Re: [PATCH V2] blk-mq: Set request mapping to NULL in blk_mq_put_driver_tag

[Jens Axboe <axboe@kernel.dk>]

On 12/18/18 10:48 AM, Kashyap Desai wrote:
>>
>> On 12/18/18 10:08 AM, Kashyap Desai wrote:
>>>>>
>>>>> Other block drivers (e.g. ib_srp, skd) do not need this to work
>>>>> reliably.
>>>>> It has been explained to you that the bug that you reported can be
>>>>> fixed by modifying the mpt3sas driver. So why to fix this by
>>>>> modifying the block layer? Additionally, what prevents that a race
>>>>> condition occurs between the block layer clearing
>>>>> hctx->tags->rqs[rq->tag] and
>>>>> scsi_host_find_tag() reading that same array element? I'm afraid
>>>>> that this is an attempt to paper over a real problem instead of
>>>>> fixing the root
>>>> cause.
>>>>
>>>> I have to agree with Bart here, I just don't see how the mpt3sas use
>>>> case is special. The change will paper over the issue in any case.
>>>
>>> Hi Jens, Bart
>>>
>>> One of the key requirement for iterating whole tagset  using
>>> scsi_host_find_tag is to block scsi host. Once we are done that, we
>>> should be good. No race condition is possible if that part is taken
>>> care.
>>> Without this patch, if driver still receive scsi command from the
>>> hctx->tags->rqs which is really not outstanding.  I am finding this is
>>> common issue for many scsi low level drivers.
>>>
>>> Just for example <fnic> - fnic_is_abts_pending() function has below
>>> code -
>>>
>>>         for (tag = 0; tag < fnic->fnic_max_tag_id; tag++) {
>>>                 sc = scsi_host_find_tag(fnic->lport->host, tag);
>>>                 /*
>>>                  * ignore this lun reset cmd or cmds that do not belong
>>> to
>>>                  * this lun
>>>                  */
>>>                 if (!sc || (lr_sc && (sc->device != lun_dev || sc ==
>>> lr_sc)))
>>>                         continue;
>>>
>>> Above code also has similar exposure of kernel panic like <mpt3sas>
>>> driver while accessing sc->device.
>>>
>>> Panic is more obvious if we have add/removal of scsi device before
>>> looping through scsi_host_find_tag().
>>>
>>> Avoiding block layer changes is also attempted in <mpt3sas> but our
>>> problem is to convert that code common for non-mq and mq.
>>> Temporary to unblock this issue, We have fixed <mpt3sas> using driver
>>> internals scsiio_tracker() instead of piggy back in scsi_command.
>>
>> For mq, the requests never go out of scope, they are always valid. So the
>> key
>> question here is WHY they have been freed. If the queue gets killed, then
>> one
>> potential solution would be to clear pointers in the tag map belonging to
>> that
>> queue. That also takes it out of the hot path.
> 
> At driver load whenever driver does scsi_add_host_with_dma(), it follows
> below code path in block layer.
> 
> scsi_mq_setup_tags
>   ->blk_mq_alloc_tag_set
>           -> blk_mq_alloc_rq_maps
>                      -> __blk_mq_alloc_rq_maps
> 
> SML create two set of request pool. One is per HBA and other is per SDEV. I
> was confused why SML creates request pool per HBA.
> 
> Example - IF HBA queue depth is 1K and there are 8 device behind that HBA,
> total request pool is created is 1K + 8 * scsi_device queue depth. 1K will
> be always static, but other request pool is managed whenever scsi device is
> added/removed.
> 
> I never observe requests allocated per HBA is used in IO path. It is always
> request allocated per scsi device is what active.
> Also, what I observed is whenever scsi_device is deleted, associated request
> is also deleted. What is missing is - "Deleted request still available in
> hctx->tags->rqs[rq->tag]."

So that sounds like the issue. If the device is deleted and its requests
go away, those pointers should be cleared. That's what your patch should
do, not do it for each IO.


-- 
Jens Axboe