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

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Message-ID: <AANLkTi=t0vFTTDr9jBZ70UmJ9YYzziA0cRP8t0T9wswW@mail.gmail.com>
Date:	Thu, 17 Feb 2011 22:19:52 -0500
From:	Mike Snitzer <snitzer@...hat.com>
To:	NeilBrown <neilb@...e.de>
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, 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?

> 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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