linux-kernel - Re: kernel BUG at ide-cd.c:1726 in 2.6.24-03863-g0ba6c33 && -g8561b089

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Message-Id: <20080201.143741.104030452.k-ueda@ct.jp.nec.com>
Date:	Fri, 01 Feb 2008 14:37:41 -0500 (EST)
From:	Kiyoshi Ueda <k-ueda@...jp.nec.com>
To:	petkovbb@...il.com, petkovbb@...glemail.com
Cc:	jens.axboe@...cle.com, nai.xia@...il.com, rdreier@...co.com,
	bzolnier@...il.com, flo@...822.org, linux-kernel@...r.kernel.org,
	j-nomura@...jp.nec.com, linux-ide@...r.kernel.org,
	k-ueda@...jp.nec.com
Subject: Re: kernel BUG at ide-cd.c:1726 in 2.6.24-03863-g0ba6c33 &&
 -g8561b089

Hi Boris,

On Fri, 1 Feb 2008 19:29:09 +0100, Borislav Petkov wrote:
> > > > end_that_request_last() is not called when __blk_end_reuqest()
> > > > returns 1.  Then, the issuer isn't waken up.
> > > > So I think the BUG() or error messages should be there.
> > > 
> > > you mean, end_that_request_last() isn't called when __end_that_request_first()
> > > returns an error and this is the case only for fs and pc requests.
> > > Otherwise it _is_ called, thus simulating somewhat the previous behavior.
> > > However, we never BUG()'ged on residual byte counts before and
> > > this driver has been in the kernel tree for ages, so what puzzles
> > > me now is how is BUG()'ing here better than before and shouldn't we
> > > simply issue a warning instead of killing the interrupt handler...
> > 
> > The Jens' patch passes the residual byte counts to __blk_end_request(),
> > so __end_that_reqeust_first() should never return 1 and we should never
> > BUG() on the residual byte counts, unless inconsistency happens such as
> > the size of remaining bios is bigger than the residual byte counts.
> 
> yep.
> 
> > So if __blk_end_request() returns 1 even with the Jens' patch,
> > it means that the block layer or the driver really have a bug.
> > And then, the request and the bios could leak or the issuer
> > would wait forever because end_that_request_last() isn't called.
> > 
> > The previous behavior might ignore such inconsistency and leak only
> > the bios because it was calling end_that_request_last() anyway.
> > I would like to BUG() in such cases personally, but I don't object
> > strongly if you prefer not to BUG().
> 
> BUG() is definitely what we should do here to catch this case of sizeof(bios) >
> rq->data_len. Putting a brown paper bag over the issue will never get it fixed
> if it really leaks bios. Thanks for clarifying that.
> 
> By the way, shouldn't we be doing a little branch prediction here:
> 
> diff --git a/drivers/ide/ide-cd.c b/drivers/ide/ide-cd.c
> index 74c6087..bee05a3 100644
> --- a/drivers/ide/ide-cd.c
> +++ b/drivers/ide/ide-cd.c
> @@ -1722,7 +1722,7 @@ static ide_startstop_t cdrom_newpc_intr(ide_drive_t *drive)
>          */
>         if ((stat & DRQ_STAT) == 0) {
>                 spin_lock_irqsave(&ide_lock, flags);
> -               if (__blk_end_request(rq, 0, 0))
> +               if (unlikely(__blk_end_request(rq, 0, rq->data_len)))
>                         BUG();
>                 HWGROUP(drive)->rq = NULL;
>                 spin_unlock_irqrestore(&ide_lock, flags);

I think it's reasonable.

Thanks,
Kiyoshi Ueda
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