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

[Tejun Heo]

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@xxxxxxxxx>
>   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@xxxxxxxxx>
>       Acked-by: Alan Cox <alan@xxxxxxxxxx>
>       Signed-off-by: Jeff Garzik <jeff@xxxxxxxxxx>
> 
> 
> 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@xxxxxxx>

NACK.

-- 
tejun
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