linux-kernel - Re: PATCH: dangling pointer when switching to noop elevator in obscure block drivers.

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Message-ID: <4C724952.3050002@kernel.dk>
Date:	Mon, 23 Aug 2010 12:11:30 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Kevin Vigor <kevin@...or.nu>
CC:	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: PATCH: dangling pointer when switching to noop elevator in obscure
  block drivers.

On 2010-08-20 23:25, Kevin Vigor wrote:
> Linux 2.6.35 introduced a test in the beginning of elevator_init(), like
> so:
> 
>      if (unlikely(q->elevator))
>          return 0;
> 
> So the following code sequence, which appears in two (obscure) block
> drivers is now a serious error:
> 
>         elevator_exit(q);
>         elevator_init(q, "noop");
> 
> The intent is to cleanup the default system elevator and replace it with
> the noop elevator. Instead, elevator_exit() frees the existing elevator
> object, but leaves q->elevator pointing to it; elevator_init() then
> silently fails since q->elevator is non-NULL, and the queue is left with
> the elevator pointer invalid, This leads to untold woe and segfaults
> later.
> 
> The fix is trivial: zero the q->elevator pointer before calling
> elevator_init(). Note that drivers/s390/block/dasd.c already follows
> this pattern.
> 
> I do not have the hardware to actually test either of the two afflicted 
> drivers, but I believe the fix to be sufficiently obvious. The following
> patches are against the current version of Linus' tree.

Thanks, this was also posted last week. I think the best course of
action is to provide an API for switching the scheduler, instead of
having drivers rely on doing it manually.

-- 
Jens Axboe

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