linux-kernel - Re: [RFC v2 1/6] fs: provide unlocked helper for freeze

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Date:   Tue, 20 Apr 2021 13:03:35 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     rafael@...nel.org, gregkh@...uxfoundation.org,
        viro@...iv.linux.org.uk, jack@...e.cz, bvanassche@....org,
        jeyu@...nel.org, ebiederm@...ssion.com, mchehab@...nel.org,
        keescook@...omium.org, linux-fsdevel@...r.kernel.org,
        kernel@...force.de, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Dave Chinner <david@...morbit.com>
Subject: Re: [RFC v2 1/6] fs: provide unlocked helper for freeze_super()

On Sat, Apr 17, 2021 at 12:10:21AM +0000, Luis Chamberlain wrote:
> freeze_super() holds a write lock, however we wish to also enable
> callers which already hold the write lock. To do this provide a helper
> and make freeze_super() use it. This way, all that freeze_super() does
> now is lock handling and active count management.

Can we take a step back and think about this a bit more?

freeze_super() has three callers:

 1) freeze_bdev
 2) ioctl_fsfreeze
 3) freeze_store (in gfs2)

The first gets its reference from get_active_super, and is the only
caller of get_active_super.  So IMHO we should just not drop the lock
in get_active_super and directly call the unlocked version.

The other two really should just call grab_super to get an active
reference and s_umount.

In other words: I don't think we need both variants, just move the
locking and s_active acquisition out of free_super.  Same for the
thaw side.