Re: [PATCH] block: Fix a deadlock related to modifying the readahead attribute

[Bart Van Assche <bvanassche@acm.org>]

On 6/25/25 10:31 PM, Nilay Shroff wrote:
> It seems that some other thread on your system acquired
> ->freeze_lock and never released it and that prevents
> the udev-worker thread to forward progress.

That's wrong. blk_mq_freeze_queue_wait() is waiting for q_usage_counter
to drop to zero as the below output shows:

(gdb) list *(blk_mq_freeze_queue_wait+0xf2)
0xffffffff823ab0b2 is in blk_mq_freeze_queue_wait (block/blk-mq.c:190).
185     }
186     EXPORT_SYMBOL_GPL(blk_freeze_queue_start);
187
188     void blk_mq_freeze_queue_wait(struct request_queue *q)
189     {
190             wait_event(q->mq_freeze_wq, 
percpu_ref_is_zero(&q->q_usage_counter));
191     }
192     EXPORT_SYMBOL_GPL(blk_mq_freeze_queue_wait);
193
194     int blk_mq_freeze_queue_wait_timeout(struct request_queue *q,

> If you haven't enabled lockdep on your system then can you
> please configure lockdep and rerun the srp/002 test?

Lockdep was enabled during the test and didn't complain.

This is my analysis of the deadlock:

* Multiple requests are pending:
# (cd /sys/kernel/debug/block && grep -aH . */*/*/*list) | head
dm-2/hctx0/cpu0/default_rq_list:0000000035c26c20 {.op=READ, 
.cmd_flags=SYNC|IDLE, .rq_flags=IO_STAT, .state=idle, .tag=137, 
.internal_tag=-1}
dm-2/hctx0/cpu0/default_rq_list:000000005060461e {.op=READ, 
.cmd_flags=SYNC|IDLE, .rq_flags=IO_STAT, .state=idle, .tag=136, 
.internal_tag=-1}
dm-2/hctx0/cpu0/default_rq_list:000000007cd295ec {.op=READ, 
.cmd_flags=SYNC|IDLE, .rq_flags=IO_STAT, .state=idle, .tag=135, 
.internal_tag=-1}
dm-2/hctx0/cpu0/default_rq_list:00000000a4a8006b {.op=READ, 
.cmd_flags=SYNC|IDLE, .rq_flags=IO_STAT, .state=idle, .tag=134, 
.internal_tag=-1}
dm-2/hctx0/cpu0/default_rq_list:000000001f93036f {.op=READ, 
.cmd_flags=SYNC|IDLE, .rq_flags=IO_STAT, .state=idle, .tag=140, 
.internal_tag=-1}
dm-2/hctx0/cpu0/default_rq_list:00000000333baffb {.op=READ, 
.cmd_flags=SYNC|IDLE, .rq_flags=IO_STAT, .state=idle, .tag=173, 
.internal_tag=-1}
dm-2/hctx0/cpu0/default_rq_list:000000002c050850 {.op=READ, 
.cmd_flags=SYNC|IDLE, .rq_flags=IO_STAT, .state=idle, .tag=141, 
.internal_tag=-1}
dm-2/hctx0/cpu0/default_rq_list:000000000668dd8b {.op=WRITE, 
.cmd_flags=SYNC|META|PRIO, .rq_flags=IO_STAT, .state=idle, .tag=133, 
.internal_tag=-1}
dm-2/hctx0/cpu0/default_rq_list:0000000079b67c9f {.op=READ, 
.cmd_flags=SYNC|IDLE, .rq_flags=IO_STAT, .state=idle, .tag=207, 
.internal_tag=-1}
dm-2/hctx0/cpu107/default_rq_list:0000000036254afb {.op=READ, 
.cmd_flags=SYNC|IDLE, .rq_flags=IO_STAT, .state=idle, .tag=1384, 
.internal_tag=-1}

* queue_if_no_path is enabled for the multipath device dm-2:
# ls -l /dev/mapper/mpatha
lrwxrwxrwx 1 root root 7 Jun 26 08:50 /dev/mapper/mpatha -> ../dm-2
# dmsetup table mpatha
0 65536 multipath 1 queue_if_no_path 1 alua 1 1 service-time 0 1 2 8:32 1 1

* The block device 8:32 is being deleted:
# grep '^8:32$' /sys/class/block/*/dev | wc -l
0

* blk_mq_freeze_queue_nomemsave() waits for the pending requests to
   finish. Because the only path in the multipath is being deleted
   and because queue_if_no_path is enabled,
   blk_mq_freeze_queue_nomemsave() hangs.

Bart.