[PATCH v2 01/40] afs: fix the usage of read_seqbegin_or_lock() in afs_lookup_volume_rcu()

[David Howells]

From: Oleg Nesterov <oleg@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
cc: Marc Dionne <marc.dionne@xxxxxxxxxxxx>
cc: linux-afs@xxxxxxxxxxxxxxxxxxx
Link: https://lore.kernel.org/r/20231130115606.GA21571@xxxxxxxxxx/
---
 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);