linux-ext4 - Re: Report in ata_scsi_port_error

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Message-Id: <1645094128-17099-1-git-send-email-byungchul.park@lge.com>
Date:   Thu, 17 Feb 2022 19:35:28 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     torvalds@...ux-foundation.org
Cc:     damien.lemoal@...nsource.wdc.com, linux-ide@...r.kernel.org,
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Subject: Re: Report in ata_scsi_port_error_handler()

<torvalds@...ux-foundation.org> wrote:
> On Tue, Feb 15, 2022 at 10:37 PM Damien Le Moal
> <damien.lemoal@...nsource.wdc.com> wrote:
> >
> > On 2/16/22 13:16, Byungchul Park wrote:
> > > [    2.051040 ] ===================================================
> > > [    2.051406 ] DEPT: Circular dependency has been detected.
> > > [    2.051730 ] 5.17.0-rc1-00014-gcf3441bb2012 #2 Tainted: G        W
> > > [    2.051991 ] ---------------------------------------------------
> > > [    2.051991 ] summary
> > > [    2.051991 ] ---------------------------------------------------
> > > [    2.051991 ] *** DEADLOCK ***
> > > [    2.051991 ]
> > > [    2.051991 ] context A
> > > [    2.051991 ]     [S] (unknown)(&(&ap->eh_wait_q)->dmap:0)
> > > [    2.051991 ]     [W] __raw_spin_lock_irq(&host->lock:0)
> > > [    2.051991 ]     [E] event(&(&ap->eh_wait_q)->dmap:0)
> > > [    2.051991 ]
> > > [    2.051991 ] context B
> > > [    2.051991 ]     [S] __raw_spin_lock_irqsave(&host->lock:0)
> > > [    2.051991 ]     [W] wait(&(&ap->eh_wait_q)->dmap:0)
> > > [    2.051991 ]     [E] spin_unlock(&host->lock:0)
> >
> > Sleeping with a spinlock held would be triggering warnings already, so
> > these reports seem bogus to me.
> 
> Yeah, Matthew pointed out the same thing for another use-case, where
> it looks like DEPT is looking at the state at the wrong point (not at
> the scheduling point, but at prepare_to_sleep()).
> 
> This ata_port_wait() is the exact same pattern, ie we have
> 
>	spin_lock_irqsave(ap->lock, flags);
> 
>	while (ap->pflags & (ATA_PFLAG_EH_PENDING | ATA_PFLAG_EH_IN_PROGRESS)) {
>		prepare_to_wait(&ap->eh_wait_q, &wait, TASK_UNINTERRUPTIBLE);
>		spin_unlock_irqrestore(ap->lock, flags);
>		schedule();
> 
> and DEPT has incorrectly taken it to mean that 'ap->lock' is held
> during the wait, when it is actually released before actually waiting.
> 
> For the spin-locks, this is all very obvious (because they'd have been
> caught long ago by much simpler debug code), but the same
> prepare_to_wait -> wait pattern can most definitely happen with
> sleeping locks too, so they are all slightly suspect.
> 
> And yes, the detailed reports are hard to read because the locations
> are given as "ata_port_wait_eh+0x52/0xc0". Running them through
> scripts/decode_stacktrace.sh to turn them into filename and line
> numbers - and also sort out inlining - would help a lot.
> 
> Byungchul, could you fix those two issues? Some of your reports may

Of couse, that's what I should do. Thanks for your feedback.

> well be entirely valid, but the hard-to-read hex offsets and the
> knowledge that at least some of them are confused about how
> prepare_to_wait -> wait actually works makes the motivation to look at
> the details much less..
> 
> 	Linus