linux-kernel - Re: perf events ring buffer memory barrier on powerpc

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <OFD776C320.26C485FF-ON43257C0D.00264F32-43257C0D.002A1BA7@il.ibm.com>
Date:	Wed, 23 Oct 2013 10:39:55 +0300
From:	Victor Kaplansky <VICTORK@...ibm.com>
To:	Michael Neuling <mikey@...ling.org>
Cc:	anton@...ba.org, benh@...nel.crashing.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org,
	Linux PPC dev <linuxppc-dev@...abs.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	michael@...erman.id.au
Subject: Re: perf events ring buffer memory barrier on powerpc

See below.

Michael Neuling <mikey@...ling.org> wrote on 10/23/2013 02:54:54 AM:

>
> diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> index cd55144..95768c6 100644
> --- a/kernel/events/ring_buffer.c
> +++ b/kernel/events/ring_buffer.c
> @@ -87,10 +87,10 @@ again:
>        goto out;
>
>     /*
> -    * Publish the known good head. Rely on the full barrier implied
> -    * by atomic_dec_and_test() order the rb->head read and this
> -    * write.
> +    * Publish the known good head. We need a memory barrier to order the
> +    * order the rb->head read and this write.
>      */
> +   smp_mb ();
>     rb->user_page->data_head = head;
>
>     /*

1. As far as I understand, smp_mb() is superfluous in this case, smp_wmb()
should be enough.
   (same for the space between the name of function and open
parenthesis :-) )

2. Again, as far as I understand from ./Documentation/atomic_ops.txt, it is
mistake in architecture independent
   code to rely on memory barriers in atomic operations, all the more so in
"local" operations.

3. The solution above is sub-optimal on architectures where memory barrier
is part of "local", since we are going to execute
   two consecutive barriers. So, maybe, it would be better to use
smp_mb__after_atomic_dec().

4. I'm not sure, but I think there is another, unrelated potential problem
in function perf_output_put_handle()
   - the write to "data_head" -

kernel/events/ring_buffer.c:

 77         /*
 78          * Publish the known good head. Rely on the full barrier
implied
 79          * by atomic_dec_and_test() order the rb->head read and this
 80          * write.
 81          */
 82         rb->user_page->data_head = head;

As data_head is 64-bit wide, the update should be done by an atomic64_set
().

Regards,
-- Victor

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/