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

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Message-ID: <20230608091130.bthttzsmdeeiagof@quack3>
Date:   Thu, 8 Jun 2023 11:11:30 +0200
From:   Jan Kara <jack@...e.cz>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Jan Kara <jack@...e.cz>, "Darrick J. Wong" <djwong@...nel.org>,
        Luis Chamberlain <mcgrof@...nel.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 Wed 07-06-23 22:29:04, Christoph Hellwig wrote:
> 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.

OK, I agree the wrappers could be confusing (they didn't confuse me but
when you spelled it out, I agree).

> > 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..

Well, we need to somehow settle on how to deal with a race when both kernel
& userspace race to freeze the filesystem and make the result consistent
with the situation when the fs is already frozen by someone.

> > 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.

OK. I'm not too hung up on BIT() macro.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR