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From: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
To: Stephen Hemminger <stephen@networkplumber.org>
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>,
	Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>,
	nd <nd@arm.com>
Subject: Re: [dpdk-dev] [PATCH] librte_eal: fix mcslock hang on weak memory
Date: Wed, 25 Nov 2020 04:50:54 +0000
Message-ID: <DBAPR08MB581461EC03BD251C6E403EB898FA0@DBAPR08MB5814.eurprd08.prod.outlook.com> (raw)
In-Reply-To: <20201123113651.46323c54@hermes.local>

<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.
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

  reply	other threads:[~2020-11-25  4:51 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
2020-11-25  4:50     ` Honnappa Nagarahalli [this message]
2020-11-25  8:41       ` Diogo Behrens
2020-11-25 14:16   ` Thomas Monjalon

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