described in
.Xr libthr 3
is reached.
.El
.Pp
Every mutex in both lists is unlocked as if the
.Dv UMTX_OP_MUTEX_UNLOCK
request is performed on it, but instead of the
.Dv UMUTEX_UNOWNED
value, the
.Dv m_owner
field is written with the
.Dv UMUTEX_RB_OWNERDEAD
value.
When a mutex in the
.Dv UMUTEX_RB_OWNERDEAD
state is locked by kernel due to the
.Dv UMTX_OP_MUTEX_TRYLOCK
and
.Dv UMTX_OP_MUTEX_LOCK
requests, the lock is granted and
.Er EOWNERDEAD
error is returned.
.Pp
Also, the kernel handles the
.Dv UMUTEX_RB_NOTRECOV
value of
.Dv the m_owner
field specially, always returning the
.Er ENOTRECOVERABLE
error for lock attempts, without granting the lock.
.Ss OPERATIONS
The following operations, requested by the
.Fa op
argument to the function, are implemented:
.Bl -tag -width indent
.It Dv UMTX_OP_WAIT
Wait.
The arguments for the request are:
.Bl -tag -width "obj"
.It Fa obj
Pointer to a variable of type
.Vt long .
.It Fa val
Current value of the
.Dv *obj .
.El
.Pp
The current value of the variable pointed to by the
.Fa obj
argument is compared with the
.Fa val .
If they are equal, the requesting thread is put to interruptible sleep
until woken up or the optionally specified timeout expires.
.Pp
The comparison and sleep are atomic.
In other words, if another thread writes a new value to
.Dv *obj
and then issues
.Dv UMTX_OP_WAKE ,
the request is guaranteed to not miss the wakeup,
which might otherwise happen between comparison and blocking.
.Pp
The physical address of memory where the
.Fa *obj
variable is located, is used as a key to index sleeping threads.
.Pp
The read of the current value of the
.Dv *obj
variable is not guarded by barriers.
In particular, it is the user's duty to ensure the lock acquire
and release memory semantics, if the
.Dv UMTX_OP_WAIT
and
.Dv UMTX_OP_WAKE
requests are used as a substrate for implementing a simple lock.
.Pp
The request is not restartable.
An unblocked signal delivered during the wait always results in sleep
interruption and
.Er EINTR
error.
.Pp
Optionally, a timeout for the request may be specified.
.It Dv UMTX_OP_WAKE
Wake the threads possibly sleeping due to
.Dv UMTX_OP_WAIT .
The arguments for the request are:
.Bl -tag -width "obj"
.It Fa obj
Pointer to a variable, used as a key to find sleeping threads.
.It Fa val
Up to
.Fa val
threads are woken up by this request.
Specify
.Dv INT_MAX
to wake up all waiters.
.El
.It Dv UMTX_OP_MUTEX_TRYLOCK
Try to lock umutex.
The arguments to the request are:
.Bl -tag -width "obj"
.It Fa obj
Pointer to the umutex.
.El
.Pp
Operates same as the
.Dv UMTX_OP_MUTEX_LOCK
request, but returns
.Er EBUSY
instead of sleeping if the lock cannot be obtained immediately.
.It Dv UMTX_OP_MUTEX_LOCK
Lock umutex.
The arguments to the request are:
.Bl -tag -width "obj"
.It Fa obj
Pointer to the umutex.
.El
.Pp
Locking is performed by writing the current thread id into the
.Dv m_owner
word of the
.Vt struct umutex .
The write is atomic, preserves the
.Dv UMUTEX_CONTESTED
contention indicator, and provides the acquire barrier for
lock entrance semantic.
.Pp
If the lock cannot be obtained immediately because another thread owns
the lock, the current thread is put to sleep, with
.Dv UMUTEX_CONTESTED
bit set before.
Upon wake up, the lock conditions are re-tested.
.Pp
The request adheres to the priority protection or inheritance protocol
of the mutex, specified by the
.Dv UMUTEX_PRIO_PROTECT
or
.Dv UMUTEX_PRIO_INHERIT
flag, respectively.
.Pp
Optionally, a timeout for the request may be specified.
.Pp
A request with a timeout specified is not restartable.
An unblocked signal delivered during the wait always results in sleep
interruption and
.Er EINTR
error.
A request without timeout specified is always restarted after return
from a signal handler.
.It Dv UMTX_OP_MUTEX_UNLOCK
Unlock umutex.
