linux-kernel - Re: block: ioc->refcount accessed twice in put_io

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Date:	Mon, 11 Apr 2011 10:53:43 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Paul Bolle <pebolle@...cali.nl>
CC:	Shaohua Li <shli@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: block: ioc->refcount accessed twice in put_io_context()?

On 2011-04-11 10:45, Paul Bolle wrote:
> 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?

It's not possible, if it was then that would be the bug - someone
releasing a reference to the ioc that they do not hold. And that is what
the BUG_ON() is there to catch, not a race between two threads.

-- 
Jens Axboe

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