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That > commit takes the lock for write, and it has to be a trylock because of > commit ce8ee8583ed8 ("block: use trylock to avoid lockdep circular > dependency in sysfs"). > > The lock covers the whole tag set, so it is easy to hit. A scheduler > write fails while another queue of the set is switching, or while a > sibling disk of the same SCSI host or NVMe controller is added or > removed. That is what breaks the udev rules that set the scheduler on > 7.2. > > Go back to taking the lock for read. The write lock was there to keep > two switches of one queue from overlapping in elevator_change_done(), > which touches the elevator sysfs and debugfs entries outside of > q->elevator_lock, so use a new mutex for that instead. It is held across > elevator_change(), and only the sysfs store takes it. The other callers > of elevator_change() run before the queue is registered, after its sysfs > files are gone, or with the lock held for write, so they cannot race > with the store. > > nr_hw_queues updates, nr_requests writes and disk removal still take the > lock for write, so a scheduler write can still get -EBUSY while one of > those holds or waits for it, as before 4ff58d6bc9dd. > > Fixes: 4ff58d6bc9dd ("block: serialize elevator changes for the same queue using a writer lock") > Cc: stable@vger.kernel.org # v7.2+ > Reported-by: Reindl Harald > Closes: https://lore.kernel.org/r/077dd42e-cbc9-4c23-a5ef-3d20e4fc7757@thelounge.net > Assisted-by: LLM > Signed-off-by: Laveesh Bansal FYI, this patch looks similar as the RFC patch that I created before [*]. It adds another mutex in similar manner as this patch. [*] https://lore.kernel.org/linux-block/aiqaXfTqCLMu2DwF@fedora/ I ran whole blktests applying this patch on top of the kernel v7.3. I observed no regression, then looks good from testing point of view. In case this patch gets applied, Tested-by: Shin'ichiro Kawasaki