linux-kernel - [PATCH AUTOSEL 5.15 10/13] afs: fix the usage of read_seqbegin_or_lock() in afs_lookup_volume

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Date: Mon, 15 Jan 2024 19:25:46 -0500
From: Sasha Levin <sashal@...nel.org>
To: linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Cc: Oleg Nesterov <oleg@...hat.com>,
	David Howells <dhowells@...hat.com>,
	Marc Dionne <marc.dionne@...istor.com>,
	linux-afs@...ts.infradead.org,
	Sasha Levin <sashal@...nel.org>
Subject: [PATCH AUTOSEL 5.15 10/13] afs: fix the usage of read_seqbegin_or_lock() in afs_lookup_volume_rcu()

From: Oleg Nesterov <oleg@...hat.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@...hat.com>
Signed-off-by: David Howells <dhowells@...hat.com>
cc: Marc Dionne <marc.dionne@...istor.com>
cc: linux-afs@...ts.infradead.org
Link: https://lore.kernel.org/r/20231130115606.GA21571@redhat.com/
Signed-off-by: Sasha Levin <sashal@...nel.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 1b4d5809808d..e0f5dcffe4e3 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