linux-kernel - Re: [PATCH 03/11] fs: add frozen sb state helpers

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Message-ID: <20180420184932.57as7jup75nx463v@garbanzo.do-not-panic.com>
Date:   Fri, 20 Apr 2018 11:49:32 -0700
From:   "Luis R. Rodriguez" <mcgrof@...nel.org>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:     Jan Kara <jack@...e.cz>, viro@...iv.linux.org.uk,
        bart.vanassche@....com, ming.lei@...hat.com, tytso@....edu,
        darrick.wong@...cle.com, jikos@...nel.org, rjw@...ysocki.net,
        pavel@....cz, len.brown@...el.com, linux-fsdevel@...r.kernel.org,
        boris.ostrovsky@...cle.com, jgross@...e.com,
        todd.e.brandt@...ux.intel.com, nborisov@...e.com,
        martin.petersen@...cle.com, ONeukum@...e.com,
        oleksandr@...alenko.name, oleg.b.antonyan@...il.com,
        yu.chen.surf@...il.com, dan.j.williams@...el.com,
        linux-pm@...r.kernel.org, linux-block@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        jlayton@...hat.com
Subject: Re: [PATCH 03/11] fs: add frozen sb state helpers

On Tue, Apr 17, 2018 at 05:59:36PM -0700, Luis R. Rodriguez wrote:
> On Thu, Dec 21, 2017 at 12:03:29PM +0100, Jan Kara wrote:
> > Hello,
> > 
> > I think I owe you a reply here... Sorry that it took so long.
> 
> Took me just as long :)
> 
> > On Fri 01-12-17 22:13:27, Luis R. Rodriguez wrote:
> > > 
> > > I'll note that its still not perfectly clear if really the semantics behind
> > > freeze_bdev() match what I described above fully. That still needs to be
> > > vetted for. For instance, does thaw_bdev() keep a superblock frozen if we
> > > an ioctl initiated freeze had occurred before? If so then great. Otherwise
> > > I think we'll need to distinguish the ioctl interface. Worst possible case
> > > is that bdev semantics and in-kernel semantics differ somehow, then that
> > > will really create a holy fucking mess.
> > 
> > I believe nobody really thought about mixing those two interfaces to fs
> > freezing and so the behavior is basically defined by the implementation.
> > That is:
> > 
> > freeze_bdev() on sb frozen by ioctl_fsfreeze() -> EBUSY
> > freeze_bdev() on sb frozen by freeze_bdev() -> success
> > ioctl_fsfreeze() on sb frozen by freeze_bdev() -> EBUSY
> > ioctl_fsfreeze() on sb frozen by ioctl_fsfreeze() -> EBUSY
> > 
> > thaw_bdev() on sb frozen by ioctl_fsfreeze() -> EINVAL
> 
> Phew, so this is what we want for the in-kernel freezing so we're good
> and *can* combine these then.

I double checked, and I don't see where you get EINVAL for this case.
We *do* keep the sb frozen though, which is good, and the worst fear
I had was that we did not. However we return 0 if there was already
a prior freeze_bdev() or ioctl_fsfreeze() other than the context that
started the prior freeze (--bdev->bd_fsfreeze_count > 0).

The -EINVAL is only returned currently if there were no freezers.

int thaw_bdev(struct block_device *bdev, struct super_block *sb)
{
	int error = -EINVAL;

	mutex_lock(&bdev->bd_fsfreeze_mutex);
	if (!bdev->bd_fsfreeze_count)
		goto out;

	error = 0;
	if (--bdev->bd_fsfreeze_count > 0)
		goto out;
	...
out:
	mutex_unlock(&bdev->bd_fsfreeze_mutex);
	return error;
}

  Luis