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

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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