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From: Diogo Behrens <diogo.behrens@huawei.com>
To: Phil Yang <phil.yang@arm.com>
Cc: <dev@dpdk.org>
Subject: [dpdk-dev] [PATCH] librte_eal: fix mcslock hang on weak memory
Date: Wed, 26 Aug 2020 11:20:02 +0200
Message-ID: <20200826092002.19395-1-diogo.behrens@huawei.com> (raw)

    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.

Signed-off-by: Diogo Behrens <diogo.behrens@huawei.com>
---
 lib/librte_eal/include/generic/rte_mcslock.h | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/lib/librte_eal/include/generic/rte_mcslock.h b/lib/librte_eal/include/generic/rte_mcslock.h
index 2bef28351..ce553f547 100644
--- a/lib/librte_eal/include/generic/rte_mcslock.h
+++ b/lib/librte_eal/include/generic/rte_mcslock.h
@@ -68,7 +68,14 @@ 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
-- 
2.28.0



             reply	other threads:[~2020-08-26  9:20 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-26  9:20 Diogo Behrens [this message]
2020-08-26 10:17 ` Phil Yang
2020-08-27  8:56   ` Diogo Behrens
2020-08-28  9:19     ` Phil Yang
2020-08-31 18:45       ` Honnappa Nagarahalli
2020-10-06 21:49         ` Thomas Monjalon
2020-10-07  9:55           ` Diogo Behrens
2020-10-20 11:56             ` Thomas Monjalon
2020-10-20 21:49               ` Honnappa Nagarahalli
2020-11-22 18:07                 ` Thomas Monjalon
2020-11-23 15:06                   ` Honnappa Nagarahalli
2020-11-23 15:44                     ` Stephen Hemminger
2020-11-23 18:16                       ` Honnappa Nagarahalli
2020-11-23 18:29 ` Honnappa Nagarahalli
2020-11-23 19:36   ` Stephen Hemminger
2020-11-25  4:50     ` Honnappa Nagarahalli
2020-11-25  8:41       ` Diogo Behrens
2020-11-25 14:16   ` Thomas Monjalon

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