Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752705AbaBZXGA (ORCPT ); Wed, 26 Feb 2014 18:06:00 -0500 Received: from mail-pa0-f42.google.com ([209.85.220.42]:41389 "EHLO mail-pa0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751172AbaBZXF7 (ORCPT ); Wed, 26 Feb 2014 18:05:59 -0500 Date: Wed, 26 Feb 2014 15:05:54 -0800 From: Kent Overstreet To: Alexander Gordeev Cc: linux-kernel@vger.kernel.org, Peter Zijlstra , Jens Axboe , "Nicholas A. Bellinger" Subject: Re: [PATCH RESEND 1/3] percpu_ida: Fix data race on cpus_have_tags cpumask Message-ID: <20140226230554.GE11655@kmo> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Feb 06, 2014 at 01:24:53PM +0100, Alexander Gordeev wrote: > Function steal_tags() might miss a bit in cpus_have_tags due to > unsynchronized access from percpu_ida_free(). As result, function > percpu_ida_alloc() might enter unwakable sleep. This update adds > memory barriers to prevent the described scenario. > > In fact, accesses to cpus_have_tags are fenced by prepare_to_wait() > and wake_up() calls at the moment and the aforementioned sequence > does not appear could hit. Nevertheless, explicit memory barriers > still seem justifiable. > > Signed-off-by: Alexander Gordeev > Cc: Kent Overstreet > Cc: Peter Zijlstra > Cc: Jens Axboe > Cc: "Nicholas A. Bellinger" > Acked-by: Kent Overstreet > --- > lib/percpu_ida.c | 12 ++++++++++-- > 1 files changed, 10 insertions(+), 2 deletions(-) > > diff --git a/lib/percpu_ida.c b/lib/percpu_ida.c > index 7be235f..fccfb28 100644 > --- a/lib/percpu_ida.c > +++ b/lib/percpu_ida.c > @@ -68,6 +68,11 @@ static inline void steal_tags(struct percpu_ida *pool, > unsigned cpus_have_tags, cpu = pool->cpu_last_stolen; > struct percpu_ida_cpu *remote; > > + /* > + * Pairs with smp_wmb() in percpu_ida_free() > + */ > + smp_rmb(); > + > for (cpus_have_tags = cpumask_weight(&pool->cpus_have_tags); > cpus_have_tags * pool->percpu_max_size > pool->nr_tags / 2; > cpus_have_tags--) { > @@ -237,8 +242,11 @@ void percpu_ida_free(struct percpu_ida *pool, unsigned tag) > spin_unlock(&tags->lock); > > if (nr_free == 1) { > - cpumask_set_cpu(smp_processor_id(), > - &pool->cpus_have_tags); > + cpumask_set_cpu(smp_processor_id(), &pool->cpus_have_tags); > + /* > + * Pairs with smp_rmb() in steal_tags() > + */ > + smp_wmb(); > wake_up(&pool->wait); I think I'm nacking this - there's a lot of code in the kernel that relies on the fact that prepare_to_wait)/wake_up() do the appropriate fences, we really shouldn't be adding to the barriers those do. If you can come up with some other reason we need the barriers I'll reconsider. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/