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CLEANUP: pools: use the regular lock for the flush operation on lockless pools

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Commit 04f5fe87d3d introduced an rwlock in the pools to deal with the risk
that pool_flush() dereferences an area being freed, and commit 899fb8abdcd
turned it into a spinlock. The pools already contain a spinlock in case of
locked pools, so let's use the same and simplify the code by removing ifdefs.

At this point I'm really suspecting that if pool_flush() would instead
rely on __pool_get_first() to pick entries from the pool, the concurrency
problem could never happen since only one user would get a given entry at
once, thus it could not be freed by another user. It's not certain this
would be faster however because of the number of atomic ops to retrieve
one entry compared to a locked batch.
This commit is contained in:
Willy Tarreau 2020-05-29 17:23:05 +02:00
parent 2f44a59c7f
commit 21072b9480
2 changed files with 6 additions and 9 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
include/common/memory.h
View File

@ -81,14 +81,15 @@ struct pool_free_list {
};
#endif
/* Note below, in case of lockless pools, we still need the lock only for
* the flush() operation.
*/
struct pool_head {
void **free_list;
#ifdef CONFIG_HAP_LOCKLESS_POOLS
uintptr_t seq;
HA_SPINLOCK_T flush_lock;
#else
__decl_hathreads(HA_SPINLOCK_T lock); /* the spin lock */
#endif
__decl_hathreads(HA_SPINLOCK_T lock); /* the spin lock */
unsigned int used; /* how many chunks are currently in use */
unsigned int needed_avg;/* floating indicator between used and allocated */
unsigned int allocated; /* how many chunks have been allocated */

8
src/memory.c
View File

@ -139,11 +139,7 @@ struct pool_head *create_pool(char *name, unsigned int size, unsigned int flags)
for (thr = 0; thr < MAX_THREADS; thr++)
pool_cache[thr][idx].size = size;
}
#ifndef CONFIG_HAP_LOCKLESS_POOLS
HA_SPIN_INIT(&pool->lock);
#else
HA_SPIN_INIT(&pool->flush_lock);
#endif
}
pool->users++;
return pool;
@ -227,7 +223,7 @@ void pool_flush(struct pool_head *pool)
if (!pool)
return;
HA_SPIN_LOCK(POOL_LOCK, &pool->flush_lock);
HA_SPIN_LOCK(POOL_LOCK, &pool->lock);
do {
cmp.free_list = pool->free_list;
cmp.seq = pool->seq;
@ -235,7 +231,7 @@ void pool_flush(struct pool_head *pool)
new.seq = cmp.seq + 1;
} while (!_HA_ATOMIC_DWCAS(&pool->free_list, &cmp, &new));
__ha_barrier_atomic_store();
HA_SPIN_UNLOCK(POOL_LOCK, &pool->flush_lock);
HA_SPIN_UNLOCK(POOL_LOCK, &pool->lock);
next = cmp.free_list;
while (next) {
temp = next;