On Thu, Apr 10, 2025 at 09:30:16PM +0800, Ming Lei wrote: > updating nr_hw_queues is usually used for error handling code, when it Capitalize the first word of each sentence, please. > doesn't make sense to allow blk-mq elevator switching, since nr_hw_queues > may change, and elevator tags depends on nr_hw_queues. I don't think it's really updated from error handling - nbd does it when starting a device - nullb can do it through debugfs - xen-blkfront does it when resuming from a suspend - nvme does it when resetting a controller. While error handling can escalate to it¸ it's basically probing and re-probing code > Prevent elevator switch during updating nr_hw_queues by setting flag of > BLK_MQ_F_UPDATE_HW_QUEUES, and use srcu to fail elevator switch during > the period. Here elevator switch code is srcu reader of nr_hw_queues, > and blk_mq_update_nr_hw_queues() is the writer. That being said as we generally are in a setup path I think the general idea is fine. No devices should be life yet at this point and thus no udev rules changing the scheduler should run yet. > This way avoids lot of trouble. Can you spell that out a bit? > Reported-by: Shinichiro Kawasaki <shinichiro.kawasaki@xxxxxxx> > Closes: https://lore.kernel.org/linux-block/mz4t4tlwiqjijw3zvqnjb7ovvvaegkqganegmmlc567tt5xj67@xal5ro544cnc/ Are we using Closes for bug reports now? I haven't really seen that anywhere. > out_cleanup_srcu: > if (set->flags & BLK_MQ_F_BLOCKING) > cleanup_srcu_struct(set->srcu); > @@ -5081,7 +5087,18 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, > void blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, int nr_hw_queues) > { > mutex_lock(&set->tag_list_lock); > + /* > + * Mark us in updating nr_hw_queues for preventing switching > + * elevator > > + * > + * Elevator switch code can _not_ acquire ->tag_list_lock Please add a . at the end of a sentences. Also this should probably be something like "Mark us as in.." but I'll leave more nitpicking to the native speakers. > struct request_queue *q = disk->queue; > + struct blk_mq_tag_set *set = q->tag_set; > > /* > * If the attribute needs to load a module, do it before freezing the > @@ -732,6 +733,13 @@ ssize_t elv_iosched_store(struct gendisk *disk, const char *buf, > > elv_iosched_load_module(name); > > + idx = srcu_read_lock(&set->update_nr_hwq_srcu); > + > + if (set->flags & BLK_MQ_F_UPDATE_HW_QUEUES) { What provides atomicity for field modifications vs reading of set->flags? i.e. does this need to switch using test/set_bit? > + struct srcu_struct update_nr_hwq_srcu; > }; > > /** > @@ -681,7 +682,14 @@ enum { > */ > BLK_MQ_F_NO_SCHED_BY_DEFAULT = 1 << 6, > > - BLK_MQ_F_MAX = 1 << 7, > + /* > + * True when updating nr_hw_queues is in-progress > + * > + * tag_set only flag, not usable for hctx > + */ > + BLK_MQ_F_UPDATE_HW_QUEUES = 1 << 7, > + > + BLK_MQ_F_MAX = 1 << 8, Also mixing internal state with driver provided flags is always a bad idea. So this should probably be a new state field in the tag_set and not reuse flags.
