linux-kernel - [PATCH 1/3] afs: fix the usage of read_seqbegin_or_lock() in afs_lookup_volume

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Message-ID: <20231130115606.GA21571@redhat.com>
Date:   Thu, 30 Nov 2023 12:56:06 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     David Howells <dhowells@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Marc Dionne <marc.dionne@...istor.com>,
        linux-afs@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: [PATCH 1/3] afs: fix the usage of read_seqbegin_or_lock() in
 afs_lookup_volume_rcu()

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>
---
 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);
-- 
2.25.1.362.g51ebf55