linux-ext4 - Re: What am I doing wrong? submit_bio() suddenly stops working...

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Date:	Fri, 22 Oct 2010 09:19:37 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Ted Ts'o <tytso@....edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: What am I doing wrong?  submit_bio() suddenly stops working...

On 2010-10-22 05:34, Ted Ts'o wrote:
> On Thu, Oct 21, 2010 at 07:46:15PM +0200, Jens Axboe wrote:
>> By the sound of things, if I were you I'd turn on the mem and slab
>> debugging to catch use-before-init and use-after-free. Mysterious hangs
>> in the IO sub system are usually caused by such bugs. And the regular
>> debugging aids, just to see if that produces anything of interest.
> 
> It looks like it was a use-after-free bug in my code.  I'm running a
> full set of set of tests now, but so far, it's gotten a lot further
> than it went before, so I think I've figured it out.
> 
> I'm not sure why it caused the weird behaviour that it did (I got as
> far as figuring out that somehow we lost the unplug timer, so after
> the queue got plugged it never got unplugged), but I'm not going to
> ask too many questions.  :-)
> 
> Maybe later on I'll try to figure out if there's any way to add some
> kind of sanity checking so that screw ups in in the bio code's caller
> cause a clearer failure (such as a BUG_ON), but that'll have to wait
> for when I have some free time.

In my experience, use-after-free bugs are best caught using the
allocator poisoning along with lock checking. I guess that didn't
trigger for you?

Out of curiousity, what was the exact bug?

-- 
Jens Axboe

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