that it operates on lockmgr and sx locks. This can be useful for tracking
down vnode deadlocks in VFS for example. Note that this command is a bit
more fragile than 'show lockchain' as we have to poke around at the
wait channel of a thread to see if it points to either a struct lock or
a condition variable inside of a struct sx. If td_wchan points to
something unmapped, then this command will terminate early due to a fault,
but no harm will be done.
'show lockchain'. The churn is because I'm about to add a new
'show sleepchain' similar to 'show lockchain' for sleep locks (lockmgr and
sx) and 'show threadchain' was a bit ambiguous as both commands show
a chain of thread dependencies, 'lockchain' is for non-sleepable locks
(mtx and rw) and 'sleepchain' is for sleepable locks.
problems in ddb:
- "show threadchain [thread]" will start with the specified thread (or the
current kdb thread by default) and show it's state. If it is blocked on
a lock, it will find the owner of the lock and show its state, etc.
- "show allchains" will find all of the threads that are blocked on a
lock (but do not have any threads blocked on a lock they hold) and show
the resulting thread chain.
- "show lockchain <lock>" takes a pointer to a lock_object (such as a
mutex or rwlock). If there is a turnstile for that lock, then it will
display all the threads blocked on the lock. In addition, for each
thread blocked on the lock, it will display any contested locks they
hold, and recurse on those locks to show any threads blocked on those
locks, etc.
a lock's priority to a sleeping thread. When we panic, dump a stack
trace of the thread that is asleep if DDB is compiled into the kernel
just before calling panic(). This is much more informative and useful
for debugging than the current behavior of getting a page fault and not
having an easy way of determining which thread caused the original problem.
MFC after: 1 week
each turnstile. Also, allow for the owner thread pointer of a turnstile
to be NULL. This is needed for the upcoming reader/writer lock
implementation.
- Add a new ddb command 'show turnstile' that will look up the turnstile
associated with the given lock argument and display useful information
like the list of threads blocked on each queue, etc. If there isn't an
active turnstile for a lock at the specified address, then the function
will see if there is an active turnstile at the specified address and
display info about it if so.
- Adjust the mutex code to handle the turnstile API changes.
Tested on: i386 (all), alpha, amd64, sparc64 (1 and 3)
schedulers a bit to ensure more correct handling of priorities and fewer
priority inversions:
- Add two functions to the sched(9) API to handle priority lending:
sched_lend_prio() and sched_unlend_prio(). The turnstile code uses these
functions to ask the scheduler to lend a thread a set priority and to
tell the scheduler when it thinks it is ok for a thread to stop borrowing
priority. The unlend case is slightly complex in that the turnstile code
tells the scheduler what the minimum priority of the thread needs to be
to satisfy the requirements of any other threads blocked on locks owned
by the thread in question. The scheduler then decides where the thread
can go back to normal mode (if it's normal priority is high enough to
satisfy the pending lock requests) or it it should continue to use the
priority specified to the sched_unlend_prio() call. This involves adding
a new per-thread flag TDF_BORROWING that replaces the ULE-only kse flag
for priority elevation.
- Schedulers now refuse to lower the priority of a thread that is currently
borrowing another therad's priority.
- If a scheduler changes the priority of a thread that is currently sitting
on a turnstile, it will call a new function turnstile_adjust() to inform
the turnstile code of the change. This function resorts the thread on
the priority list of the turnstile if needed, and if the thread ends up
at the head of the list (due to having the highest priority) and its
priority was raised, then it will propagate that new priority to the
owner of the lock it is blocked on.
Some additional fixes specific to the 4BSD scheduler include:
- Common code for updating the priority of a thread when the user priority
of its associated kse group has been consolidated in a new static
function resetpriority_thread(). One change to this function is that
it will now only adjust the priority of a thread if it already has a
time sharing priority, thus preserving any boosts from a tsleep() until
the thread returns to userland. Also, resetpriority() no longer calls
maybe_resched() on each thread in the group. Instead, the code calling
resetpriority() is responsible for calling resetpriority_thread() on
any threads that need to be updated.