The arguments to the request are:
.Bl -tag -width "obj"
.It Fa obj
Pointer to the umutex.
.El
.Pp
Unlocks the mutex, by writing
.Dv UMUTEX_UNOWNED
(zero) value into
.Dv m_owner
word of the
.Vt struct umutex .
The write is done with a release barrier, to provide lock leave semantic.
.Pp
If there are threads sleeping in the sleep queue associated with the
umutex, one thread is woken up.
If more than one thread sleeps in the sleep queue, the
.Dv UMUTEX_CONTESTED
bit is set together with the write of the
.Dv UMUTEX_UNOWNED
value into
.Dv m_owner .
.Pp
The request adheres to the priority protection or inheritance protocol
of the mutex, specified by the
.Dv UMUTEX_PRIO_PROTECT
or
.Dv UMUTEX_PRIO_INHERIT
flag, respectively.
See description of the
.Dv m_ceilings
member of the
.Vt struct umutex
structure for additional details of the request operation on the
priority protected protocol mutex.
.It Dv UMTX_OP_SET_CEILING
Set ceiling for the priority protected umutex.
The arguments to the request are:
.Bl -tag -width "uaddr"
.It Fa obj
Pointer to the umutex.
.It Fa val
New ceiling value.
.It Fa uaddr
Address of a variable of type
.Vt uint32_t .
If not
.Dv NULL
and the update was successful, the previous ceiling value is
written to the location pointed to by
.Fa uaddr .
.El
.Pp
The request locks the umutex pointed to by the
.Fa obj
parameter, waiting for the lock if not immediately available.
After the lock is obtained, the new ceiling value
.Fa val
is written to the
.Dv m_ceilings[0]
member of the
.Vt struct umutex,
after which the umutex is unlocked.
.Pp
The locking does not adhere to the priority protect protocol,
to conform to the
.Tn POSIX
requirements for the
.Xr pthread_mutex_setprioceiling 3
interface.
.It Dv UMTX_OP_CV_WAIT
Wait for a condition.
The arguments to the request are:
.Bl -tag -width "uaddr2"
.It Fa obj
Pointer to the
.Vt struct ucond .
.It Fa val
Request flags, see below.
.It Fa uaddr
Pointer to the umutex.
.It Fa uaddr2
Optional pointer to a
.Vt struct timespec
for timeout specification.
.El
.Pp
The request must be issued by the thread owning the mutex pointed to
by the
.Fa uaddr
argument.
The
.Dv c_hash_waiters
member of the
.Vt struct ucond ,
pointed to by the
.Fa obj
argument, is set to an arbitrary non-zero value, after which the
.Fa uaddr
mutex is unlocked (following the appropriate protocol), and
the current thread is put to sleep on the sleep queue keyed by
the
.Fa obj
argument.
The operations are performed atomically.
It is guaranteed to not miss a wakeup from
.Dv UMTX_OP_CV_SIGNAL
or
.Dv UMTX_OP_CV_BROADCAST
sent between mutex unlock and putting the current thread on the sleep queue.
.Pp
Upon wakeup, if the timeout expired and no other threads are sleeping in
the same sleep queue, the
.Dv c_hash_waiters
member is cleared.
After wakeup, the
.Fa uaddr
umutex is not relocked.
.Pp
The following flags are defined:
.Bl -tag -width "CVWAIT_CLOCKID"
.It Dv CVWAIT_ABSTIME
Timeout is absolute.
.It Dv CVWAIT_CLOCKID
Clockid is provided.
.El
.Pp
Optionally, a timeout for the request may be specified.
Unlike other requests, the timeout value is specified directly by a
.Vt struct timespec ,
pointed to by the
.Fa uaddr2
argument.
If the
.Dv CVWAIT_CLOCKID
flag is provided, the timeout uses the clock from the
.Dv c_clockid
member of the
.Vt struct ucond ,
pointed to by
.Fa obj
argument.
Otherwise,
.Dv CLOCK_REALTIME
is used, regardless of the clock identifier possibly specified in the
.Vt struct _umtx_time .
If the
.Dv CVWAIT_ABSTIME
flag is supplied, the timeout specifies absolute time value, otherwise
it denotes a relative time interval.
.Pp
The request is not restartable.
An unblocked signal delivered during
the wait always results in sleep interruption and
.Er EINTR
error.
.It Dv UMTX_OP_CV_SIGNAL
Wake up one condition waiter.
