From: Stephen Hemminger <stephen@networkplumber.org>
To: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
Cc: Diogo Behrens <diogo.behrens@huawei.com>,
"thomas@monjalon.net" <thomas@monjalon.net>,
"david.marchand@redhat.com" <david.marchand@redhat.com>,
"dev@dpdk.org" <dev@dpdk.org>, nd <nd@arm.com>
Subject: Re: [dpdk-dev] [PATCH] librte_eal: fix mcslock hang on weak memory
Date: Mon, 23 Nov 2020 11:36:51 -0800 [thread overview]
Message-ID: <20201123113651.46323c54@hermes.local> (raw)
In-Reply-To: <DBAPR08MB581416E1D0835A1BBCAB042998FC0@DBAPR08MB5814.eurprd08.prod.outlook.com>
On Mon, 23 Nov 2020 18:29:32 +0000
Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com> wrote:
> <snip>
>
> >
> > 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>
> The change looks fine to me. I have tested this on few x86 and Arm machines.
> Acked-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Maybe a simpler alternative would be as fast and safer.
By using compare_exchange you can get same effect in one operation.
Like the following UNTESTED.
diff --git a/lib/librte_eal/include/generic/rte_mcslock.h b/lib/librte_eal/include/generic/rte_mcslock.h
index 78b0df295e2d..9c537ce577e6 100644
--- a/lib/librte_eal/include/generic/rte_mcslock.h
+++ b/lib/librte_eal/include/generic/rte_mcslock.h
@@ -48,23 +48,23 @@ rte_mcslock_lock(rte_mcslock_t **msl, rte_mcslock_t *me)
rte_mcslock_t *prev;
/* Init me node */
- __atomic_store_n(&me->locked, 1, __ATOMIC_RELAXED);
- __atomic_store_n(&me->next, NULL, __ATOMIC_RELAXED);
+ me->locked = 1;
- /* If the queue is empty, the exchange operation is enough to acquire
- * the lock. Hence, the exchange operation requires acquire semantics.
- * The store to me->next above should complete before the node is
- * visible to other CPUs/threads. Hence, the exchange operation requires
- * release semantics as well.
+ /*
+ * Atomic insert into single linked list
*/
- prev = __atomic_exchange_n(msl, me, __ATOMIC_ACQ_REL);
+ do {
+ prev = __atomic_load_n(msl, __ATOMIC_RELAXED);
+ me->next = prev;
+ } while (!__atomic_compare_exchange_n(&msl, me, prev,
+ __ATOMIC_ACQUIRE, __ATOMIC_RELAXED));
+
if (likely(prev == NULL)) {
/* Queue was empty, no further action required,
* proceed with lock taken.
*/
return;
}
- __atomic_store_n(&prev->next, me, __ATOMIC_RELAXED);
/* The while-load of me->locked should not move above the previous
* store to prev->next. Otherwise it will cause a deadlock. Need a
next prev parent reply other threads:[~2020-11-23 19:37 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-26 9:20 Diogo Behrens
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 [this message]
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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