linux-kernel - Re: [PATCH v4] fs/namespace: defer RCU sync for MNT

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Message-ID: <ubimdwyutm47mlmtn43mbobtnwaxkmuse3vpgosbsh4yd7f73t@bbt7az62ybbb>
Date: Wed, 23 Apr 2025 11:04:48 -0400
From: Eric Chanudet <echanude@...hat.com>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Clark Williams <clrkwllms@...nel.org>, 
	Steven Rostedt <rostedt@...dmis.org>, Ian Kent <ikent@...hat.com>, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev, 
	Alexander Larsson <alexl@...hat.com>, Lucas Karpinski <lkarpins@...hat.com>
Subject: Re: [PATCH v4] fs/namespace: defer RCU sync for MNT_DETACH umount

On Wed, Apr 23, 2025 at 03:15:47AM +0100, Al Viro wrote:
> On Tue, Apr 22, 2025 at 03:53:43PM -0400, Eric Chanudet wrote:
> 
> > I'm not quite following. With umount -l, I thought there is no guaranty
> > that the file-system is shutdown. Doesn't "shutdown -r now" already
> > risks loses without any of these changes today?
> 
> Busy filesystems might stay around after umount -l, for as long as they
> are busy.  I.e. if there's a process with cwd on one of the affected
> filesystems, it will remain active until that process chdirs away or
> gets killed, etc.  Assuming that your userland kills all processes before
> rebooting the kernel, everything ought to be shut down, TYVM...
> 
> If not for that, the damn thing would be impossible to use safely...
> 

Right, that ties up with Christian's earlier reply and was also stated
in 9ea459e110df ("delayed mntput") description.

Thanks for your patience and explanations.

-- 
Eric Chanudet