On Wed, Feb 26, 2025 at 06:09:56PM +0530, Nilay Shroff wrote: > There're few sysfs attributes in block layer which don't really need > acquiring q->sysfs_lock while accessing it. The reason being, reading/ > writing a value from/to such attributes are either atomic or could be > easily protected using READ_ONCE()/WRITE_ONCE(). Moreover, sysfs > attributes are inherently protected with sysfs/kernfs internal locking. > > So this change help segregate all existing sysfs attributes for which > we could avoid acquiring q->sysfs_lock. For all read-only attributes > we removed the q->sysfs_lock from show method of such attributes. In > case attribute is read/write then we removed the q->sysfs_lock from > both show and store methods of these attributes. > > We audited all block sysfs attributes and found following list of > attributes which shouldn't require q->sysfs_lock protection: > > 1. io_poll: > Write to this attribute is ignored. So, we don't need q->sysfs_lock. > > 2. io_poll_delay: > Write to this attribute is NOP, so we don't need q->sysfs_lock. > > 3. io_timeout: > Write to this attribute updates q->rq_timeout and read of this > attribute returns the value stored in q->rq_timeout Moreover, the > q->rq_timeout is set only once when we init the queue (under blk_mq_ > init_allocated_queue()) even before disk is added. So that means > that we don't need to protect it with q->sysfs_lock. As this > attribute is not directly correlated with anything else simply using > READ_ONCE/WRITE_ONCE should be enough. > > 4. nomerges: > Write to this attribute file updates two q->flags : QUEUE_FLAG_ > NOMERGES and QUEUE_FLAG_NOXMERGES. These flags are accessed during > bio-merge which anyways doesn't run with q->sysfs_lock held. > Moreover, the q->flags are updated/accessed with bitops which are > atomic. So, protecting it with q->sysfs_lock is not necessary. > > 5. rq_affinity: > Write to this attribute file makes atomic updates to q->flags: > QUEUE_FLAG_SAME_COMP and QUEUE_FLAG_SAME_FORCE. These flags are > also accessed from blk_mq_complete_need_ipi() using test_bit macro. > As read/write to q->flags uses bitops which are atomic, protecting > it with q->stsys_lock is not necessary. > > 6. nr_zones: > Write to this attribute happens in the driver probe method (except > nvme) before disk is added and outside of q->sysfs_lock or any other > lock. Moreover nr_zones is defined as "unsigned int" and so reading > this attribute, even when it's simultaneously being updated on other > cpu, should not return torn value on any architecture supported by > linux. So we can avoid using q->sysfs_lock or any other lock/ > protection while reading this attribute. > > 7. discard_zeroes_data: > Reading of this attribute always returns 0, so we don't require > holding q->sysfs_lock. > > 8. write_same_max_bytes > Reading of this attribute always returns 0, so we don't require > holding q->sysfs_lock. > > Reviewed-by: Christoph Hellwig <hch@xxxxxx> > Reviewed-by: Hannes Reinecke <hare@xxxxxxx> > Signed-off-by: Nilay Shroff <nilay@xxxxxxxxxxxxx> Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx> Thanks, Ming
