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Fix missing wake-ups in pthread_rwlock_rdlock.

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This adds wake-ups that would be missing if assuming that for a
non-writer-preferring rwlock, if one thread has acquired a rdlock and
does not release it, another thread will eventually acquire a rdlock too
despite concurrent write lock acquisition attempts.  BZ 14958 is about
supporting this assumption.  Strictly speaking, this isn't a valid
test case, but nonetheless worth supporting (see comment 7 of BZ 14958).
This commit is contained in:
Torvald Riegel
2015-04-28 23:24:36 +02:00
parent 3c9c61febe
commit b634486d57
9 changed files with 297 additions and 18 deletions
Download Patch File Download Diff File Expand all files Collapse all files

40
nptl/pthread_rwlock_rdlock.c
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@ -23,6 +23,7 @@
#include <pthreadP.h>
#include <stap-probe.h>
#include <elide.h>
#include <stdbool.h>
/* Acquire read lock for RWLOCK. Slow path. */
@ -30,6 +31,7 @@ static int __attribute__((noinline))
__pthread_rwlock_rdlock_slow (pthread_rwlock_t *rwlock)
{
int result = 0;
bool wake = false;
/* Lock is taken in caller. */
@ -81,7 +83,17 @@ __pthread_rwlock_rdlock_slow (pthread_rwlock_t *rwlock)
result = EAGAIN;
}
else
LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
{
LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
/* See pthread_rwlock_rdlock. */
if (rwlock->__data.__nr_readers == 1
&& rwlock->__data.__nr_readers_queued > 0
&& rwlock->__data.__nr_writers_queued > 0)
{
++rwlock->__data.__readers_wakeup;
wake = true;
}
}
break;
}
@ -90,6 +102,10 @@ __pthread_rwlock_rdlock_slow (pthread_rwlock_t *rwlock)
/* We are done, free the lock. */
lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
if (wake)
lll_futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX,
rwlock->__data.__shared);
return result;
}
@ -100,6 +116,7 @@ int
__pthread_rwlock_rdlock (pthread_rwlock_t *rwlock)
{
int result = 0;
bool wake = false;
LIBC_PROBE (rdlock_entry, 1, rwlock);
@ -126,11 +143,30 @@ __pthread_rwlock_rdlock (pthread_rwlock_t *rwlock)
result = EAGAIN;
}
else
LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
{
LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
/* If we are the first reader, and there are blocked readers and
writers (which we don't prefer, see above), then it can be the
case that we stole the lock from a writer that was already woken
to acquire it. That means that we need to take over the writer's
responsibility to wake all readers (see pthread_rwlock_unlock).
Thus, wake all readers in this case. */
if (rwlock->__data.__nr_readers == 1
&& rwlock->__data.__nr_readers_queued > 0
&& rwlock->__data.__nr_writers_queued > 0)
{
++rwlock->__data.__readers_wakeup;
wake = true;
}
}
/* We are done, free the lock. */
lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
if (wake)
lll_futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX,
rwlock->__data.__shared);
return result;
}