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New pthread_barrier algorithm to fulfill barrier destruction requirements.

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The previous barrier implementation did not fulfill the POSIX requirements
for when a barrier can be destroyed.  Specifically, it was possible that
threads that haven't noticed yet that their round is complete still access
the barrier's memory, and that those accesses can happen after the barrier
has been legally destroyed.
The new algorithm does not have this issue, and it avoids using a lock
internally.
This commit is contained in:
Torvald Riegel
2015-06-24 14:37:32 +02:00
parent a3e5b4feeb
commit b02840bacd
18 changed files with 417 additions and 762 deletions
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47
nptl/pthread_barrier_destroy.c
View File

@ -18,25 +18,44 @@
#include <errno.h>
#include "pthreadP.h"
#include <lowlevellock.h>
#include <atomic.h>
#include <futex-internal.h>
int
pthread_barrier_destroy (pthread_barrier_t *barrier)
{
struct pthread_barrier *ibarrier;
int result = EBUSY;
struct pthread_barrier *bar = (struct pthread_barrier *) barrier;
ibarrier = (struct pthread_barrier *) barrier;
/* Destroying a barrier is only allowed if no thread is blocked on it.
Thus, there is no unfinished round, and all modifications to IN will
have happened before us (either because the calling thread took part
in the most recent round and thus synchronized-with all other threads
entering, or the program ensured this through other synchronization).
We must wait until all threads that entered so far have confirmed that
they have exited as well. To get the notification, pretend that we have
reached the reset threshold. */
unsigned int count = bar->count;
unsigned int max_in_before_reset = BARRIER_IN_THRESHOLD
- BARRIER_IN_THRESHOLD % count;
/* Relaxed MO sufficient because the program must have ensured that all
modifications happen-before this load (see above). */
unsigned int in = atomic_load_relaxed (&bar->in);
/* Trigger reset. The required acquire MO is below. */
if (atomic_fetch_add_relaxed (&bar->out, max_in_before_reset - in) < in)
{
/* Not all threads confirmed yet that they have exited, so another
thread will perform a reset. Wait until that has happened. */
while (in != 0)
{
futex_wait_simple (&bar->in, in, bar->shared);
in = atomic_load_relaxed (&bar->in);
}
}
/* We must ensure that memory reuse happens after all prior use of the
barrier (specifically, synchronize-with the reset of the barrier or the
confirmation of threads leaving the barrier). */
atomic_thread_fence_acquire ();
lll_lock (ibarrier->lock, ibarrier->private ^ FUTEX_PRIVATE_FLAG);
if (__glibc_likely (ibarrier->left == ibarrier->init_count))
/* The barrier is not used anymore. */
result = 0;
else
/* Still used, return with an error. */
lll_unlock (ibarrier->lock, ibarrier->private ^ FUTEX_PRIVATE_FLAG);
return result;
return 0;
}