Re: [PATCH] libata: Forcing PIO0 mode on reset must not freeze system

Tejun Heo
2008-02-11T02:37:35Z

Hello,

Holger Macht wrote:
> Calling ap->ops->set_piomode(ap, dev) on a device/controller which got
> already removed, locks the system hard. Reproducibly on an X60 attached to
> a dock station containing a cdrom device with doing
>
> $ echo 1 > /sys/devices/platform/dock.0/undock && echo 123 > /dev/sr0
>
> This calls ata_eh_reset(...) which in turn tries to force PIO mode 0. But
> the device is already gone.
>
> Bisecting revealed the following commit as culprit:
>
> commit cdeab1140799f09c5f728a5ff85e0bdfa5679cd2
> Author: Tejun Heo <htejun@gmail.com>
> Date: Mon Oct 29 16:41:09 2007 +0900
>
> libata: relocate forcing PIO0 on reset
>
> Forcing PIO0 on reset was done inside ata_bus_softreset(), which is a
> bit out of place as it should be applied to all resets - hard, soft
> and implementation which don't use ata_bus_softreset(). Relocate it
> such that...
>
> * For new EH, it's done in ata_eh_reset() before calling prereset.
>
> * For old EH, it's done before calling ap->ops->phy_reset() in
> ata_bus_probe().
>
> This makes PIO0 forced after all resets. Another difference is that
> reset itself is done after PIO0 is forced.
>
> Signed-off-by: Tejun Heo <htejun@gmail.com>
> Acked-by: Alan Cox <alan@redhat.com>
> Signed-off-by: Jeff Garzik <jeff@garzik.org>
>
>
> ATTENTION! The following patch solves the problem on my system, but please
> be aware that I don't really know what I'm doing because I don't have the
> big picture. There's surely a better way to check if the device/controller
> is still functional than calling ata_link_{online,offline}.

In the above example, even the reset sequence itself can cause hang if
the hardware is implemented slightly differently. The reason why
set_piomode() locks up but reset sequence doesn't is simple dumb luck.
I think the proper fix is to tell libata to detach the cdrom before
undocking.

> Signed-off-by: Holger Macht <hmacht@suse.de>

NACK.

--
tejun