On Tue, Aug 25, 2020 at 03:22:03PM -0700, Bart Van Assche wrote: > 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. Ah, perfect. So in blk_queue_enter(), pm should be defined in terms of RQF_PM rather than BLK_MQ_REQ_PREEMPT. The difficulty is that the flags argument is the wrong type; RQF_PM is defined as req_flags_t, not blk_mq_req_flags_t. It is associated with a particular request after the request has been created, so after blk_queue_enter() has been called. How can we solve this? Alan Stern
