linux-kernel - [PATCH v3 06/21] fast_dput(): having ->d_delete() is not reason to delay refcount decrement

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Message-Id: <20231124060422.576198-6-viro@zeniv.linux.org.uk>
Date:   Fri, 24 Nov 2023 06:04:07 +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 06/21] fast_dput(): having ->d_delete() is not reason to delay refcount decrement

->d_delete() is a way for filesystem to tell that dentry is not worth
keeping cached.  It is not guaranteed to be called every time a dentry
has refcount drop down to zero; it is not guaranteed to be called before
dentry gets evicted.  In other words, it is not suitable for any kind
of keeping track of dentry state.

None of the in-tree filesystems attempt to use it that way, fortunately.

So the contortions done by fast_dput() (as well as dentry_kill()) are
not warranted.  fast_dput() certainly should treat having ->d_delete()
instance as "can't assume we'll be keeping it", but that's not different
from the way we treat e.g. DCACHE_DONTCACHE (which is rather similar
to making ->d_delete() returns true when called).

Reviewed-by: Christian Brauner <brauner@...nel.org>
Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
---
 fs/dcache.c | 12 ++----------
 1 file changed, 2 insertions(+), 10 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index c1025921f8d3..00c19041adf3 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -768,15 +768,7 @@ static inline bool fast_dput(struct dentry *dentry)
 	unsigned int d_flags;
 
 	/*
-	 * If we have a d_op->d_delete() operation, we sould not
-	 * let the dentry count go to zero, so use "put_or_lock".
-	 */
-	if (unlikely(dentry->d_flags & DCACHE_OP_DELETE))
-		return lockref_put_or_lock(&dentry->d_lockref);
-
-	/*
-	 * .. otherwise, we can try to just decrement the
-	 * lockref optimistically.
+	 * try to decrement the lockref optimistically.
 	 */
 	ret = lockref_put_return(&dentry->d_lockref);
 
@@ -830,7 +822,7 @@ static inline bool fast_dput(struct dentry *dentry)
 	 */
 	smp_rmb();
 	d_flags = READ_ONCE(dentry->d_flags);
-	d_flags &= DCACHE_REFERENCED | DCACHE_LRU_LIST |
+	d_flags &= DCACHE_REFERENCED | DCACHE_LRU_LIST | DCACHE_OP_DELETE |
 			DCACHE_DISCONNECTED | DCACHE_DONTCACHE;
 
 	/* Nothing to do? Dropping the reference was all we needed? */
-- 
2.39.2