Re: [PATCH 3/8] 9p: free the session after a grace period

[sashiko-bot@kernel.org]

> v9fs_vfs_get_link_dotl() reads ->cache from the v9fs_session_info in
> sb->s_fs_info to decide whether the symlink target sits in the page
> cache. It does that before it looks at the dentry, so it also does it in
> rcu pathwalk mode. Without loose or metadata caching the dentries have
> no ->d_revalidate() that would make the walk leave rcu mode before it
> gets there.
> 
> v9fs_kill_super() frees the session and clears sb->s_fs_info without
> waiting for a grace period. Right now a pathwalk can't still be in
> v9fs_vfs_get_link_dotl() at that point, but only by accident. umount(2)
> waits for a grace period in namespace_unlock() before it drops the
> mount. And a mount whose last reference goes away elsewhere, like the
> chdir out of a lazily unmounted directory, still gets a grace period
> from bdi_unregister(). v9fs_fill_super() sets up a private bdi, so
> generic_shutdown_super() unregisters it from kill_anon_super() and
> bdi_remove_from_list() waits in synchronize_rcu_expedited() before
> v9fs_kill_super() frees the session. That grace period protects the rcu
> readers of bdi_list in the writeback code. It has nothing to do with
> pathwalk.
> 
> Don't rely on it. Keep sb->s_fs_info and free the session after a grace
> period.
> 
> Fixes: 712da38d134b ("9p: Enable symlink caching in page cache")
> Cc: stable@vger.kernel.org
> Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

Sashiko has reviewed this patch and found no issues. It looks great!

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Sashiko AI review · https://sashiko.dev/#/patchset/20261009-work-fixes-rcu-v1-0-7446d2d335e6@kernel.org?part=3