afs: fix the usage of read_seqbegin_or_lock() in afs_lookup_volume_rcu()

[ 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>

diff --git a/fs/afs/callback.c b/fs/afs/callback.c
index a484fa6..90f9b2a 100644 (file)
--- 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);