linux-kernel - Re: [PATCH 1/6] block: export blkdev_reread

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Date:	Mon, 6 Apr 2015 09:42:27 -0400
From:	Jarod Wilson <jarod@...hat.com>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Jens Axboe <axboe@...nel.dk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	David Herrmann <dh.herrmann@...il.com>,
	Markus Pargmann <mpa@...gutronix.de>,
	"nbd-general@...ts.sourceforge.net" 
	<nbd-general@...ts.sourceforge.net>,
	Stefan Haberland <stefan.haberland@...ibm.com>,
	Sebastian Ott <sebott@...ux.vnet.ibm.com>,
	Fabian Frederick <fabf@...net.be>, linux-s390@...r.kernel.org
Subject: Re: [PATCH 1/6] block: export blkdev_reread_part()

On Mon, Apr 06, 2015 at 12:40:12AM +0800, Ming Lei wrote:
> On Mon, Apr 6, 2015 at 12:12 AM, Christoph Hellwig <hch@...radead.org> wrote:
> >> +/*
> >> + * This is exported as API for block driver, can be called
> >> + * with requiring bd_mutex or not.
> >> + */
> >> +int __blkdev_reread_part(struct block_device *bdev, bool lock)
> >>  {
> >>       struct gendisk *disk = bdev->bd_disk;
> >>       int res;
> >> @@ -159,12 +163,14 @@ static int blkdev_reread_part(struct block_device *bdev)
> >>               return -EINVAL;
> >>       if (!capable(CAP_SYS_ADMIN))
> >>               return -EACCES;
> >> -     if (!mutex_trylock(&bdev->bd_mutex))
> >> +     if (lock && !mutex_trylock(&bdev->bd_mutex))
> >>               return -EBUSY;
> >
> > Please don't add funtions that do conditional locking, instead move
> > all the code into blkdev_reread_part_nolock, and then wrap it:
> >
> > int blkdev_reread_part(struct block_device *bdev)
> > {
> >         if (!mutex_trylock(&bdev->bd_mutex))
> >                 return -EBUSY;
> >         blkdev_reread_part_nolock(bdev);
> >         mutex_unlock(&bdev->bd_mutex);
> > }
> 
> Yes, it is more clean, but with extra acquiring lock cost for the
> failure cases, especially when we replace trylock with mutex_lock().

I was working on a version of this myself over the past few days, I
actually removed blkdev_reread_part() entirely, renamed
fs/partition-generic.c::reread_partitions() to __reread_partitions(), then
moved the locking from blkdev_reread_part() into a new reread_partitions()
that wrapped around __reread_partitions(). Same difference, I guess.

> > Please also add a lockdep_assert_held to blkdev_reread_part_nolock to
> > ensure callers actually do hold the lock.
> 
> Good point!

Looks like fs/block_dev.c::__blkdev_get() is the only thing that would be
calling the _nolock variant of whichever route, as it handles bd_mutex
acquisition within __blkdev_get().

As an aside, there's a piece of that function that could be worth
duplicating over into loop.c as well:

        if (bdev->bd_invalidated) {
                if (!ret)
                        rescan_partitions(bdev);
                else if (ret == -ENOMEDIUM)
                        invalidate_partitions(disk, bdev);

Might this possibly be put to use to help with the problem commit
8761a3dc1f07b163414e2215a2cadbb4cfe2a107 was trying to solve?

-- 
Jarod Wilson
jarod@...hat.com

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