Re: block: ioc->refcount accessed twice in put_io_context()?

From: Jens Axboe
Date: Mon Apr 11 2011 - 04:53:46 EST

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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