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

Ming Lei
2020-11-17T01:05:36Z

On Mon, Nov 16, 2020 at 06:26:58PM +0100, Christoph Hellwig wrote:
> 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.

blk_mq_hctx_set_fq_lock_class() is just one-shot thing, do you really
care the cost?

thanks,
Ming