Re: [PATCH] llist: Fix missing memory barrier

[Peter Hurley <peter@hurleysoftware.com>]

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.

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
>>
>