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

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Message-ID: <20250423021547.GD2023217@ZenIV>
Date: Wed, 23 Apr 2025 03:15:47 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Eric Chanudet <echanude@...hat.com>
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 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...