From 4121b4337146b64560d1e46ebec77196d9287802 Mon Sep 17 00:00:00 2001
From: Oleg Nesterov <oleg@redhat.com>
Date: Thu, 30 Nov 2023 12:56:06 +0100
Subject: [PATCH] afs: fix the usage of read_seqbegin_or_lock() in
afs_lookup_volume_rcu()
Git-commit: 4121b4337146b64560d1e46ebec77196d9287802
Patch-mainline: v6.8-rc1
References: git-fixes
David Howells says:
(2) afs_lookup_volume_rcu().
There can be a lot of volumes known by a system. A thousand would
require a 10-step walk and this is drivable by remote operation, so I
think this should probably take a lock on the second pass too.
Make the "seq" counter odd on the 2nd pass, otherwise read_seqbegin_or_lock()
never takes the lock.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Cc: Marc Dionne <marc.dionne@auristor.com>
Cc: linux-afs@lists.infradead.org
Link: https://lore.kernel.org/r/20231130115606.GA21571@redhat.com/
Acked-by: Anthony Iliopoulos <ailiop@suse.com>
---
fs/afs/callback.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/fs/afs/callback.c b/fs/afs/callback.c
index a484fa642808..90f9b2a46ff4 100644
--- a/fs/afs/callback.c
+++ b/fs/afs/callback.c
@@ -110,13 +110,14 @@ static struct afs_volume *afs_lookup_volume_rcu(struct afs_cell *cell,
{
struct afs_volume *volume = NULL;
struct rb_node *p;
- int seq = 0;
+ int seq = 1;
do {
/* Unfortunately, rbtree walking doesn't give reliable results
* under just the RCU read lock, so we have to check for
* changes.
*/
+ seq++; /* 2 on the 1st/lockless path, otherwise odd */
read_seqbegin_or_lock(&cell->volume_lock, &seq);
p = rcu_dereference_raw(cell->volumes.rb_node);
--
2.43.0