linux-kernel - Re: [PATCH 3/6] fs: distinguish between user initiated freeze and kernel initiated freeze

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Date:   Wed, 7 Jun 2023 22:29:04 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Jan Kara <jack@...e.cz>
Cc:     "Darrick J. Wong" <djwong@...nel.org>,
        Luis Chamberlain <mcgrof@...nel.org>, hch@...radead.org,
        sandeen@...deen.net, song@...nel.org, rafael@...nel.org,
        gregkh@...uxfoundation.org, viro@...iv.linux.org.uk,
        jikos@...nel.org, bvanassche@....org, ebiederm@...ssion.com,
        mchehab@...nel.org, keescook@...omium.org, p.raghav@...sung.com,
        da.gomez@...sung.com, linux-fsdevel@...r.kernel.org,
        kernel@...force.de, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] fs: distinguish between user initiated freeze and
 kernel initiated freeze

On Thu, May 25, 2023 at 04:14:30PM +0200, Jan Kara wrote:
> Yes, this is exactly how I'd imagine it. Thanks for writing the patch!
> 
> I'd just note that this would need rebasing on top of Luis' patches 1 and
> 2. Also:

I'd not do that for now.  1 needs a lot more work, and 2 seems rather
questionable.

> Now the only remaining issue with the code is that the two different
> holders can be attempting to freeze the filesystem at once and in that case
> one of them has to wait for the other one instead of returning -EBUSY as
> would happen currently. This can happen because we temporarily drop
> s_umount in freeze_super() due to lock ordering issues. I think we could
> do something like:
> 
> 	if (!sb_unfrozen(sb)) {
> 		up_write(&sb->s_umount);
> 		wait_var_event(&sb->s_writers.frozen,
> 			       sb_unfrozen(sb) || sb_frozen(sb));
> 		down_write(&sb->s_umount);
> 		goto retry;
> 	}
> 
> and then sprinkle wake_up_var(&sb->s_writers.frozen) at appropriate places
> in freeze_super().

Let's do that separately as a follow on..

> 
> BTW, when reading this code, I've spotted attached cleanup opportunity but
> I'll queue that separately so that is JFYI.
> 
> > +#define FREEZE_HOLDER_USERSPACE	(1U << 1)	/* userspace froze fs */
> > +#define FREEZE_HOLDER_KERNEL	(1U << 2)	/* kernel froze fs */
> 
> Why not start from 1U << 0? And bonus points for using BIT() macro :).

BIT() is a nasty thing and actually makes code harder to read. And it
doesn't interact very well with the __bitwise annotation that might
actually be useful here.