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

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Date:   Wed, 28 Nov 2018 14:43:24 +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.
> 
> 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.

Having discussed it with Paul McKenney and thought about it some more, I think
relaxed is probably okay since there isn't a pair of variables that need
ordering.

David