Re: [PATCHv7 2/3] block: fix lockdep warning caused by lock dependency in elv_iosched_store

[Hannes Reinecke <hare@suse.de>]

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@linux.ibm.com/
> 
> Reported-by: Stefan Haberland <sth@linux.ibm.com>
> Closes: https://lore.kernel.org/all/0659ea8d-a463-47c8-9180-43c719e106eb@linux.ibm.com/
> Reviewed-by: Ming Lei <ming.lei@redhat.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
> ---
>   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@suse.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@suse.de                                +49 911 74053 688
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