The arguments to the request are:
.Bl -tag -width "obj"
.It Fa obj
Pointer to
.Vt struct ucond .
.El
.Pp
The request wakes up at most one thread sleeping on the sleep queue keyed
by the
.Fa obj
argument.
If the woken up thread was the last on the sleep queue, the
.Dv c_has_waiters
member of the
.Vt struct ucond
is cleared.
.It Dv UMTX_OP_CV_BROADCAST
Wake up all condition waiters.
The arguments to the request are:
.Bl -tag -width "obj"
.It Fa obj
Pointer to
.Vt struct ucond .
.El
.Pp
The request wakes up all threads sleeping on the sleep queue keyed by the
.Fa obj
argument.
The
.Dv c_has_waiters
member of the
.Vt struct ucond
is cleared.
.It Dv UMTX_OP_WAIT_UINT
Same as
.Dv UMTX_OP_WAIT ,
but the type of the variable pointed to by
.Fa obj
is
.Vt u_int
.Pq a 32-bit integer .
.It Dv UMTX_OP_RW_RDLOCK
Read-lock a
.Vt struct rwlock
lock.
The arguments to the request are:
.Bl -tag -width "obj"
.It Fa obj
Pointer to the lock (of type
.Vt struct rwlock )
to be read-locked.
.It Fa val
Additional flags to augment locking behaviour.
The valid flags in the
.Fa val
argument are:
.Bl -tag -width indent
.It Dv URWLOCK_PREFER_READER
.El
.El
.Pp
The request obtains the read lock on the specified
.Vt struct rwlock
by incrementing the count of readers in the
.Dv rw_state
word of the structure.
If the
.Dv URWLOCK_WRITE_OWNER
bit is set in the word
.Dv rw_state ,
the lock was granted to a writer which has not yet relinquished
its ownership.
In this case the current thread is put to sleep until it makes sense to
retry.
.Pp
If the
.Dv URWLOCK_PREFER_READER
flag is set either in the
.Dv rw_flags
word of the structure, or in the
.Fa val
argument of the request, the presence of the threads trying to obtain
the write lock on the same structure does not prevent the current thread
from trying to obtain the read lock.
Otherwise, if the flag is not set, and the
.Dv URWLOCK_WRITE_WAITERS
flag is set in
.Dv rw_state ,
the current thread does not attempt to obtain read-lock.
Instead it sets the
.Dv URWLOCK_READ_WAITERS
in the
.Dv rw_state
word and puts itself to sleep on corresponding sleep queue.
Upon wakeup, the locking conditions are re-evaluated.
.Pp
Optionally, a timeout for the request may be specified.
.Pp
The request is not restartable.
An unblocked signal delivered during the wait always results in sleep
interruption and
.Er EINTR
error.
.It Dv UMTX_OP_RW_WRLOCK
Write-lock a
.Vt struct rwlock
lock.
The arguments to the request are:
.Bl -tag -width "obj"
.It Fa obj
Pointer to the lock (of type
.Vt struct rwlock )
to be write-locked.
.El
.Pp
The request obtains a write lock on the specified
.Vt struct rwlock ,
by setting the
.Dv URWLOCK_WRITE_OWNER
bit in the
.Dv rw_state
word of the structure.
If there is already a write lock owner, as indicated by the
.Dv URWLOCK_WRITE_OWNER
bit being set, or there are read lock owners, as indicated
by the read-lock counter, the current thread does not attempt to
obtain the write-lock.
Instead it sets the
.Dv URWLOCK_WRITE_WAITERS
in the
.Dv rw_state
word and puts itself to sleep on corresponding sleep queue.
Upon wakeup, the locking conditions are re-evaluated.
.Pp
Optionally, a timeout for the request may be specified.
.Pp
The request is not restartable.
An unblocked signal delivered during the wait always results in sleep
interruption and
.Er EINTR
error.
.It Dv UMTX_OP_RW_UNLOCK
Unlock rwlock.
The arguments to the request are:
.Bl -tag -width "obj"
.It Fa obj
Pointer to the lock (of type
.Vt struct rwlock )
to be unlocked.
.El
.Pp
The unlock type (read or write) is determined by the
current lock state.
Note that the
.Vt struct rwlock
does not save information about the identity of the thread which
acquired the lock.
