[SMB,client] two multichannel patches

Commit Message

Steve French Nov. 16, 2023, 7:09 p.m. UTC

Any thoughts on these two multichannel patches from Shyam (attached)?

The first fixes: "cifs: account for primary channel in the interface list"
which fixes a refcounting issue in channel deallocation.  The second fixes
a lock ordering problem in the recent patch: "cifs: handle when server
stops supporting multichannel"

The code to handle the case of server disabling multichannel
was picking iface_lock with chan_lock held. This goes against
the lock ordering rules, as iface_lock is a higher order lock
(even if it isn't so obvious).

This change fixes the lock ordering by doing the following in
that order for each secondary channel:
1. store iface and server pointers in local variable
2. remove references to iface and server in channels
3. unlock chan_lock
4. lock iface_lock
5. dec ref count for iface
6. unlock iface_lock
7. dec ref count for server
8. lock chan_lock again

Let me know if any test feedback or reviews

Patch

From 5eef12c4e3230f2025dc46ad8c4a3bc19978e5d7 Mon Sep 17 00:00:00 2001
From: Shyam Prasad N <[e-mail]>
Date: Tue, 14 Nov 2023 04:58:23 +0000
Subject: [PATCH 2/2] cifs: fix lock ordering while disabling multichannel

The code to handle the case of server disabling multichannel
was picking iface_lock with chan_lock held. This goes against
the lock ordering rules, as iface_lock is a higher order lock
(even if it isn't so obvious).

This change fixes the lock ordering by doing the following in
that order for each secondary channel:
1. store iface and server pointers in local variable
2. remove references to iface and server in channels
3. unlock chan_lock
4. lock iface_lock
5. dec ref count for iface
6. unlock iface_lock
7. dec ref count for server
8. lock chan_lock again

Since this function can only be called in smb2_reconnect, and
that cannot be called by two parallel processes, we should not
have races due to dropping chan_lock between steps 3 and 8.

Fixes: ee1d21794e55 ("cifs: handle when server stops supporting multichannel")
Reported-by: Paulo Alcantara <[e-mail]>
Signed-off-by: Shyam Prasad N <[e-mail]>
Signed-off-by: Steve French <[e-mail]>
---
 fs/smb/client/sess.c | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/fs/smb/client/sess.c b/fs/smb/client/sess.c
index 0bb2ac929061..8b2d7c1ca428 100644
--- a/fs/smb/client/sess.c
+++ b/fs/smb/client/sess.c
@@ -322,28 +322,32 @@  cifs_disable_secondary_channels(struct cifs_ses *ses)
 		iface = ses->chans[i].iface;
 		server = ses->chans[i].server;
 
+		/*
+		 * remove these references first, since we need to unlock
+		 * the chan_lock here, since iface_lock is a higher lock
+		 */
+		ses->chans[i].iface = NULL;
+		ses->chans[i].server = NULL;
+		spin_unlock(&ses->chan_lock);
+
 		if (iface) {
 			spin_lock(&ses->iface_lock);
 			kref_put(&iface->refcount, release_iface);
-			ses->chans[i].iface = NULL;
 			iface->num_channels--;
 			if (iface->weight_fulfilled)
 				iface->weight_fulfilled--;
 			spin_unlock(&ses->iface_lock);
 		}
 
-		spin_unlock(&ses->chan_lock);
-		if (server && !server->terminate) {
-			server->terminate = true;
-			cifs_signal_cifsd_for_reconnect(server, false);
-		}
-		spin_lock(&ses->chan_lock);
-
 		if (server) {
-			ses->chans[i].server = NULL;
+			if (!server->terminate) {
+				server->terminate = true;
+				cifs_signal_cifsd_for_reconnect(server, false);
+			}
 			cifs_put_tcp_session(server, false);
 		}
 
+		spin_lock(&ses->chan_lock);
 	}
 
 done:
-- 
2.39.2