linux-kernel - [PATCH] fs: dodge an atomic in putname if ref == 1

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Message-ID: <20250311181804.1165758-1-mjguzik@gmail.com>
Date: Tue, 11 Mar 2025 19:18:04 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: brauner@...nel.org,
	viro@...iv.linux.org.uk
Cc: jack@...e.cz,
	linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	io-uring@...r.kernel.org,
	audit@...r.kernel.org,
	axboe@...nel.dk,
	Mateusz Guzik <mjguzik@...il.com>
Subject: [PATCH] fs: dodge an atomic in putname if ref == 1

While the structure is refcounted, the only consumer incrementing it is
audit and even then the atomic operation is only needed when it
interacts with io_uring.

If putname spots a count of 1, there is no legitimate way for anyone to
bump it.

If audit is disabled, the count is guaranteed to be 1, which
consistently elides the atomic for all path lookups. If audit is
enabled, it still manages to elide the last decrement.

Note the patch does not do anything to prevent audit from suffering
atomics. See [1] and [2] for a different approach.

Benchmarked on Sapphire Rapids issuing access() (ops/s):
before: 5106246
after:  5269678 (+3%)

Link 1:	https://lore.kernel.org/linux-fsdevel/20250307161155.760949-1-mjguzik@gmail.com/
Link 2: https://lore.kernel.org/linux-fsdevel/20250307164216.GI2023217@ZenIV/
Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
---

This is an alternative to the patch I linked above.

I think the improved commit message should also cover the feedback
Christian previously shared concerning it.

This is a trivial win until the atomic issue gets resolved, I don't see
any reason to NOT include it. At the same time I don't have that much
interest arguing about it either.

That is to say, here is a different take, if you don't like it, I'm
dropping the subject.

cheers

 fs/namei.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 06765d320e7e..add90981cfcd 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -275,14 +275,19 @@ EXPORT_SYMBOL(getname_kernel);

 void putname(struct filename *name)
 {
+	int refcnt;
+
 	if (IS_ERR_OR_NULL(name))
 		return;

-	if (WARN_ON_ONCE(!atomic_read(&name->refcnt)))
-		return;
+	refcnt = atomic_read(&name->refcnt);
+	if (refcnt != 1) {
+		if (WARN_ON_ONCE(!refcnt))
+			return;

-	if (!atomic_dec_and_test(&name->refcnt))
-		return;
+		if (!atomic_dec_and_test(&name->refcnt))
+			return;
+	}

 	if (name->name != name->iname) {
 		__putname(name->name);
-- 
2.43.0