.Pp
If there are pending writers after the unlock, and the
.Dv URWLOCK_PREFER_READER
flag is not set in the
.Dv rw_flags
member of the
.Fa *obj
structure, one writer is woken up, selected as described in the
.Sx SLEEP QUEUES
subsection.
If the
.Dv URWLOCK_PREFER_READER
flag is set, a pending writer is woken up only if there is
no pending readers.
.Pp
If there are no pending writers, or, in the case that the
.Dv URWLOCK_PREFER_READER
flag is set, then all pending readers are woken up by unlock.
.It Dv UMTX_OP_WAIT_UINT_PRIVATE
Same as
.Dv UMTX_OP_WAIT_UINT ,
but unconditionally select the process-private sleep queue.
.It Dv UMTX_OP_WAKE_PRIVATE
Same as
.Dv UMTX_OP_WAKE ,
but unconditionally select the process-private sleep queue.
.It Dv UMTX_OP_MUTEX_WAIT
Wait for mutex availability.
The arguments to the request are:
.Bl -tag -width "obj"
.It Fa obj
Address of the mutex.
.El
.Pp
Similarly to the
.Dv UMTX_OP_MUTEX_LOCK ,
put the requesting thread to sleep if the mutex lock cannot be obtained
immediately.
The
.Dv UMUTEX_CONTESTED
bit is set in the
.Dv m_owner
word of the mutex to indicate that there is a waiter, before the thread
is added to the sleep queue.
Unlike the
.Dv UMTX_OP_MUTEX_LOCK
request, the lock is not obtained.
.Pp
The operation is not implemented for priority protected and
priority inherited protocol mutexes.
.Pp
Optionally, a timeout for the request may be specified.
.Pp
A request with a timeout specified is not restartable.
An unblocked signal delivered during the wait always results in sleep
interruption and
.Er EINTR
error.
A request without a timeout automatically restarts if the signal disposition
requested restart via the
.Dv SA_RESTART
flag in
.Vt struct sigaction
member
.Dv sa_flags .
.It Dv UMTX_OP_NWAKE_PRIVATE
Wake up a batch of sleeping threads.
The arguments to the request are:
.Bl -tag -width "obj"
.It Fa obj
Pointer to the array of pointers.
.It Fa val
Number of elements in the array pointed to by
.Fa obj .
.El
.Pp
For each element in the array pointed to by
.Fa obj ,
wakes up all threads waiting on the
.Em private
sleep queue with the key
being the byte addressed by the array element.
.It Dv UMTX_OP_MUTEX_WAKE
Check if a normal umutex is unlocked and wake up a waiter.
The arguments for the request are:
.Bl -tag -width "obj"
.It Fa obj
Pointer to the umutex.
.El
.Pp
If the
.Dv m_owner
word of the mutex pointed to by the
.Fa obj
argument indicates unowned mutex, which has its contention indicator bit
.Dv UMUTEX_CONTESTED
set, clear the bit and wake up one waiter in the sleep queue associated
with the byte addressed by the
.Fa obj ,
if any.
Only normal mutexes are supported by the request.
The sleep queue is always one for a normal mutex type.
.Pp
This request is deprecated in favor of
.Dv UMTX_OP_MUTEX_WAKE2
since mutexes using it cannot synchronize their own destruction.
That is, the
.Dv m_owner
word has already been set to
.Dv UMUTEX_UNOWNED
when this request is made,
so that another thread can lock, unlock and destroy the mutex
(if no other thread uses the mutex afterwards).
Clearing the
.Dv UMUTEX_CONTESTED
bit may then modify freed memory.
.It Dv UMTX_OP_MUTEX_WAKE2
Check if a umutex is unlocked and wake up a waiter.
The arguments for the request are:
.Bl -tag -width "obj"
.It Fa obj
Pointer to the umutex.
.It Fa val
The umutex flags.
.El
.Pp
The request does not read the
.Dv m_flags
member of the
.Vt struct umutex ;
instead, the
.Fa val
argument supplies flag information, in particular, to determine the
sleep queue where the waiters are found for wake up.
.Pp
If the mutex is unowned, one waiter is woken up.
.Pp
If the mutex memory cannot be accessed, all waiters are woken up.
.Pp
If there is more than one waiter on the sleep queue, or there is only
one waiter but the mutex is owned by a thread, the
.Dv UMUTEX_CONTESTED
bit is set in the
.Dv m_owner
word of the
.Vt struct umutex .
.It Dv UMTX_OP_SEM2_WAIT
Wait until semaphore is available.
