From 370ac285f23aecae40600851fb4a1a9e75e50973 Mon Sep 17 00:00:00 2001 From: Nilay Shroff Date: Thu, 14 Aug 2025 13:54:59 +0530 Subject: block: avoid cpu_hotplug_lock depedency on freeze_lock A recent lockdep[1] splat observed while running blktest block/005 reveals a potential deadlock caused by the cpu_hotplug_lock dependency on ->freeze_lock. This dependency was introduced by commit 033b667a823e ("block: blk-rq-qos: guard rq-qos helpers by static key"). That change added a static key to avoid fetching q->rq_qos when neither blk-wbt nor blk-iolatency is configured. The static key dynamically patches kernel text to a NOP when disabled, eliminating overhead of fetching q->rq_qos in the I/O hot path. However, enabling a static key at runtime requires acquiring both cpu_hotplug_lock and jump_label_mutex. When this happens after the queue has already been frozen (i.e., while holding ->freeze_lock), it creates a locking dependency from cpu_hotplug_lock to ->freeze_lock, which leads to a potential deadlock reported by lockdep [1]. To resolve this, replace the static key mechanism with q->queue_flags: QUEUE_FLAG_QOS_ENABLED. This flag is evaluated in the fast path before accessing q->rq_qos. If the flag is set, we proceed to fetch q->rq_qos; otherwise, the access is skipped. Since q->queue_flags is commonly accessed in IO hotpath and resides in the first cacheline of struct request_queue, checking it imposes minimal overhead while eliminating the deadlock risk. This change avoids the lockdep splat without introducing performance regressions. [1] https://lore.kernel.org/linux-block/4fdm37so3o4xricdgfosgmohn63aa7wj3ua4e5vpihoamwg3ui@fq42f5q5t5ic/ Reported-by: Shinichiro Kawasaki Closes: https://lore.kernel.org/linux-block/4fdm37so3o4xricdgfosgmohn63aa7wj3ua4e5vpihoamwg3ui@fq42f5q5t5ic/ Fixes: 033b667a823e ("block: blk-rq-qos: guard rq-qos helpers by static key") Tested-by: Shin'ichiro Kawasaki Signed-off-by: Nilay Shroff Reviewed-by: Ming Lei Reviewed-by: Yu Kuai Link: https://lore.kernel.org/r/20250814082612.500845-4-nilay@linux.ibm.com Signed-off-by: Jens Axboe --- include/linux/blkdev.h | 1 + 1 file changed, 1 insertion(+) (limited to 'include/linux/blkdev.h') diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h index 95886b404b16..fe1797bbec42 100644 --- a/include/linux/blkdev.h +++ b/include/linux/blkdev.h @@ -656,6 +656,7 @@ enum { QUEUE_FLAG_SQ_SCHED, /* single queue style io dispatch */ QUEUE_FLAG_DISABLE_WBT_DEF, /* for sched to disable/enable wbt */ QUEUE_FLAG_NO_ELV_SWITCH, /* can't switch elevator any more */ + QUEUE_FLAG_QOS_ENABLED, /* qos is enabled */ QUEUE_FLAG_MAX }; -- cgit