linux-kernel - [PATCH] vfs: use RCU in ilookup

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Message-ID: <20240715071324.265879-1-mjguzik@gmail.com>
Date: Mon, 15 Jul 2024 09:13:24 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: brauner@...nel.org
Cc: viro@...iv.linux.org.uk,
	jack@...e.cz,
	linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	bharata@....com,
	Mateusz Guzik <mjguzik@...il.com>
Subject: [PATCH] vfs: use RCU in ilookup

A soft lockup in ilookup was reported when stress-testing a 512-way
system [1] (see [2] for full context) and it was verified that not
taking the lock shifts issues back to mm.

[1] https://lore.kernel.org/linux-mm/56865e57-c250-44da-9713-cf1404595bcc@amd.com/
[2] https://lore.kernel.org/linux-mm/d2841226-e27b-4d3d-a578-63587a3aa4f3@amd.com/

Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
---

fwiw the originally sent patch to the reporter performs a lockless
lookup first and falls back to the locked variant, but that was me
playing overfly safe.

I would add tested-by but patches are not the same in the end.

This is the only spot which can get this fixup, everything else taking
the lock is also using custom callbacks, so filesystems invoking such
code will need to get patched up on case-by-case basis (but
realistically they probably already can do RCU-only operation).

 fs/inode.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index f356fe2ec2b6..52ca063c552c 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -1525,9 +1525,7 @@ struct inode *ilookup(struct super_block *sb, unsigned long ino)
 	struct hlist_head *head = inode_hashtable + hash(sb, ino);
 	struct inode *inode;
 again:
-	spin_lock(&inode_hash_lock);
-	inode = find_inode_fast(sb, head, ino, true);
-	spin_unlock(&inode_hash_lock);
+	inode = find_inode_fast(sb, head, ino, false);

 	if (inode) {
 		if (IS_ERR(inode))
-- 
2.43.0