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Date: Wed, 23 May 2018 14:36:15 +0200
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 3/8] md: raid5: use refcount_t for reference counting
 instead atomic_t
Message-ID: <20180523123615.GY12217@hirez.programming.kicks-ass.net>
References: <20180509193645.830-1-bigeasy@linutronix.de>
 <20180509193645.830-4-bigeasy@linutronix.de>
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To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: linux-kernel@vger.kernel.org, tglx@linutronix.de, Ingo Molnar <mingo@redhat.com>, linux-mm@kvack.org, Shaohua Li <shli@kernel.org>, linux-raid@vger.kernel.org, Anna-Maria Gleixner <anna-maria@linutronix.de>

> 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.