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

[Honnappa Nagarahalli]

<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 at huawei.com>
> > The change looks fine to me.  I have tested this on few x86 and Arm machines.
> > Acked-by: Honnappa Nagarahalli <honnappa.nagarahalli at arm.com>
> 
> Maybe a simpler alternative would be as fast and safer.
Why is this 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;
This needs to be __atomic_store_n(__ATOMIC_RELEASE) as it can sink below the following line.

> +	} 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