- schedcpu() now uses resetpriority_thread() instead of just calling
sched_prio() directly after it updates a kse group's user priority.
- sched_clock() now uses resetpriority_thread() rather than writing
directly to td_priority.
- sched_nice() now updates all the priorities of the threads after the
group priority has been adjusted.
Discussed with: bde
Reviewed by: ups, jeffr
Tested on: 4bsd, ule
Tested on: i386, alpha, sparc64
- Add a new _lock() call to each API that locks the associated chain lock
for a lock_object pointer or wait channel. The _lookup() functions now
require that the chain lock be locked via _lock() when they are called.
- Change sleepq_add(), turnstile_wait() and turnstile_claim() to lookup
the associated queue structure internally via _lookup() rather than
accepting a pointer from the caller. For turnstiles, this means that
the actual lookup of the turnstile in the hash table is only done when
the thread actually blocks rather than being done on each loop iteration
in _mtx_lock_sleep(). For sleep queues, this means that sleepq_lookup()
is no longer used outside of the sleep queue code except to implement an
assertion in cv_destroy().
- Change sleepq_broadcast() and sleepq_signal() to require that the chain
lock is already required. For condition variables, this lets the
cv_broadcast() and cv_signal() functions lock the sleep queue chain lock
while testing the waiters count. This means that the waiters count
internal to condition variables is no longer protected by the interlock
mutex and cv_broadcast() and cv_signal() now no longer require that the
interlock be held when they are called. This lets consumers of condition
variables drop the lock before waking other threads which can result in
fewer context switches.
MFC after: 1 month
turnstile chain lock until after making all the awakened threads
runnable. First, this fixes a priority inversion race. Second, this
attempts to finish waking up all of the threads waiting on a turnstile
before doing a preemption.
Reviewed by: Stephan Uphoff (who found the priority inversion race)
switch to. If a non-NULL thread pointer is passed in, then the CPU will
switch to that thread directly rather than calling choosethread() to pick
a thread to choose to.
- Make sched_switch() aware of idle threads and know to do
TD_SET_CAN_RUN() instead of sticking them on the run queue rather than
requiring all callers of mi_switch() to know to do this if they can be
called from an idlethread.
- Move constants for arguments to mi_switch() and thread_single() out of
the middle of the function prototypes and up above into their own
section.
creation of the sysctl tree for the turnstile profiling stats until a
SI_SUB_LOCK sysinit. Doing it in init_turnstiles() is too early as it is
called before mi_startup().
hash tables used in the sleep queue and turnstile code. Each option adds
a sysctl tree under debug containing the maximum depth of any bucket in
the hash table as well as a separate node for each bucket (or chain)
containing the current depth and maximum depth for that bucket.
more consistent with other APIs. sleepq and cv's use signal/broadcast, and
msleep uses wakeup_one/wakeup. Prior to this turnstiles were using a
signal/wakeup mixture.
to queue threads sleeping on a wait channel similar to how turnstiles are
used to queue threads waiting for a lock. This subsystem will be used as
the backend for sleep/wakeup and condition variables initially. Eventually
it will also be used to replace the ithread-specific iwait thread
inhibitor.
Sleep queues are also not locked by sched_lock, so this splits sched_lock
up a bit further increasing concurrency within the scheduler. Sleep queues
also natively support timeouts on sleeps and interruptible sleeps allowing
for the reduction of a lot of duplicated code between the sleep/wakeup and
condition variable implementations. For more details on the sleep queue
implementation, check the comments in sys/sleepqueue.h and
kern/subr_sleepqueue.c.
SW_INVOL. Assert that one of these is set in mi_switch() and propery
adjust the rusage statistics. This is to simplify the large number of
users of this interface which were previously all required to adjust the
proper counter prior to calling mi_switch(). This also facilitates more
switch and locking optimizations.
