linux-kernel - Re: [syzbot] [fs?] KCSAN: data-race in choose_mountpoint_rcu / umount

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Message-ID: <20250422171237.GA2023217@ZenIV>
Date: Tue, 22 Apr 2025 18:12:37 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Marco Elver <elver@...gle.com>
Cc: Christian Brauner <brauner@...nel.org>,
	syzbot <syzbot+81fdaf0f522d5c5e41fb@...kaller.appspotmail.com>,
	jack@...e.cz, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [fs?] KCSAN: data-race in choose_mountpoint_rcu /
 umount_tree

On Tue, Apr 22, 2025 at 04:42:52PM +0200, Marco Elver wrote:

> Seqlocks are generally supported, but have caused headaches in the
> past, esp. if the reader-side seqlock critical section does not follow
> the typical do-seqbegin-while-retry pattern, or the critical section
> is too large. If I read this right, the
> 
>   struct dentry *mountpoint = m->mnt_mountpoint;
> 
> is before the seqlock-reader beginning with "*seqp =
> read_seqcount_begin(&mountpoint->d_seq);" ?

Different seqlock - mount_lock protects mount tree and it's been sampled
all way back in the beginning of RCU pathwalk...