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:   Mon, 26 Nov 2018 17:56:06 +0100
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: <20181126165606.GA11282@andrea>
References: <20181017164848.GA9795@andrea>
 <20181017151134.GA8966@andrea>
 <153978619457.8478.3813964117489247515.stgit@warthog.procyon.org.uk>
 <153978621809.8478.2198040871218302573.stgit@warthog.procyon.org.uk>
 <14408.1539790333@warthog.procyon.org.uk>
 <26942.1543249596@warthog.procyon.org.uk>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <26942.1543249596@warthog.procyon.org.uk>
User-Agent: Mutt/1.9.4 (2018-02-28)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, Nov 26, 2018 at 04:26:36PM +0000, David Howells wrote:
> Andrea Parri <andrea.parri@amarulasolutions.com> wrote:
> 
> > > > > Fix this by using atomic_sub_return() instead of two calls.
> > > > 
> > > > Seems a case for atomic_sub_return_relaxed()... why not?
> > > 
> > > Ummm...  In that case, should it be atomic_sub_return_release()?
> > 
> > Hard to tell for me: your diff./changelog is all I know about fs-cache
> > ... (and this suggests -no-, given that atomic_sub() and atomic_read()
> > provide no ordering...); good question though. ;-)
> 
> Yeah, that doesn't mean that it shouldn't be stricter than 'relaxed'.  It's
> kind of like an unlock/release operation, so I think 'release' is probably the
> minimum requirement.

Sure.  My point was: those operations are currently not atomic _and_
they provide no ordering; I think that the above commit message does
a good work in explaining *why* we need atomicity, but can't say the
same for the memory-ordering requirement.

  Andrea


> 
> David