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);