RE: [PATCH] scsi: ufs: Fix proper API to send HPB pre-request

[Daejun Park]

Hi James,

> On Thu, 2021-10-28 at 08:59 -0700, Bart Van Assche wrote:
> > On 10/28/21 7:28 AM, James Bottomley wrote:
> > > If the block people are happy with this, then I'm OK with it, but
> > > it
> > > doesn't look like you've solved the fanout deadlock problem because
> > > this new mechanism is still going to allocate a new tag.
> > 
> > (+Jens, Christoph and linux-block)
> > 
> > Hi James,
> > 
> > My understanding is that the UFS HPB code makes ufshcd_queuecommand()
> > return SCSI_MLQUEUE_HOST_BUSY if the pool with pre-allocated requests
> > is exhausted. This will make the SCSI core reissue a SCSI command
> > until completion of another command has freed up one of the pre-
> > allocated requests. This is not the most efficient approach but
> > should not trigger a deadlock.
>  
> I think the deadlock is triggered if the system is down to its last
> reserved request on the memory clearing device and the next entry in
> the queue for this device is one which does a fanout so we can't
> service it with the single reserved request we have left for the
> purposes of making forward progress.  Sending it back doesn't help,
> assuming this is the only memory clearing path, because retrying it
> won't help ... we have to succeed with a request on this path to move
> forward with clearing memory.
The above approach can retry several times (before the HPB timeout) but, it
gives up to allocate pre-request and it sends as just READ. So deadlock can
be avoided.

Thanks,
Daejun