The commit '245618f8e45f ("block: protect wbt_lat_usec using q->
elevator_lock")' introduced q->elevator_lock to protect updates
to blk-wbt parameters when writing to the sysfs attribute wbt_
lat_usec and the cgroup attribute io.cost.qos. However, both
these attributes also acquire q->rq_qos_mutex, leading to the
following lockdep warning:
======================================================
WARNING: possible circular locking dependency detected
6.14.0-rc5+ #138 Not tainted
------------------------------------------------------
bash/5902 is trying to acquire lock:
c000000085d495a0 (&q->rq_qos_mutex){+.+.}-{4:4}, at: wbt_init+0x164/0x238
but task is already holding lock:
c000000085d498c8 (&q->elevator_lock){+.+.}-{4:4}, at: queue_wb_lat_store+0xb0/0x20c
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #1 (&q->elevator_lock){+.+.}-{4:4}:
__mutex_lock+0xf0/0xa58
ioc_qos_write+0x16c/0x85c
cgroup_file_write+0xc4/0x32c
kernfs_fop_write_iter+0x1b8/0x29c
vfs_write+0x410/0x584
ksys_write+0x84/0x140
system_call_exception+0x134/0x360
system_call_vectored_common+0x15c/0x2ec
-> #0 (&q->rq_qos_mutex){+.+.}-{4:4}:
__lock_acquire+0x1b6c/0x2ae0
lock_acquire+0x140/0x430
__mutex_lock+0xf0/0xa58
wbt_init+0x164/0x238
queue_wb_lat_store+0x1dc/0x20c
queue_attr_store+0x12c/0x164
sysfs_kf_write+0x6c/0xb0
kernfs_fop_write_iter+0x1b8/0x29c
vfs_write+0x410/0x584
ksys_write+0x84/0x140
system_call_exception+0x134/0x360
system_call_vectored_common+0x15c/0x2ec
other info that might help us debug this:
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(&q->elevator_lock);
lock(&q->rq_qos_mutex);
lock(&q->elevator_lock);
lock(&q->rq_qos_mutex);
*** DEADLOCK ***
6 locks held by bash/5902:
#0: c000000051122400 (sb_writers#3){.+.+}-{0:0}, at: ksys_write+0x84/0x140
#1: c00000007383f088 (&of->mutex#2){+.+.}-{4:4}, at: kernfs_fop_write_iter+0x174/0x29c
#2: c000000008550428 (kn->active#182){.+.+}-{0:0}, at: kernfs_fop_write_iter+0x180/0x29c
#3: c000000085d493a8 (&q->q_usage_counter(io)#5){++++}-{0:0}, at: blk_mq_freeze_queue_nomemsave+0x28/0x40
#4: c000000085d493e0 (&q->q_usage_counter(queue)#5){++++}-{0:0}, at: blk_mq_freeze_queue_nomemsave+0x28/0x40
#5: c000000085d498c8 (&q->elevator_lock){+.+.}-{4:4}, at: queue_wb_lat_store+0xb0/0x20c
stack backtrace:
CPU: 17 UID: 0 PID: 5902 Comm: bash Kdump: loaded Not tainted 6.14.0-rc5+ #138
Hardware name: IBM,9043-MRX POWER10 (architected) 0x800200 0xf000006 of:IBM,FW1060.00 (NM1060_028) hv:phyp pSeries
Call Trace:
[c0000000721ef590] [c00000000118f8a8] dump_stack_lvl+0x108/0x18c (unreliable)
[c0000000721ef5c0] [c00000000022563c] print_circular_bug+0x448/0x604
[c0000000721ef670] [c000000000225a44] check_noncircular+0x24c/0x26c
[c0000000721ef740] [c00000000022bf28] __lock_acquire+0x1b6c/0x2ae0
[c0000000721ef870] [c000000000229240] lock_acquire+0x140/0x430
[c0000000721ef970] [c0000000011cfbec] __mutex_lock+0xf0/0xa58
[c0000000721efaa0] [c00000000096c46c] wbt_init+0x164/0x238
[c0000000721efaf0] [c0000000008f8cd8] queue_wb_lat_store+0x1dc/0x20c
[c0000000721efb50] [c0000000008f8fa0] queue_attr_store+0x12c/0x164
[c0000000721efc60] [c0000000007c11cc] sysfs_kf_write+0x6c/0xb0
[c0000000721efca0] [c0000000007bfa4c] kernfs_fop_write_iter+0x1b8/0x29c
[c0000000721efcf0] [c0000000006a281c] vfs_write+0x410/0x584
[c0000000721efdc0] [c0000000006a2cc8] ksys_write+0x84/0x140
[c0000000721efe10] [c000000000031b64] system_call_exception+0x134/0x360
[c0000000721efe50] [c00000000000cedc] system_call_vectored_common+0x15c/0x2ec
