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TCP: post buffer semaphore before re-acquiring both

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TCP uses two semaphores for buffers, one for the entire buffer pool,
and one for the number of allowed buffers for readahead.  To avoid
double taking a semaphore under heavy load, release the semaphore
after waiting before trying to acquire both again.
This commit is contained in:
Andrew Webster 2016-01-22 16:15:10 -06:00 committed by Gregory Nutt
parent 4875c6bbe3
commit e221777a7a
1 changed files with 36 additions and 7 deletions
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43
net/iob/iob_alloc.c
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@ -121,16 +121,45 @@ static FAR struct iob_s *iob_allocwait(bool throttled)
*/
ret = sem_wait(sem);
if (ret < 0)
{
int errcode = get_errno();
/* When we wake up from wait, an I/O buffer was returned to
* the free list. However, if there are concurrent allocations
* from interrupt handling, then I suspect that there is a
* race condition. But no harm, we will just wait again in
* that case.
*/
/* EINTR is not an error! EINTR simply means that we were
* awakened by a signal and we should try again.
*/
if (errcode == EINTR)
{
/* Force a success indication so that we will continue
* looping.
*/
ret = 0;
}
}
else
{
/* When we wake up from wait successfully, an I/O buffer was
* returned to the free list. However, if there are concurrent
* allocations from interrupt handling, then I suspect that
* there is a race condition. But no harm, we will just wait
* again in that case.
*
* We need release our count so that it is available to
* iob_tryalloc(), perhaps allowing another thread to take our
* count. In that event, iob_tryalloc() will fail above and
* we will have to wait again.
*
* TODO: Consider a design modification to permit us to
* complete the allocation without losing our count.
*/
sem_post(sem);
}
}
}
while (ret == OK && !iob);
while (ret == OK && iob == NULL);
irqrestore(flags);
return iob;