The arguments to the request are:
.Bl -tag -width "obj"
.It Fa obj
Pointer to the semaphore (of type
.Vt struct _usem2 ) .
.It Fa uaddr
Size of the memory passed in via the
.Fa uaddr2
argument.
.It Fa uaddr2
Optional pointer to a structure of type
.Vt struct _umtx_time ,
which may be followed by a structure of type
.Vt struct timespec .
.El
.Pp
Put the requesting thread onto a sleep queue if the semaphore counter
is zero.
If the thread is put to sleep, the
.Dv USEM_HAS_WAITERS
bit is set in the
.Dv _count
word to indicate waiters.
The function returns either due to
.Dv _count
indicating the semaphore is available (non-zero count due to post),
or due to a wakeup.
The return does not guarantee that the semaphore is available,
nor does it consume the semaphore lock on successful return.
.Pp
Optionally, a timeout for the request may be specified.
.Pp
A request with non-absolute timeout value is not restartable.
An unblocked signal delivered during such wait results in sleep
interruption and
.Er EINTR
error.
.Pp
If
.Dv UMTX_ABSTIME
was not set, and the operation was interrupted and the caller passed in a
.Fa uaddr2
large enough to hold a
.Vt struct timespec
following the initial
.Vt struct _umtx_time ,
then the
.Vt struct timespec
is updated to contain the unslept amount.
.It Dv UMTX_OP_SEM2_WAKE
Wake up waiters on semaphore lock.
The arguments to the request are:
.Bl -tag -width "obj"
.It Fa obj
Pointer to the semaphore (of type
.Vt struct _usem2 ) .
.El
.Pp
The request wakes up one waiter for the semaphore lock.
The function does not increment the semaphore lock count.
If the
.Dv USEM_HAS_WAITERS
bit was set in the
.Dv _count
word, and the last sleeping thread was woken up, the bit is cleared.
.It Dv UMTX_OP_SHM
Manage anonymous
.Tn POSIX
shared memory objects (see
.Xr shm_open 2 ) ,
which can be attached to a byte of physical memory, mapped into the
process address space.
The objects are used to implement process-shared locks in
.Dv libthr .
.Pp
The
.Fa val
argument specifies the sub-request of the
.Dv UMTX_OP_SHM
request:
.Bl -tag -width indent
.It Dv UMTX_SHM_CREAT
Creates the anonymous shared memory object, which can be looked up
with the specified key
.Fa uaddr .
If the object associated with the
.Fa uaddr
key already exists, it is returned instead of creating a new object.
The object's size is one page.
On success, the file descriptor referencing the object is returned.
The descriptor can be used for mapping the object using
.Xr mmap 2 ,
or for other shared memory operations.
.It Dv UMTX_SHM_LOOKUP
Same as
.Dv UMTX_SHM_CREATE
request, but if there is no shared memory object associated with
the specified key
.Fa uaddr ,
an error is returned, and no new object is created.
.It Dv UMTX_SHM_DESTROY
De-associate the shared object with the specified key
.Fa uaddr .
The object is destroyed after the last open file descriptor is closed
and the last mapping for it is destroyed.
.It Dv UMTX_SHM_ALIVE
Checks whether there is a live shared object associated with the
supplied key
.Fa uaddr .
Returns zero if there is, and an error otherwise.
This request is an optimization of the
.Dv UMTX_SHM_LOOKUP
request.
It is cheaper when only the liveness of the associated object is asked
for, since no file descriptor is installed in the process fd table
on success.
.El
.Pp
The
.Fa uaddr
argument specifies the virtual address, which backing physical memory
byte identity is used as a key for the anonymous shared object
creation or lookup.
.It Dv UMTX_OP_ROBUST_LISTS
Register the list heads for the current thread's robust mutex lists.
The arguments to the request are:
.Bl -tag -width "uaddr"
.It Fa val
Size of the structure passed in the
.Fa uaddr
argument.
.It Fa uaddr
Pointer to the structure of type
.Vt struct umtx_robust_lists_params .
.El
.Pp
The structure is defined as
.Bd -literal
struct umtx_robust_lists_params {
	uintptr_t	robust_list_offset;
	uintptr_t	robust_priv_list_offset;
	uintptr_t	robust_inact_offset;
};
.Ed
.Pp
The
.Dv robust_list_offset
member contains address of the first element in the list of locked
robust shared mutexes.
