Re: [PATCH] llist: Fix missing memory barrier

[Mathieu Desnoyers]

----- Original Message -----
> From: "Pranith Kumar" <bobby.prani@xxxxxxxxx>
> To: "Mathieu Desnoyers" <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: "Huang Ying" <ying.huang@xxxxxxxxx>, "LKML" <linux-kernel@xxxxxxxxxxxxxxx>, "Paul McKenney"
> <paulmck@xxxxxxxxxxxxxxxxxx>, "David Howells" <dhowells@xxxxxxxxxx>
> Sent: Thursday, February 5, 2015 10:44:07 PM
> Subject: Re: [PATCH] llist: Fix missing memory barrier
> 
> Hi Mathieu,
> 
> On Thu, Feb 5, 2015 at 10:06 PM, Mathieu Desnoyers
> <mathieu.desnoyers@xxxxxxxxxxxx> 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?

Since llist.h has been introduced prior to 3.18, I'm wondering if
it would be worthwhile to submit 2 patches for the purpose of
backporting to stable branches:

1) Fix introducing smp_read_barrier_depends() (for master and
   stable branches)
2) Move master from smp_read_barrier_depends() to
   lockless_dereference(),

Thoughts ?

Thanks!

Mathieu


> 
> Thanks!
> 
> [1] http://lkml.iu.edu/hypermail/linux/kernel/1410.3/04561.html
> 
> >
> > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > CC: Huang Ying <ying.huang@xxxxxxxxx>
> > CC: Paul McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > CC: David Howells <dhowells@xxxxxxxxxx>
> > ---
> >  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
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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