linux-kernel - Re: [PATCH 2/4] fscache: Fix race in fscache_op_complete() due to split atomic

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Message-ID: <26942.1543249596@warthog.procyon.org.uk>
Date:   Mon, 26 Nov 2018 16:26:36 +0000
From:   David Howells <dhowells@...hat.com>
To:     Andrea Parri <andrea.parri@...rulasolutions.com>
Cc:     dhowells@...hat.com, gregkh@...ux-foundation.org,
        Kiran Kumar Modukuri <kiran.modukuri@...il.com>,
        viro@...iv.linux.org.uk, sandeen@...hat.com,
        linux-cachefs@...hat.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] fscache: Fix race in fscache_op_complete() due to split atomic_sub & read

Andrea Parri <andrea.parri@...rulasolutions.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.

David