[PATCH 6.6 039/322] afs: fix the usage of read_seqbegin_or_lock() in afs_lookup_volume_rcu()

Greg Kroah-Hartman
2024-02-03T04:11:51Z

From: Oleg Nesterov <oleg@redhat.com>

[ Upstream commit 4121b4337146b64560d1e46ebec77196d9287802 ]

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/
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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