Re: [PATCH v11 6/9] Make scsi_remove_host() wait until error handling finished

[Bart Van Assche <bvanassche@acm.org>]

On 06/25/13 18:13, Michael Christie wrote:
> On Jun 25, 2013, at 10:31 AM, Bart Van Assche <bvanassche@acm.org> wrote:
>> On 06/25/13 15:45, James Bottomley wrote:
>>> On Tue, 2013-06-25 at 11:01 +0200, Bart Van Assche wrote:
>>>> There is a difference though between moving the EH kthread_stop() call
>>>> and the patch at the start of this thread: moving the EH kthread_stop()
>>>> call does not prevent that an ioctl like SG_SCSI_RESET triggers an eh_*
>>>> callback after scsi_remove_host() has finished. However, the
>>>> scsi_begin_eh() / scsi_end_eh() functions do prevent that an ioctl can
>>>> cause an eh_* callback to be invoked after scsi_remove_device() finished.
>>>
>>> OK, but this doesn't tell me what you're trying to achieve.
>>>
>>> An eh function is allowable as long as the host hadn't had the release
>>> callback executed.  That means you must have to have a reference to the
>>> device/host to execute the eh function, which is currently guaranteed
>>> for all invocations.
>>
>> That raises a new question: how is an LLD expected to clean up resources
>> without triggering a race condition ? What you wrote means that it's not
>> safe for an LLD to start cleaning up the resources needed by the eh_*
>> callbacks immediately after scsi_remove_device() returns since it it not
>> guaranteed that at that time all references to the device have already
>> been dropped.
>
> A callback in the device/target/host (whatever is needed) release function
> would do this right? If I understand James right, I think he suggested
> something like this in another mail.

(replying to an e-mail of seven months ago - see also
http://thread.gmane.org/gmane.linux.scsi/82572/focus=82822)

Hello Mike,

Sorry but I'm afraid that making the SCSI core invoke a callback
function from a device, target or host release function would create a
new challenge, namely making sure that all these callback functions have
finished before the module is unloaded that contains the SCSI host
template and the code implementing such a callback function. That
challenge is not specific to the SCSI infrastructure but is a general
question that has not yet been solved in the Linux kernel (see e.g.
"[PATCH] kobject: provide kobject_put_wait to fix module unload race"
for a more general discussion about how to ensure that kobject release
functions are invoked before the module is unloaded that owns the
release function - http://thread.gmane.org/gmane.linux.kernel/1622885).

Or maybe this just means that I misunderstood you ?

In case it is not clear why I'm reviving this discussion: now that the
"improved eh timeout handler" patch is upstream (commit
e494f6a728394ab0df194342549ee20e6f0752df) there is an additional way in
which the SCSI core can invoke an EH function concurrently with or after
scsi_remove_host() has finished, namely from the TMF work queue
(tmf_work_q).

Thanks,

Bart.