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9357873641
When we try to kill a session, the shard must be locked before decrementing
the ref count on the session. Otherwise, the ref count can fall to 0 and a
purge task (stktable_trash_oldest or process_table_expire) may release the
session before we have the opportunity to acquire the lock on the shard to
effectively kill the session. This could lead to a double free.
Here is the scenario:
Thread 1 Thread 2
sktsess_kill(ts)
if (ATOMIC_DEC(&ts->ref_cnt) != 0)
return
/* here the ref count is 0 */
stktable_trash_oldest()
LOCK(&sh_lock)
if (!ATOMIC_LOAD(&ts->ref_cnf))
__stksess_free(ts)
UNLOCK(&sh_lock)
/* here the session was released */
LOCK(&sh_lock)
__stksess_free(ts) <--- double free
UNLOCK(&sh_lock)
The bug was introduced in 2.9 by the commit 7968fe3889 ("MEDIUM:
stick-table: change the ref_cnt atomically"). The ref count must be
decremented inside the lock for stksess_kill() and sktsess_kill_if_expired()
function.
This patch should fix the issue #2611. It must be backported as far as 2.9. On
the 2.9, there is no sharding. All the table is locked. The patch will have to
be adapted.