Patch "afs: fix the usage of read_seqbegin_or_lock() in afs_find_server*()" has been added to the 5.15-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    afs: fix the usage of read_seqbegin_or_lock() in afs_find_server*()

to the 5.15-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     afs-fix-the-usage-of-read_seqbegin_or_lock-in-afs_fi.patch
and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit dde374c140553555823ee237e0eae349bb0c6779
Author: Oleg Nesterov <oleg@xxxxxxxxxx>
Date:   Thu Nov 30 12:56:14 2023 +0100

    afs: fix the usage of read_seqbegin_or_lock() in afs_find_server*()
    
    [ Upstream commit 1702e0654ca9a7bcd7c7619c8a5004db58945b71 ]
    
    David Howells says:
    
     (5) afs_find_server().
    
         There could be a lot of servers in the list and each server can have
         multiple addresses, so I think this would be better with an exclusive
         second pass.
    
         The server list isn't likely to change all that often, but when it does
         change, there's a good chance several servers are going to be
         added/removed one after the other.  Further, this is only going to be
         used for incoming cache management/callback requests from the server,
         which hopefully aren't going to happen too often - but it is remotely
         drivable.
    
     (6) afs_find_server_by_uuid().
    
         Similarly to (5), there could be a lot of servers to search through, but
         they are in a tree not a flat list, so it should be faster to process.
         Again, it's not likely to change that often and, again, when it does
         change it's likely to involve multiple changes.  This can be driven
         remotely by an incoming cache management request but is mostly going to
         be driven by setting up or reconfiguring a volume's server list -
         something that also isn't likely to happen often.
    
    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/20231130115614.GA21581@xxxxxxxxxx/
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/fs/afs/server.c b/fs/afs/server.c
index ffed828622b6..34b47218129e 100644
--- a/fs/afs/server.c
+++ b/fs/afs/server.c
@@ -27,7 +27,7 @@ struct afs_server *afs_find_server(struct afs_net *net,
 	const struct afs_addr_list *alist;
 	struct afs_server *server = NULL;
 	unsigned int i;
-	int seq = 0, diff;
+	int seq = 1, diff;
 
 	rcu_read_lock();
 
@@ -35,6 +35,7 @@ struct afs_server *afs_find_server(struct afs_net *net,
 		if (server)
 			afs_unuse_server_notime(net, server, afs_server_trace_put_find_rsq);
 		server = NULL;
+		seq++; /* 2 on the 1st/lockless path, otherwise odd */
 		read_seqbegin_or_lock(&net->fs_addr_lock, &seq);
 
 		if (srx->transport.family == AF_INET6) {
@@ -90,7 +91,7 @@ struct afs_server *afs_find_server_by_uuid(struct afs_net *net, const uuid_t *uu
 {
 	struct afs_server *server = NULL;
 	struct rb_node *p;
-	int diff, seq = 0;
+	int diff, seq = 1;
 
 	_enter("%pU", uuid);
 
@@ -102,7 +103,7 @@ struct afs_server *afs_find_server_by_uuid(struct afs_net *net, const uuid_t *uu
 		if (server)
 			afs_unuse_server(net, server, afs_server_trace_put_uuid_rsq);
 		server = NULL;
-
+		seq++; /* 2 on the 1st/lockless path, otherwise odd */
 		read_seqbegin_or_lock(&net->fs_lock, &seq);
 
 		p = net->fs_servers.rb_node;