freebsd-skq/share/man/man9/rmlock.9

.\" Copyright (c) 2007 Stephan Uphoff <ups@FreeBSD.org>
.\" Copyright (c) 2006 Gleb Smirnoff <glebius@FreeBSD.org>
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.\" Based on rwlock.9 man page
.Dd November 16, 2011
.Dt RMLOCK 9
.Os
.Sh NAME
.Nm rmlock ,
.Nm rm_init ,
.Nm rm_init_flags ,
.Nm rm_destroy ,
.Nm rm_rlock ,
.Nm rm_try_rlock ,
.Nm rm_wlock ,
.Nm rm_runlock ,
.Nm rm_wunlock ,
.Nm rm_wowned ,
.Nm RM_SYSINIT
.Nd kernel reader/writer lock optimized for mostly read access patterns
.Sh SYNOPSIS
.In sys/param.h
.In sys/lock.h
.In sys/rmlock.h
.Ft void
.Fn rm_init "struct rmlock *rm" "const char *name"
.Ft void
.Fn rm_init_flags "struct rmlock *rm" "const char *name" "int opts"
.Ft void
.Fn rm_destroy "struct rmlock *rm"
.Ft void
.Fn rm_rlock "struct rmlock *rm"  "struct rm_priotracker* tracker"
.Ft int
.Fn rm_try_rlock "struct rmlock *rm"  "struct rm_priotracker* tracker"
.Ft void
.Fn rm_wlock "struct rmlock *rm"
.Ft void
.Fn rm_runlock "struct rmlock *rm" "struct rm_priotracker* tracker"
.Ft void
.Fn rm_wunlock "struct rmlock *rm"
.Ft int
.Fn rm_wowned "const struct rmlock *rm"
.In sys/kernel.h
.Fn RM_SYSINIT "name" "struct rmlock *rm" "const char *desc" "int opts"
.Sh DESCRIPTION
Mostly reader locks allow shared access to protected data by multiple threads,
or exclusive access by a single thread.
The threads with shared access are known as
.Em readers
since they only read the protected data.
A thread with exclusive access is known as a
.Em writer
since it can modify protected data.
.Pp
Read mostly locks are designed to be efficient for locks almost exclusively
used as reader locks and as such should be used for protecting data that
rarely changes.
Acquiring an exclusive lock after the lock had been locked for shared access
is an expensive operation. 
.Pp
Although reader/writer locks look very similar to
.Xr sx 9
locks, their usage pattern is different.
Reader/writer locks can be treated as mutexes (see
.Xr mutex 9 )
with shared/exclusive semantics unless initialized with
.Dv RM_SLEEPABLE .
Unlike
.Xr sx 9 ,
an
.Nm
can be locked while holding a non-spin mutex, and an
.Nm
cannot be held while sleeping, again unless initialized with
.Dv RM_SLEEPABLE .
The
.Nm
locks have full priority propagation like mutexes.
The
.Va rm_priotracker
structure argument supplied in
.Fn rm_rlock
and
.Fn rm_runlock
is used to keep track of the read owner(s).
Another important property is that shared holders of
.Nm
can recurse if the lock has been initialized with the
.Dv LO_RECURSABLE
option, however exclusive locks are not allowed to recurse.
.Ss Macros and Functions
.Bl -tag -width indent
.It Fn rm_init "struct rmlock *rm" "const char *name"
Initialize structure located at
.Fa rm
as mostly reader lock, described by
.Fa name .
The name description is used solely for debugging purposes.
This function must be called before any other operations
on the lock.
.It Fn rm_init_flags "struct rmlock *rm" "const char *name" "int opts"
Initialize the rm lock just like the
.Fn rm_init
function, but specifying a set of optional flags to alter the
behaviour of
.Fa rm ,
through the
.Fa opts
argument.
It contains one or more of the following flags:
.Bl -tag -width ".Dv RM_NOWITNESS"
.It Dv RM_NOWITNESS
Instruct
.Xr witness 4
to ignore this lock.
.It Dv RM_RECURSE
Allow threads to recursively acquire exclusive locks for
.Fa rm .
.It Dv RM_SLEEPABLE
Allow writers to sleep while holding the lock.
Readers must not sleep while holding the lock and can avoid to sleep on
taking the lock by using
.Fn rm_try_rlock
instead of
.Fn rm_rlock .
.El
.It Fn rm_rlock "struct rmlock *rm" "struct rm_priotracker* tracker"
Lock
.Fa rm
as a reader.
Using
.Fa tracker
to track read owners of a lock for priority propagation.
This data structure is only used internally by
.Nm
and must persist until
.Fn rm_runlock
has been called.
This data structure can be allocated on the stack since
rmlocks cannot be held while sleeping.
If any thread holds this lock exclusively, the current thread blocks,
and its priority is propagated to the exclusive holder.
If the lock was initialized with the
.Dv LO_RECURSABLE
option the
.Fn rm_rlock
function can be called when the thread has already acquired reader
access on
.Fa rm .
This is called
.Dq "recursing on a lock" .
.It Fn rm_try_rlock "struct rmlock *rm" "struct rm_priotracker* tracker"
Try to lock
.Fa rm
as a reader.
.Fn rm_try_rlock
will return 0 if the lock cannot be acquired immediately;
otherwise the lock will be acquired and a non-zero value will be returned.
Note that
.Fn rm_try_rlock
may fail even while the lock is not currently held by a writer.
.It Fn rm_wlock "struct rmlock *rm"
Lock
.Fa rm
as a writer.
If there are any shared owners of the lock, the current thread blocks.
The
.Fn rm_wlock
function cannot be called recursively.
.It Fn rm_runlock "struct rmlock *rm" "struct rm_priotracker* tracker"
This function releases a shared lock previously acquired by
.Fn rm_rlock .
The
.Fa tracker
argument must match the 
.Fa tracker 
argument used for acquiring the shared lock 
.It Fn rm_wunlock "struct rmlock *rm"
This function releases an exclusive lock previously acquired by
.Fn rm_wlock .
.It Fn rm_destroy "struct rmlock *rm"
This functions destroys a lock previously initialized with
.Fn rm_init .
The
.Fa rm
lock must be unlocked.
.It Fn rm_wowned "const struct rmlock *rm"
This function returns a non-zero value if the current thread owns an
exclusive lock on
.Fa rm .
.El
.Sh SEE ALSO
.Xr locking 9 ,
.Xr mutex 9 ,
.Xr panic 9 ,
.Xr rwlock 9 ,
.Xr sema 9 ,
.Xr sx 9
.Sh HISTORY
These
functions appeared in
.Fx 7.0 .
.Sh AUTHORS
.An -nosplit
The
.Nm
facility was written by
.An "Stephan Uphoff" .
This manual page was written by
.An "Gleb Smirnoff"
for rwlock and modified to reflect rmlock by
.An "Stephan Uphoff" .
.Sh BUGS
The
.Nm
implementation is currently not optimized for single processor systems.
.Pp
.Fn rm_try_rlock
can fail transiently even when there is no writer, while another reader
updates the state on the local CPU.
.Pp
The
.Nm
implementation uses a single per CPU list shared by all
rmlocks in the system.
If rmlocks become popular, hashing to multiple per CPU queues may
be needed to speed up the writer lock process. 
.Pp
The
.Nm
can currently not be used as a lock argument for condition variable
wait functions.