Re: [PATCH] block: Fix a race in the runtime power management code

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

On 2020-08-25 11:24, Alan Stern wrote:
> A related question concerns the BLK_MQ_REQ_PREEMPT flag.  If it is set 
> then the request is allowed while rpm_status is RPM_SUSPENDING.  But in 
> fact, the only requests which should be allowed at that time are those 
> created by the lower-layer driver as part of its runtime-suspend 
> handling; all other requests should be deferred.  The BLK_MQ_REQ_PREEMPT 
> flag doesn't seem like the right way to achieve this.  Should we be 
> using a different flag?

I think there is already a flag that is used to mark power management
commands, namely RQF_PM.

My understanding is that the BLK_MQ_REQ_PREEMPT/RQF_PREEMPT flags are used
for the following purposes:
* In the SCSI core, scsi_execute() sets the BLK_MQ_PREEMPT flag. As a
  result, SCSI commands submitted with scsi_execute() are processed in
  the SDEV_QUIESCE SCSI device state. Since scsi_execute() is only used
  to submit other than medium access commands, in the SDEV_QUIESCE state
  medium access commands are paused and commands that do not access the
  storage medium (e.g. START/STOP commands) are processed.
* In the IDE-driver, for making one request preempt another request. From
  an old IDE commit (not sure if this is still relevant):
  + * If action is ide_preempt, then the rq is queued at the head of
  + * the request queue, displacing the currently-being-processed
  + * request and this function returns immediately without waiting
  + * for the new rq to be completed.  This is VERY DANGEROUS, and is
  + * intended for careful use by the ATAPI tape/cdrom driver code.

Bart.