Subject: Re: block: ioc->refcount accessed twice in put_io_context()?
From: Paul Bolle <pebolle@tiscali.nl>
To: Jens Axboe <axboe@kernel.dk>
Cc: Shaohua Li <shli@kernel.org>, linux-kernel@vger.kernel.org
Date: Mon, 11 Apr 2011 10:45:24 +0200
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On Mon, 2011-04-11 at 09:42 +0200, Jens Axboe wrote:
> Indeed, there is nothing wrong with having the BUG_ON() there first and
> doing the decrement later.

But what makes sure then that refcount doesn't get decremented by
something else just before the atomic_long_dec_and_test() call. Eg:

Thread 1			Thread 2
========                        ========
BUG_ON()
				BUG_ON()
atomic_long_dec_and_test()
				atomic_long_dec_and_test()
				/* refcount drops to -1 here */

Or is this not possible?

> If the BUG_ON() is hit, then it's not a race
> conditon - it's a plain bug in the code.

I haven't hit that BUG_ON(), I'm just wondering why an atomic variable
is accessed twice in the same function.


Paul Bolle

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