From ca0d620b0afae20a7bcd5182606eba6860b2dbf2 Mon Sep 17 00:00:00 2001 From: Mateusz Guzik Date: Thu, 27 Nov 2025 14:15:26 +0100 Subject: [PATCH] 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 Link: https://patch.msgid.link/20251127131526.4137768-1-mjguzik@gmail.com Reviewed-by: Jan Kara Signed-off-by: Christian Brauner --- fs/dcache.c | 15 ++++++++++++--- 1 file changed, 12 insertions(+), 3 deletions(-) diff --git a/fs/dcache.c b/fs/dcache.c index 25131f105a60..ddc5ee49694d 100644 --- 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; -- 2.47.3