linux-kernel - Re: [PATCH] llist: Fix missing memory barrier

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Message-ID: <CAJhHMCDDG2Woiv1E3OnPCWv33k7+1FVRbmmc5EN3Gxim3zoJsA@mail.gmail.com>
Date:	Thu, 5 Feb 2015 22:44:07 -0500
From:	Pranith Kumar <bobby.prani@...il.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Huang Ying <ying.huang@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	David Howells <dhowells@...hat.com>
Subject: Re: [PATCH] llist: Fix missing memory barrier

Hi Mathieu,

On Thu, Feb 5, 2015 at 10:06 PM, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
> A smp_read_barrier_depends() appears to be missing in llist_del_first().
> It should only matter for Alpha in practice. Adding it after the check
> of entry against NULL allows skipping the barrier in a common case.

We recently decided on using lockless_dereference() instead of
hard-coding smp_read_barrier_depends()[1]. The advantage is that
lockless_dereference() clearly shows what loads are being ordered.
Could you resend the patch using that API?

Thanks!

[1] http://lkml.iu.edu/hypermail/linux/kernel/1410.3/04561.html

>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> CC: Huang Ying <ying.huang@...el.com>
> CC: Paul McKenney <paulmck@...ux.vnet.ibm.com>
> CC: David Howells <dhowells@...hat.com>
> ---
>  lib/llist.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/lib/llist.c b/lib/llist.c
> index f76196d..72861f3 100644
> --- a/lib/llist.c
> +++ b/lib/llist.c
> @@ -26,6 +26,7 @@
>  #include <linux/export.h>
>  #include <linux/interrupt.h>
>  #include <linux/llist.h>
> +#include <asm/barrier.h>
>
>
>  /**
> @@ -72,6 +73,12 @@ struct llist_node *llist_del_first(struct llist_head *head)
>                 if (entry == NULL)
>                         return NULL;
>                 old_entry = entry;
> +               /*
> +                * Load entry before entry->next. Matches the implicit
> +                * memory barrier before the cmpxchg in llist_add_batch(),
> +                * which ensures entry->next is stored before entry.
> +                */
> +               smp_read_barrier_depends();
>                 next = entry->next;
>                 entry = cmpxchg(&head->first, old_entry, next);
>                 if (entry == old_entry)
> --
> 2.1.4
>
> --
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-- 
Pranith
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