mcslock: fix hang in weak memory model

The initialization me->locked=1 in lock() must happen before
next->locked=0 in unlock(), otherwise a thread may hang forever,
waiting me->locked become 0. On weak memory systems (such as ARMv8),
the current implementation allows me->locked=1 to be reordered with
announcing the node (pred->next=me) and, consequently, to be
reordered with next->locked=0 in unlock().

This fix adds a release barrier to pred->next=me, forcing
me->locked=1 to happen before this operation.

Fixes: 2173f3333b61 ("mcslock: add MCS queued lock implementation")
Cc: stable@dpdk.org
Signed-off-by: Diogo Behrens <diogo.behrens@huawei.com>
Acked-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

diff --git a/lib/librte_eal/include/generic/rte_mcslock.h b/lib/librte_eal/include/generic/rte_mcslock.h
index 78b0df2..d370bef 100644 (file)
--- a/lib/librte_eal/include/generic/rte_mcslock.h
+++ b/lib/librte_eal/include/generic/rte_mcslock.h
@@ -64,7 +64,14 @@ rte_mcslock_lock(rte_mcslock_t **msl, rte_mcslock_t *me)
                 */
                return;
        }
-       __atomic_store_n(&prev->next, me, __ATOMIC_RELAXED);
+       /* The store to me->next above should also complete before the node is
+        * visible to predecessor thread releasing the lock. Hence, the store
+        * prev->next also requires release semantics. Note that, for example,
+        * on ARM, the release semantics in the exchange operation is not
+        * strong as a release fence and is not sufficient to enforce the
+        * desired order here.
+        */
+       __atomic_store_n(&prev->next, me, __ATOMIC_RELEASE);
 
        /* The while-load of me->locked should not move above the previous
         * store to prev->next. Otherwise it will cause a deadlock. Need a