Re: [dpdk-dev] [PATCH] librte_eal: fix mcslock hang on weak memory

[Stephen Hemminger <stephen@networkplumber.org>]

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