dcache: touch up predicts in __d_lookup_rcu()

Rationale is that if the parent dentry is the same and the length is the
same, then you have to be unlucky for the name to not match.

At the same time the dentry was literally just found on the hash, so you
have to be even more unlucky to determine it is unhashed.

While here add commentary while d_unhashed() is necessary. It was
already removed once and brought back in:
2e321806b681b192 ("Revert "vfs: remove unnecessary d_unhashed() check from __d_lookup_rcu"")

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://patch.msgid.link/20251127131526.4137768-1-mjguzik@gmail.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/dcache.c b/fs/dcache.c
index 25131f105a60ec8bc935fbacf70bff9198ad3ab9..ddc5ee49694d3a1c9e8bb0065ef911f3108ecd33 100644 (file)
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2300,11 +2300,20 @@ struct dentry *__d_lookup_rcu(const struct dentry *parent,
                seq = raw_seqcount_begin(&dentry->d_seq);
                if (dentry->d_parent != parent)
                        continue;
-               if (d_unhashed(dentry))
-                       continue;
                if (dentry->d_name.hash_len != hashlen)
                        continue;
-               if (dentry_cmp(dentry, str, hashlen_len(hashlen)) != 0)
+               if (unlikely(dentry_cmp(dentry, str, hashlen_len(hashlen)) != 0))
+                       continue;
+               /*
+                * Check for the dentry being unhashed.
+                *
+                * As tempting as it is, we *can't* skip it because of a race window
+                * between us finding the dentry before it gets unhashed and loading
+                * the sequence counter after unhashing is finished.
+                *
+                * We can at least predict on it.
+                */
+               if (unlikely(d_unhashed(dentry)))
                        continue;
                *seqp = seq;
                return dentry;