Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Wed, 17 Oct 2018 17:11:34 +0200
From:   Andrea Parri <andrea.parri@amarulasolutions.com>
To:     David Howells <dhowells@redhat.com>
Cc:     gregkh@linux-foundation.org,
        Kiran Kumar Modukuri <kiran.modukuri@gmail.com>,
        viro@zeniv.linux.org.uk, sandeen@redhat.com,
        linux-cachefs@redhat.com, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/4] fscache: Fix race in fscache_op_complete() due to
 split atomic_sub & read
Message-ID: <20181017151134.GA8966@andrea>
References: <153978619457.8478.3813964117489247515.stgit@warthog.procyon.org.uk>
 <153978621809.8478.2198040871218302573.stgit@warthog.procyon.org.uk>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <153978621809.8478.2198040871218302573.stgit@warthog.procyon.org.uk>
User-Agent: Mutt/1.9.4 (2018-02-28)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Hi David,

On Wed, Oct 17, 2018 at 03:23:38PM +0100, David Howells wrote:
> From: kiran.modukuri <kiran.modukuri@gmail.com>
> 
> The code in fscache_retrieval_complete is using atomic_sub followed by an
> atomic_read:
> 
>         atomic_sub(n_pages, &op->n_pages);
>         if (atomic_read(&op->n_pages) <= 0)
>                 fscache_op_complete(&op->op, true);
> 
> This causes two threads doing a decrement of n_pages to race with each
> other seeing the op->refcount 0 at same time - and they end up calling
> fscache_op_complete() in both the threads leading to an assertion failure.
> 
> Fix this by using atomic_sub_return() instead of two calls.

Seems a case for atomic_sub_return_relaxed()... why not?

  Andrea