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

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Message-ID: <20250417-zappeln-angesagt-f172a71839d3@brauner>
Date: Thu, 17 Apr 2025 17:28:20 +0200
From: Christian Brauner <brauner@...nel.org>
To: Ian Kent <raven@...maw.net>
Cc: Mark Brown <broonie@...nel.org>, Eric Chanudet <echanude@...hat.com>, 
	Alexander Viro <viro@...iv.linux.org.uk>, 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>, Aishwarya.TCV@....com
Subject: Re: [PATCH v4] fs/namespace: defer RCU sync for MNT_DETACH umount

>     So if there's some userspace process with a broken NFS server and it
>     does umount(MNT_DETACH) it will end up hanging every other
>     umount(MNT_DETACH) on the system because the dealyed_mntput_work
>     workqueue (to my understanding) cannot make progress.

Ok, "to my understanding" has been updated after going back and reading
the delayed work code. Luckily it's not as bad as I thought it is
because it's queued on system_wq which is multi-threaded so it's at
least not causing everyone with MNT_DETACH to get stuck. I'm still
skeptical how safe this all is.