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

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

<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>

> ---
>  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
>