Re: [PATCH 2/2] scsi: ufs: Fix a deadlock in the error handler

[Adrian Hunter <adrian.hunter@intel.com>]

On 03/11/2021 09:40, Christoph Hellwig wrote:
> On Tue, Nov 02, 2021 at 05:05:29PM -0700, Bart Van Assche wrote:
>> -	req = blk_get_request(q, REQ_OP_DRV_OUT, 0);
>> +	req = blk_mq_alloc_request(q, REQ_OP_DRV_OUT, BLK_MQ_REQ_RESERVED);
> 
> blk_get_request will be gone in 5.16-rc, so this won't apply.
> 
> But more importantly: SCSI LLDDs have absolutel no business calling
> blk_get_request or blk_mq_alloc_request directly, but as usual UFS is
> completely fucked up here.

The UFS driver does not issue device commands to the block layer.
blk_get_request() is used only to get a free slot.

> Please add a SCSI midlayer helper to allocate the reserved tags, and
> switch _all_ of this UFS we're sending our own commands crap to it
> so it doesn't mix with the actual SCSI requests.

It doesn't mix them.

> We might or might not
> want a separate request_queue for them as well as non-SCSI requests
> really should not show up in ->queuecommand.

Already has one (hba->cmd_queue), but as noted above, device commands
aren't issued to it at the moment - they are sent directly.

> Hannes and John have been
> looking into this for a while and we need to sort this out properly.

To avoid involving SCSI, since device commands aren't issued to the
block layer, the UFS driver could instead keep 1 slot for itself
(i.e. reduce the number of tags by one), and drop blk_get_request();
noting that currently only 1 device command is sent at a time due to
hba->dev_cmd.lock mutex.