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

[Kashyap Desai]

> On Tue, Nov 17, 2020 at 09:05:04AM +0800, Ming Lei wrote:
> > 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?
>
> Forget to mention, 'blk_flush_queue' is one private structure inside
block
> layer, so we can't define as inline.

Original issue reported by Qian is fixed by this patch series. Tested on
MegaRaid h/w.

Tested-by: Kashyap Desai <kashyap.desai at broadcom.com>

>
> thanks,
> Ming
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