The
.Dv robust_priv_list_offset
member contains address of the first element in the list of locked
robust private mutexes.
The private and shared robust locked lists are split to allow fast
termination of the shared list on fork, in the child.
.Pp
The
.Dv robust_inact_offset
contains a pointer to the mutex which might be locked in nearby future,
or might have been just unlocked.
It is typically set by the lock or unlock mutex implementation code
around the whole operation, since lists can be only changed race-free
when the thread owns the mutex.
The kernel inspects the
.Dv robust_inact_offset
in addition to walking the shared and private lists.
Also, the mutex pointed to by
.Dv robust_inact_offset
is handled more loosely at the thread termination time,
than other mutexes on the list.
That mutex is allowed to be not owned by the current thread,
in which case list processing is continued.
See
.Sx ROBUST UMUTEXES
subsection for details.
.It Dv UMTX_OP_GET_MIN_TIMEOUT
Writes out the current value of minimal umtx operations timeout,
in nanoseconds, into the long integer variable pointed to by
.Fa uaddr1 .
.It Dv UMTX_OP_SET_MIN_TIMEOUT
Set the minimal amount of time, in nanoseconds, the thread is required
to sleep for umtx operations specifying a timeout using absolute clocks.
The value is taken from the
.Fa val
argument of the call.
Zero means no minimum.
.El
.Pp
The
.Fa op
argument may be a bitwise OR of a single command from above with one or more of
the following flags:
.Bl -tag -width indent
.It Dv UMTX_OP__I386
Request i386 ABI compatibility from the native
.Nm
system call.
Specifically, this implies that:
.Bl -hang -offset indent
.It
.Fa obj
arguments that point to a word, point to a 32-bit integer.
.It
The
.Dv UMTX_OP_NWAKE_PRIVATE
.Fa obj
argument is a pointer to an array of 32-bit pointers.
.It
The
.Dv m_rb_lnk
member of
.Vt struct umutex
is a 32-bit pointer.
.It
.Vt struct timespec
uses a 32-bit time_t.
.El
.Pp
.Dv UMTX_OP__32BIT
has no effect if this flag is set.
This flag is valid for all architectures, but it is ignored on i386.
.It Dv UMTX_OP__32BIT
Request non-i386, 32-bit ABI compatibility from the native
.Nm
system call.
Specifically, this implies that:
.Bl -hang -offset indent
.It
.Fa obj
arguments that point to a word, point to a 32-bit integer.
.It
The
.Dv UMTX_OP_NWAKE_PRIVATE
.Fa obj
argument is a pointer to an array of 32-bit pointers.
.It
The
.Dv m_rb_lnk
member of
.Vt struct umutex
is a 32-bit pointer.
.It
.Vt struct timespec
uses a 64-bit time_t.
.El
.Pp
This flag has no effect if
.Dv UMTX_OP__I386
is set.
This flag is valid for all architectures.
.El
.Pp
Note that if any 32-bit ABI compatibility is being requested, then care must be
taken with robust lists.
A single thread may not mix 32-bit compatible robust lists with native
robust lists.
The first
.Dv UMTX_OP_ROBUST_LISTS
call in a given thread determines which ABI that thread will use for robust
lists going forward.
.Sh RETURN VALUES
If successful,
all requests, except
.Dv UMTX_SHM_CREAT
and
.Dv UMTX_SHM_LOOKUP
sub-requests of the
.Dv UMTX_OP_SHM
request, will return zero.
The
.Dv UMTX_SHM_CREAT
and
.Dv UMTX_SHM_LOOKUP
return a shared memory file descriptor on success.
On error \-1 is returned, and the
.Va errno
variable is set to indicate the error.
.Sh ERRORS
The
.Fn _umtx_op
operations can fail with the following errors:
.Bl -tag -width "[ETIMEDOUT]"
.It Bq Er EFAULT
One of the arguments point to invalid memory.
.It Bq Er EINVAL
The clock identifier, specified for the
.Vt struct _umtx_time
timeout parameter, or in the
.Dv c_clockid
member of
.Vt struct ucond,
is invalid.
.It Bq Er EINVAL
The type of the mutex, encoded by the
.Dv m_flags
member of
.Vt struct umutex ,
is invalid.
.It Bq Er EINVAL
The
.Dv m_owner
member of the
.Vt struct umutex
has changed the lock owner thread identifier during unlock.
