[PATCH 1/3] blk-mq: add new API of blk_mq_hctx_set_fq_lock_class

[Christoph Hellwig]

On Thu, Nov 12, 2020 at 03:55:24PM +0800, Ming Lei wrote:
> flush_end_io() may be called recursively from some driver, such as
> nvme-loop, so lockdep may complain 'possible recursive locking'.
> Commit b3c6a5997541("block: Fix a lockdep complaint triggered by
> request queue flushing") tried to address this issue by assigning
> dynamically allocated per-flush-queue lock class. This solution
> adds synchronize_rcu() for each hctx's release handler, and causes
> horrible SCSI MQ probe delay(more than half an hour on megaraid sas).
> 
> Add new API of blk_mq_hctx_set_fq_lock_class() for these drivers, so
> we just need to use driver specific lock class for avoiding the
> lockdep warning of 'possible recursive locking'.

I'd turn this into an inline function to avoid the (although very
minimal) cost when LOCKDEP is not enabled.