linux-kernel - [PATCH v3 12/21] dentry_kill(): don't bother with retain

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Message-Id: <20231124060422.576198-12-viro@zeniv.linux.org.uk>
Date:   Fri, 24 Nov 2023 06:04:13 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     linux-fsdevel@...r.kernel.org
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Christian Brauner <brauner@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: [PATCH v3 12/21] dentry_kill(): don't bother with retain_dentry() on slow path

We have already checked it and dentry used to look not worthy
of keeping.  The only hard obstacle to evicting dentry is
non-zero refcount; everything else is advisory - e.g. memory
pressure could evict any dentry found with refcount zero.
On the slow path in dentry_kill() we had dropped and regained
->d_lock; we must recheck the refcount, but everything else
is not worth bothering with.

Note that filesystem can not count upon ->d_delete() being
called for dentry - not even once.  Again, memory pressure
(as well as d_prune_aliases(), or attempted rmdir() of ancestor,
or...) will not call ->d_delete() at all.

So from the correctness point of view we are fine doing the
check only once.  And it makes things simpler down the road.

Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
---
 fs/dcache.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index b527db8e5901..80992e49561c 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -739,14 +739,10 @@ static struct dentry *dentry_kill(struct dentry *dentry)
 	spin_lock(&dentry->d_lock);
 	parent = lock_parent(dentry);
 got_locks:
-	if (unlikely(dentry->d_lockref.count != 1)) {
-		dentry->d_lockref.count--;
-	} else if (likely(!retain_dentry(dentry))) {
-		dentry->d_lockref.count--;
+	dentry->d_lockref.count--;
+	if (likely(dentry->d_lockref.count == 0)) {
 		__dentry_kill(dentry);
 		return parent;
-	} else {
-		dentry->d_lockref.count--;
 	}
 	/* we are keeping it, after all */
 	if (inode)
-- 
2.39.2