linux-kernel - Re: [PATCHSET RFC] block/SCSI: implement in-kernel disk event handling

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Date:	Thu, 04 Nov 2010 13:07:06 -0400
From:	Douglas Gilbert <dgilbert@...erlog.com>
To:	Tejun Heo <tj@...nel.org>
CC:	axboe@...nel.dk, linux-kernel@...r.kernel.org,
	linux-scsi@...r.kernel.org, kay.sievers@...y.org, jack@...e.cz,
	James.Bottomley@...senPartnership.com
Subject: Re: [PATCHSET RFC] block/SCSI: implement in-kernel disk event handling

On 10-11-01 02:44 PM, Tejun Heo wrote:
> This patchset implements in-kernel disk event handling framework and
> add support for it to sr and sd.  This is largely to move media
> presence polling into kernel as userspace implementation turned out to
> be quite problematic over the years.
>
>> From the patch description of the third patch,
>
>   Currently, media presence polling for removeable block devices is done
>   from userland.  There are several issues with this.
>
>   * Polling is done by periodically opening the device.  For SCSI
>     devices, the command sequence generated by such action involves a
>     few different commands including TEST_UNIT_READY.  This behavior,
>     while perfectly legal, is different from Windows which only issues
>     single command, GET_EVENT_STATUS_NOTIFICATION.  Unfortunately, some
>     ATAPI devices lock up after being periodically queried such command
>     sequences.
>   * There is no reliable and unintrusive way for a userland program to
>     tell whether the target device is safe for media presence polling.
>     For example, polling for media presence during an on-going burning
>     session can make it fail.  The polling program can avoid this by
>     opening the device with O_EXCL but then it risks making a valid
>     exclusive user of the device fail w/ -EBUSY.

Tejun,
Yes. You could add the SCSI FORMAT UNIT and the ATA
(and soon SCSI) SANITIZE commands to that example.
Compliant SCSI units should respond to such an untimely
TEST UNIT READY with the additional sense of "logical
unit not ready, format in progress" but many devices
don't leading to error recovery and device resets.


Ideally I would like to see a way from the user space
to uncouple a unit from the sd (or sr) driver, in a
similar way to how an LLD can use
scsi_device::no_uld_attach . That would restrict user
space access to the bsg or sg driver. Another approach
is to flag the sd (or sr) driver to not send any !@...^&
commands to a device until further notice.

Doug Gilbert

>   * Userland polling is unnecessarily heavy and in-kernel implementation
>     is lighter and better coordinated (workqueue, timer slack).

<snip>

> --
> tejun

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