.It Bq Er EINVAL
The
.Dv timeout.tv_sec
or
.Dv timeout.tv_nsec
member of
.Vt struct _umtx_time
is less than zero, or
.Dv timeout.tv_nsec
is greater than 1000000000.
.It Bq Er EINVAL
The
.Fa op
argument specifies invalid operation.
.It Bq Er EINVAL
The
.Fa uaddr
argument for the
.Dv UMTX_OP_SHM
request specifies invalid operation.
.It Bq Er EINVAL
The
.Dv UMTX_OP_SET_CEILING
request specifies non priority protected mutex.
.It Bq Er EINVAL
The new ceiling value for the
.Dv UMTX_OP_SET_CEILING
request, or one or more of the values read from the
.Dv m_ceilings
array during lock or unlock operations, is greater than
.Dv RTP_PRIO_MAX .
.It Bq Er EPERM
Unlock attempted on an object not owned by the current thread.
.It Bq Er EOWNERDEAD
The lock was requested on an umutex where the
.Dv m_owner
field was set to the
.Dv UMUTEX_RB_OWNERDEAD
value, indicating terminated robust mutex.
The lock was granted to the caller, so this error in fact
indicates success with additional conditions.
.It Bq Er ENOTRECOVERABLE
The lock was requested on an umutex which
.Dv m_owner
field is equal to the
.Dv UMUTEX_RB_NOTRECOV
value, indicating abandoned robust mutex after termination.
The lock was not granted to the caller.
.It Bq Er ENOTTY
The shared memory object, associated with the address passed to the
.Dv UMTX_SHM_ALIVE
sub-request of
.Dv UMTX_OP_SHM
request, was destroyed.
.It Bq Er ESRCH
For the
.Dv UMTX_SHM_LOOKUP ,
.Dv UMTX_SHM_DESTROY ,
and
.Dv UMTX_SHM_ALIVE
sub-requests of the
.Dv UMTX_OP_SHM
request, there is no shared memory object associated with the provided key.
.It Bq Er ENOMEM
The
.Dv UMTX_SHM_CREAT
sub-request of the
.Dv UMTX_OP_SHM
request cannot be satisfied, because allocation of the shared memory object
would exceed the
.Dv RLIMIT_UMTXP
resource limit, see
.Xr setrlimit 2 .
.It Bq Er EAGAIN
The maximum number of readers
.Dv ( URWLOCK_MAX_READERS )
were already granted ownership of the given
.Vt struct rwlock
for read.
.It Bq Er EBUSY
A try mutex lock operation was not able to obtain the lock.
.It Bq Er ETIMEDOUT
The request specified a timeout in the
.Fa uaddr
and
.Fa uaddr2
arguments, and timed out before obtaining the lock or being woken up.
.It Bq Er EINTR
A signal was delivered during wait, for a non-restartable operation.
Operations with timeouts are typically non-restartable, but timeouts
specified in absolute time may be restartable.
.It Bq Er ERESTART
A signal was delivered during wait, for a restartable operation.
Mutex lock requests without timeout specified are restartable.
The error is not returned to userspace code since restart
is handled by usual adjustment of the instruction counter.
.El
.Sh SEE ALSO
.Xr clock_gettime 2 ,
.Xr mmap 2 ,
.Xr setrlimit 2 ,
.Xr shm_open 2 ,
.Xr sigaction 2 ,
.Xr thr_exit 2 ,
.Xr thr_kill 2 ,
.Xr thr_kill2 2 ,
.Xr thr_new 2 ,
.Xr thr_self 2 ,
.Xr thr_set_name 2 ,
.Xr signal 3
.Sh STANDARDS
The
.Fn _umtx_op
system call is non-standard and is used by the
.Lb libthr
to implement
.St -p1003.1-2001
.Xr pthread 3
functionality.
.Sh BUGS
A window between a unlocking robust mutex and resetting the pointer in the
.Dv robust_inact_offset
member of the registered
.Vt struct umtx_robust_lists_params
allows another thread to destroy the mutex, thus making the kernel inspect
freed or reused memory.
The
.Li libthr
implementation is only vulnerable to this race when operating on
a shared mutex.
A possible fix for the current implementation is to strengthen the checks
for shared mutexes before terminating them, in particular, verifying
that the mutex memory is mapped from a shared memory object allocated
by the
.Dv UMTX_OP_SHM
request.
This is not done because it is believed that the race is adequately
covered by other consistency checks, while adding the check would
prevent alternative implementations of
.Li libpthread .