linux-kernel - Re: [PATCH] Fix open/close race in saa7134

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Message-ID: <20080623125348.3133b30b@infradead.org>
Date:	Mon, 23 Jun 2008 12:53:48 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Marcin Slusarz <marcin.slusarz@...il.com>
Cc:	mchehab@...radead.org, linux-kernel@...r.kernel.org,
	Al Viro <viro@....linux.org.uk>
Subject: Re: [PATCH] Fix open/close race in saa7134

On Mon, 23 Jun 2008 21:22:03 +0200
Marcin Slusarz <marcin.slusarz@...il.com> wrote:

> 
> If dev->empress_users could be > 1 then ok - it could break, but it
> can only be 1 or 0. If it's 1 you won't open the device. If it's 0
> you won't reach ts_close.
> 
> If you still see the race, please show me the sequence, because I
> don't (of course when decrementing is the last operation of ts_close).

a decrement in C, without locking, is NOT atomic.
It in fact is 
"load variable into register"
"increment register value"
"write register to variable"

which the compiler then takes and orders for maximum performance....



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