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Message-ID: <20181017151134.GA8966@andrea>
Date: Wed, 17 Oct 2018 17:11:34 +0200
From: Andrea Parri <andrea.parri@...rulasolutions.com>
To: David Howells <dhowells@...hat.com>
Cc: 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
Hi David,
On Wed, Oct 17, 2018 at 03:23:38PM +0100, David Howells wrote:
> From: kiran.modukuri <kiran.modukuri@...il.com>
>
> The code in fscache_retrieval_complete is using atomic_sub followed by an
> atomic_read:
>
> atomic_sub(n_pages, &op->n_pages);
> if (atomic_read(&op->n_pages) <= 0)
> fscache_op_complete(&op->op, true);
>
> This causes two threads doing a decrement of n_pages to race with each
> other seeing the op->refcount 0 at same time - and they end up calling
> fscache_op_complete() in both the threads leading to an assertion failure.
>
> Fix this by using atomic_sub_return() instead of two calls.
Seems a case for atomic_sub_return_relaxed()... why not?
Andrea
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