linux-kernel - Re: [PATCH 3/8] md: raid5: use refcount_t for reference counting instead atomic

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Message-ID: <20180523125542.GT12198@hirez.programming.kicks-ass.net>
Date:   Wed, 23 May 2018 14:55:42 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de,
        Ingo Molnar <mingo@...hat.com>, linux-mm@...ck.org,
        Shaohua Li <shli@...nel.org>, linux-raid@...r.kernel.org,
        Anna-Maria Gleixner <anna-maria@...utronix.de>
Subject: Re: [PATCH 3/8] md: raid5: use refcount_t for reference counting
 instead atomic_t

On Wed, May 23, 2018 at 02:50:07PM +0200, Sebastian Andrzej Siewior wrote:
> On 2018-05-23 14:36:15 [+0200], Peter Zijlstra wrote:
> > > Most changes are 1:1 replacements except for
> > > 	BUG_ON(atomic_inc_return(&sh->count) != 1);
> > 
> > That doesn't look right, 'inc_return == 1' implies inc-from-zero, which
> > is not allowed by refcount.
> > 
> > > which has been turned into
> > >         refcount_inc(&sh->count);
> > >         BUG_ON(refcount_read(&sh->count) != 1);
> > 
> > And that is racy, you can have additional increments in between.
> 
> so do we stay with the atomic* API here or do we extend refcount* API?

Stay with the atomic; I'll look at the rest of these patches, but raid5
looks like a usage-count, not a reference count.

I'll probably ack your initial set and parts of this.. but let me get to
the end of this first.