- Change all callers of mi_switch() to pass the appropriate paramter and
remove direct references to the process statistics.
turnstile_unpend(). A racing thread that does not have TDI_LOCK set may
either be running on another CPU or it may be sitting on a run queue if it
was preempted during the very small window in turnstile_wait() between
unlocking the turnstile chain lock and locking sched_lock.
case of a turnstile having no threads is just one instance of the more
general case where the thread we are examining has been partially awakened
already in that it has been removed from the turnstile's blocked list but
still has TDI_LOCK set. We detect that case by checking to see if the
thread has already had a turnstile reassigned to it.
and empty its turnstile while the blocking threads still pointed to the
turnstile. If the thread on the first CPU blocked on a lock owned by
one of the threads blocked on the turnstile just woken up, then the
first CPU could try to manipulate a bogus thread queue in the turnstile
during priority propagation.
- Update locking notes for ts_owner and always clear ts_owner, not just
under INVARIANTS.
Tested by: sam (1)
turnstiles to implement blocking isntead of implementing a thread queue
directly. These turnstiles are somewhat similar to those used in Solaris 7
as described in Solaris Internals but are also different.
Turnstiles do not come out of a fixed-sized pool. Rather, each thread is
assigned a turnstile when it is created that it frees when it is destroyed.
When a thread blocks on a lock, it donates its turnstile to that lock to
serve as queue of blocked threads. The queue associated with a given lock
is found by a lookup in a simple hash table. The turnstile itself is
protected by a lock associated with its entry in the hash table. This
means that sched_lock is no longer needed to contest on a mutex. Instead,
sched_lock is only used when manipulating run queues or thread priorities.
Turnstiles also implement priority propagation inherently.
Currently turnstiles only support mutexes. Eventually, however, turnstiles
may grow two queue's to support a non-sleepable reader/writer lock
implementation. For more details, see the comments in sys/turnstile.h and
kern/subr_turnstile.c.
The two primary advantages from the turnstile code include: 1) the size
of struct mutex shrinks by four pointers as it no longer stores the
thread queue linkages directly, and 2) less contention on sched_lock in
SMP systems including the ability for multiple CPUs to contend on different
locks simultaneously (not that this last detail is necessarily that much of
a big win). Note that 1) means that this commit is a kernel ABI breaker,
so don't mix old modules with a new kernel and vice versa.
Tested on: i386 SMP, sparc64 SMP, alpha SMP
thread's pid to make debugging easier for people who don't want to have to
use the intended tool for these panics (witness).
Indirectly prodded by: kris
locks held by each thread.
- Fix a bug in the original BSD/OS code where a contested lock was not
properly handed off from the old thread to the new thread when a
contested lock with more than one blocked thread was transferred from
one thread to another.
- Don't use an atomic operation to write the MTX_CONTESTED value to
mtx_lock in the aforementioned special case. The memory barriers and
exclusion provided by sched_lock are sufficient.
Spotted by: alc (2)
already own. The mtx_trylock() will fail however. Enhance the comment
at the top of the try lock function to explain this.
Requested by: jlemon and his evil netisr locking
- Declare some local variables at the top of the function instead of in a
nested block.
- Use mtx_owned() instead of masking off bits from mtx_lock manually.
- Read the value of mtx_lock into 'v' as a separate line rather than inside
an if statement for clarity. This code is hairy enough as it is.
owned. Previously the KASSERT would only trigger if we successfully
acquired a lock that we already held. However, _obtain_lock() fails to
acquire locks that we already hold, so the KASSERT was never checked in
the case it was supposed to fail.
o Always check for null when dereferencing the filename component.
o Implement a try-and-backoff method for allocating memory to
dump stats to avoid a spin-lock -> sleep-lock mutex lock order
panic with WITNESS.
Approved by: des, markm (mentor)
Not objected: jhb
does not require Giant.
This means that we may miss panics on a class of mutex programming bugs,
but only if running with a Chernobyl setting of debug-flags.
Spotted by: Pete Carah <pete@ns.altadena.net>
