Re: [PATCH 2/2] blk: Fix lock inversion between ioc lock and bfqd lock

On Thu, May 20, 2021 at 5:57 PM Ming Lei [off-list ref] wrote:

On Fri, May 21, 2021 at 12:33:53AM +0200, Jan Kara wrote:

quoted

Lockdep complains about lock inversion between ioc->lock and bfqd->lock:

bfqd -> ioc:
 put_io_context+0x33/0x90 -> ioc->lock grabbed
 blk_mq_free_request+0x51/0x140
 blk_put_request+0xe/0x10
 blk_attempt_req_merge+0x1d/0x30
 elv_attempt_insert_merge+0x56/0xa0
 blk_mq_sched_try_insert_merge+0x4b/0x60
 bfq_insert_requests+0x9e/0x18c0 -> bfqd->lock grabbed

We could move blk_put_request() into scheduler code, then the lock
inversion is avoided. So far only mq-deadline and bfq calls into
blk_mq_sched_try_insert_merge(), and this change should be small.

We'd potentially be putting multiple requests if we keep the recursive merge.

Could we move backmerge loop to the schedulers, perhaps?

Thanks,
Ming