On 7/1/25 10:18, Nilay Shroff wrote:
Recent lockdep reports [1] have revealed a potential deadlock caused by a lock dependency between the percpu allocator lock and the elevator lock. This issue can be avoided by ensuring that the allocation and release of scheduler tags (sched_tags) are performed outside the elevator lock. Furthermore, the queue does not need to be remain frozen during these operations. To address this, move all sched_tags allocations and deallocations outside of both the ->elevator_lock and the ->freeze_lock. Since the lifetime of the elevator queue and its associated sched_tags is closely tied, the allocated sched_tags are now stored in the elevator queue structure. Then, during the actual elevator switch (which runs under ->freeze_lock and ->elevator_lock), the pre-allocated sched_tags are assigned to the appropriate q->hctx. Once the elevator switch is complete and the locks are released, the old elevator queue and its associated sched_tags are freed. This commit specifically addresses the allocation/deallocation of sched_ tags during elevator switching. Note that sched_tags may also be allocated in other contexts, such as during nr_hw_queues updates. Supporting that use case will require batch allocation/deallocation, which will be handled in a follow-up patch. This restructuring ensures that sched_tags memory management occurs entirely outside of the ->elevator_lock and ->freeze_lock context, eliminating the lock dependency problem seen during scheduler updates. [1] https://lore.kernel.org/all/0659ea8d-a463-47c8-9180-43c719e106eb@xxxxxxxxxxxxx/ Reported-by: Stefan Haberland <sth@xxxxxxxxxxxxx> Closes: https://lore.kernel.org/all/0659ea8d-a463-47c8-9180-43c719e106eb@xxxxxxxxxxxxx/ Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx> Reviewed-by: Christoph Hellwig <hch@xxxxxx> Signed-off-by: Nilay Shroff <nilay@xxxxxxxxxxxxx> --- block/blk-mq-sched.c | 155 +++++++++++++++++++++++-------------------- block/blk-mq-sched.h | 8 ++- block/elevator.c | 47 +++++++++++-- block/elevator.h | 14 +++- 4 files changed, 144 insertions(+), 80 deletions(-)
Reviewed-by: Hannes Reinecke <hare@xxxxxxx> Cheers, Hannes -- Dr. Hannes Reinecke Kernel Storage Architect hare@xxxxxxx +49 911 74053 688 SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich
