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MEDIUM: buffers: relax the buffer lock a little bit

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In lock profiles it's visible that there is a huge contention on the
buffer lock. The reason is that when offer_buffers() is called, it
systematically takes the lock before verifying if there is any
waiter. However doing so doesn't protect against races since a
waiter can happen just after we release the lock as well. Similarly
in h2 we take the lock every time an h2c is going to be released,
even without checking that the h2c belongs to a wait list. These
two have now been addressed by verifying non-emptiness of the list
prior to taking the lock.
This commit is contained in:
Willy Tarreau 2019-05-28 17:21:18 +02:00
parent a8b2ce02b8
commit 186e96ece0
2 changed files with 10 additions and 8 deletions
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10
include/common/buffer.h
View File

@ -201,12 +201,12 @@ void __offer_buffer(void *from, unsigned int threshold);
static inline void offer_buffers(void *from, unsigned int threshold)
{
HA_SPIN_LOCK(BUF_WQ_LOCK, &buffer_wq_lock);
if (LIST_ISEMPTY(&buffer_wq)) {
HA_SPIN_UNLOCK(BUF_WQ_LOCK, &buffer_wq_lock);
if (LIST_ISEMPTY(&buffer_wq))
return;
}
__offer_buffer(from, threshold);
HA_SPIN_LOCK(BUF_WQ_LOCK, &buffer_wq_lock);
if (!LIST_ISEMPTY(&buffer_wq))
__offer_buffer(from, threshold);
HA_SPIN_UNLOCK(BUF_WQ_LOCK, &buffer_wq_lock);
}

8
src/mux_h2.c
View File

@ -662,9 +662,11 @@ static void h2_release(struct h2c *h2c)
hpack_dht_free(h2c->ddht);
HA_SPIN_LOCK(BUF_WQ_LOCK, &buffer_wq_lock);
LIST_DEL(&h2c->buf_wait.list);
HA_SPIN_UNLOCK(BUF_WQ_LOCK, &buffer_wq_lock);
if (LIST_ADDED(&h2c->buf_wait.list)) {
HA_SPIN_LOCK(BUF_WQ_LOCK, &buffer_wq_lock);
LIST_DEL(&h2c->buf_wait.list);
HA_SPIN_UNLOCK(BUF_WQ_LOCK, &buffer_wq_lock);
}
h2_release_buf(h2c, &h2c->dbuf);
h2_release_mbuf(h2c);