linux-kernel - Re: ata: BUG in ata_sff_hsm

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Date:	Fri, 29 Jan 2016 14:18:57 +0100
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	syzkaller <syzkaller@...glegroups.com>
Cc:	linux-ide@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Jeff Garzik <jgarzik@...hat.com>,
	Sergei Shtylyov <sshtylyov@...mvista.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: ata: BUG in ata_sff_hsm_move

On Fri, Jan 29, 2016 at 1:23 PM, Tejun Heo <tj@...nel.org> wrote:
> Hello, Dmitry.
>
> On Fri, Jan 29, 2016 at 12:59:49PM +0100, Dmitry Vyukov wrote:
>> > Hmmm... the port interrupt handler checks for IDLE before calling into
>> > hsm_move, so the only explanation would be that something is resetting
>> > it to IDLE inbetween.  ce7514526742 ("libata: prevent HSM state change
>> > race between ISR and PIO") describes and fixes the same problem.  The
>> > fix seems correct and I can't find anywhere else where this can
>> > happen.  :(
>> >
>> > Can you please post the kernel log leading to the BUG?  Also, I don't
>> > think that condition needs to be BUG.  I'll change it to WARN.
>>
>> Here are two logs, in both cases no kernel messages prior to the bug:
>> https://gist.githubusercontent.com/dvyukov/5087d633e3620280b6c7/raw/31c9ab1ced92ac5f85cfb15eaf48ec5793c2c3a1/gistfile1.txt
>> https://gist.githubusercontent.com/dvyukov/825b2e3d5fb80ae08a9a/raw/03c5a4f4c4bd9d0a304a71cda2da4c92f4b7f1ba/gistfile1.txt
>
> lol, this is kinda embarrassing.  It looks like the poll path wasn't
> doing any locking.  Can you please verify the following patch at least
> doesn't crash the machine immediately and if so keep it applied to the
> test kernel so that we can verify that the problem actually goes away?


Great that you managed to debug it without a repro!
I've applied this patch to my tree and will rerun fuzzer. I will
notify you if I see this warning again.
Thanks

> diff --git a/drivers/ata/libata-sff.c b/drivers/ata/libata-sff.c
> index 608677d..6991efc 100644
> --- a/drivers/ata/libata-sff.c
> +++ b/drivers/ata/libata-sff.c
> @@ -1362,12 +1362,14 @@ static void ata_sff_pio_task(struct work_struct *work)
>         u8 status;
>         int poll_next;
>
> +       spin_lock_irq(ap->lock);
> +
>         BUG_ON(ap->sff_pio_task_link == NULL);
>         /* qc can be NULL if timeout occurred */
>         qc = ata_qc_from_tag(ap, link->active_tag);
>         if (!qc) {
>                 ap->sff_pio_task_link = NULL;
> -               return;
> +               goto out_unlock;
>         }
>
>  fsm_start:
> @@ -1382,11 +1384,14 @@ static void ata_sff_pio_task(struct work_struct *work)
>          */
>         status = ata_sff_busy_wait(ap, ATA_BUSY, 5);
>         if (status & ATA_BUSY) {
> +               spin_unlock_irq(ap->lock);
>                 ata_msleep(ap, 2);
> +               spin_lock_irq(ap->lock);
> +
>                 status = ata_sff_busy_wait(ap, ATA_BUSY, 10);
>                 if (status & ATA_BUSY) {
>                         ata_sff_queue_pio_task(link, ATA_SHORT_PAUSE);
> -                       return;
> +                       goto out_unlock;
>                 }
>         }
>
> @@ -1403,6 +1408,8 @@ static void ata_sff_pio_task(struct work_struct *work)
>          */
>         if (poll_next)
>                 goto fsm_start;
> +out_unlock:
> +       spin_unlock_irq(ap->lock);
>  }
>
>  /**
>
> --
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