Re: [PATCH] llist: Fix missing memory barrier

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- Original Message -----
> From: "Peter Hurley" <peter@hurleysoftware.com>
> To: "Mathieu Desnoyers" <mathieu.desnoyers@efficios.com>, "Pranith Kumar" <bobby.prani@gmail.com>
> Cc: "Huang Ying" <ying.huang@intel.com>, "LKML" <linux-kernel@vger.kernel.org>, "Paul McKenney"
> <paulmck@linux.vnet.ibm.com>, "David Howells" <dhowells@redhat.com>, "Greg Kroah-Hartman"
> <gregkh@linuxfoundation.org>
> Sent: Friday, February 6, 2015 10:17:37 AM
> Subject: Re: [PATCH] llist: Fix missing memory barrier
> 
> On 02/06/2015 09:12 AM, Mathieu Desnoyers wrote:
> > ----- Original Message -----
> >> From: "Pranith Kumar" <bobby.prani@gmail.com>
> >> To: "Mathieu Desnoyers" <mathieu.desnoyers@efficios.com>
> >> Cc: "Huang Ying" <ying.huang@intel.com>, "LKML"
> >> <linux-kernel@vger.kernel.org>, "Paul McKenney"
> >> <paulmck@linux.vnet.ibm.com>, "David Howells" <dhowells@redhat.com>
> >> 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@efficios.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?
> > 
> > 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 ?
> 
> Other way around.
> 
> The first patch should use lockless_dereference() for mainline.
> 
> Then once that's been picked up and has a SHA, then a backport
> patch for stable using smp_read_barrier_depends() instead
> before lockless_dereference() was introduced.

That sounds like a good approach. I'll wait for a final word from
Greg and proceed this way unless there are objections.

Thanks!

Mathieu

> 
> Regards,
> Peter Hurley
> 
> > Thanks!
> > 
> > Mathieu
> > 
> > 
> >>
> >> Thanks!
> >>
> >> [1] http://lkml.iu.edu/hypermail/linux/kernel/1410.3/04561.html
> >>
> >>>
> >>> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> >>> CC: Huang Ying <ying.huang@intel.com>
> >>> CC: Paul McKenney <paulmck@linux.vnet.ibm.com>
> >>> CC: David Howells <dhowells@redhat.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
> >>
> > 
> 
> 

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