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This patch solves two issues. The first one is cancellation handling
when a cancellation request is sent before reaching a cancellation
point (namely, pthread_cond_{timed,}wait). Cancellation is implemented
by pushing an appropriate cleanup handler and switching to
PTHREAD_CANCEL_ASYNCHRONOUS type. The main problem is that it doesn't
handle pending requests, only a cancellation that occurs while blocking.
Other problems occur when trying to correctly handle a timeout and a
cancellation request through the cleanup routine.
The other issue is correctly handling timeouts. This problem was already
well known, as explained by the following comment :
"FIXME: What do we do if we get a wakeup message before we disconnect
ourself? It may remain until the next time we block."
In addition, the prevp thread member is inconsistently used. It is
sometimes accessed while protected by the appropriate queue lock to
determine whether a thread is still queued, while at times, threads
are unqueued without holding a lock, as in pthread_cond_broadcast :
/* We can safely walk the list of waiting threads without holding
the lock since it is now decoupled from the condition. */
__pthread_dequeuing_iterate (wakeup, wakeup)
__pthread_wakeup (wakeup);
This is the root cause that triggers some assertion failures.
The solution brought by this patch is to consistently use the prevp link
to determine if both a thread has been unqueued and if a wakeup message
has been sent (both are needed to wake up a thread). A thread unblocked
because of a timeout can now accurately determine if it needs to drain
its message queue. A direct improvement is that the message queue size
can be limited to one message, and wakeups are guaranteed to be
non-blocking, which allows safely calling __pthread_wakeup from critical
sections.
As it now affects the cleanup cancellation routine of
__pthread_cond_timedwait_internal, cancellation is reworked as well.
Cancellation type is forced to PTHREAD_CANCEL_DEFERRED during the call,
and actually checked on both entry and return. A hook is set by the
blocking thread so that the waker doesn't need to know about the call
implementation. Cancellation members are now protected with a mutex for
truely safe access.
* pthread/pt-alloc.c (initialize_pthread): Initialize the new `cancel_lock',
`cancel_hook' and `cancel_hook_args' fields.
* pthread/pt-cancel.c (pthread_cancel): Rework cancellation handling.
* pthread/pt-internal.h (struct __pthread): Add `cancel_lock', `cancel_hook'
and `cancel_hook_args' fields.
(__pthread_dequeue): Assert thread->prevp isn't NULL.
* pthread/pt-join.c (pthread_join): Describe how the cancellation point is
implemented.
* pthread/pt-setcancelstate.c (__pthread_setcancelstate): Lock the given
thread cancellation lock when switching state.
* pthread/pt-setcanceltype.c (__pthread_setcanceltype): Likewise for
cancellation type.
* pthread/pt-testcancel.c (pthread_testcancel): Likewise for pending
cancellations.
* sysdeps/generic/pt-cond-brdcast.c (__pthread_cond_broadcast): Dequeue
and wake up threads with condition locked.
* sysdeps/generic/pt-cond-signal.c (cond_signal): Remove function, move
implementation to ...
(__pthread_cond_signal): ... this function. Remove unused `unblocked'
variable.
* sysdeps/generic/pt-cond-timedwait.c (struct cancel_ctx): New structure.
(cancel_hook): New static function.
(__pthread_cond_timedwait_internal): Fix cancellation and timeout handling.
* sysdeps/generic/pt-mutex-timedlock.c
(__pthread_mutex_timedlock_internal): Fix timeout handling.
* sysdeps/generic/pt-rwlock-timedrdlock.c
(__pthread_rwlock_timedrdlock_internal): Likewise.
* sysdeps/generic/pt-rwlock-timedwrlock.c
(__pthread_rwlock_timedwrlock_internal): Likewise.
* sysdeps/generic/pt-rwlock-unlock.c (pthread_rwlock_unlock): Dequeue and
wake up threads with rwlock internal lock held.
* sysdeps/generic/sem-timedwait.c (__sem_timedwait_internal): Fix timeout
handling.
* sysdeps/mach/hurd/pt-docancel.c (__pthread_do_cancel): Unlock the given
thread cancellation lock.
* sysdeps/mach/pt-thread-alloc.c (create_wakeupmsg): Limit the message
queue size of the wakeup port to 1.
* sysdeps/mach/pt-wakeup.c (__pthread_wakeup): Call __mach_msg in a
non-blocking way.
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