linux-kernel - Re: blk_throtl_exit taking q->queue

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Date:	Fri, 18 Feb 2011 14:33:25 +1100
From:	NeilBrown <neilb@...e.de>
To:	Mike Snitzer <snitzer@...hat.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>, Jens Axboe <jaxboe@...ionio.com>,
	linux-kernel@...r.kernel.org
Subject: Re: blk_throtl_exit taking q->queue_lock is problematic

On Thu, 17 Feb 2011 22:19:52 -0500 Mike Snitzer <snitzer@...hat.com> wrote:

> On Thu, Feb 17, 2011 at 9:40 PM, NeilBrown <neilb@...e.de> wrote:
> > On Thu, 17 Feb 2011 11:59:06 -0500 Vivek Goyal <vgoyal@...hat.com> wrote:
> >> So if we do this change for performance reasons, it still makes sense
> >> but doing this change because md provided a q->queue_lock and took away that
> >> lock without notifying block layer hence we do this change, is still not
> >> the right reason, IMHO.
> >
> > Well...I like that patch, as it makes my life easier....
> >
> > But I agree that md is doing something wrong.  Now that ->queue_lock is
> > always initialised, it is wrong to leave it in a state where it not defined.
> >
> > So maybe I'll apply this (after testing it a bit.  The only reason for taking
> > the lock queue_lock in a couple of places is to silence some warnings.
> >
> > Thanks,
> > NeilBrown
> >
> >
> > diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
> > index a23ffa3..909282d 100644
> > --- a/drivers/md/raid1.c
> > +++ b/drivers/md/raid1.c
> > @@ -959,7 +961,9 @@ static int make_request(mddev_t *mddev, struct bio * bio)
> >                atomic_inc(&r1_bio->remaining);
> >                spin_lock_irqsave(&conf->device_lock, flags);
> >                bio_list_add(&conf->pending_bio_list, mbio);
> > +               spin_lock(mddev->queue->queue_lock);
> >                blk_plug_device(mddev->queue);
> > +               spin_unlock(mddev->queue->queue_lock);
> >                spin_unlock_irqrestore(&conf->device_lock, flags);
> >        }
> >        r1_bio_write_done(r1_bio, bio->bi_vcnt, behind_pages, behind_pages != NULL);
> 
> Noticed an inconsistency, raid10.c's additional locking also protects
> the bio_list_add() whereas raid1.c's doesn't.  Seems the additional
> protection in raid10 isn't needed?

Correct - not needed at all.
I put it there because it felt a little cleaner keeping the two 'lock's
together like the two 'unlock's.  Probably confusing though...

My other though is to stop using the block-layer plugging altogether like I
have in RAID5 (Which I needed to do to make it work with DM).  Then I
wouldn't need to touch queue_lock at all - very tempting.


Thanks for the review.

NeilBrown


> 
> > diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> > index 3b607b2..60e6cb1 100644
> > --- a/drivers/md/raid10.c
> > +++ b/drivers/md/raid10.c
> > @@ -970,8 +972,10 @@ static int make_request(mddev_t *mddev, struct bio * bio)
> >
> >                atomic_inc(&r10_bio->remaining);
> >                spin_lock_irqsave(&conf->device_lock, flags);
> > +               spin_lock(mddev->queue->queue_lock);
> >                bio_list_add(&conf->pending_bio_list, mbio);
> >                blk_plug_device(mddev->queue);
> > +               spin_unlock(mddev->queue->queue_lock);
> >                spin_unlock_irqrestore(&conf->device_lock, flags);
> >        }
> >

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