Patch "net: give more chances to rcu in netdev_wait_allrefs_any()" has been added to the 6.9-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

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

    net: give more chances to rcu in netdev_wait_allrefs_any()

to the 6.9-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:
     net-give-more-chances-to-rcu-in-netdev_wait_allrefs_.patch
and it can be found in the queue-6.9 subdirectory.

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



commit 61ef3cec3a5603d2be0dd2650634bb966dca36d4
Author: Eric Dumazet <edumazet@xxxxxxxxxx>
Date:   Fri Apr 26 06:42:22 2024 +0000

    net: give more chances to rcu in netdev_wait_allrefs_any()
    
    [ Upstream commit cd42ba1c8ac9deb9032add6adf491110e7442040 ]
    
    This came while reviewing commit c4e86b4363ac ("net: add two more
    call_rcu_hurry()").
    
    Paolo asked if adding one synchronize_rcu() would help.
    
    While synchronize_rcu() does not help, making sure to call
    rcu_barrier() before msleep(wait) is definitely helping
    to make sure lazy call_rcu() are completed.
    
    Instead of waiting ~100 seconds in my tests, the ref_tracker
    splats occurs one time only, and netdev_wait_allrefs_any()
    latency is reduced to the strict minimum.
    
    Ideally we should audit our call_rcu() users to make sure
    no refcount (or cascading call_rcu()) is held too long,
    because rcu_barrier() is quite expensive.
    
    Fixes: 0e4be9e57e8c ("net: use exponential backoff in netdev_wait_allrefs")
    Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
    Link: https://lore.kernel.org/all/28bbf698-befb-42f6-b561-851c67f464aa@xxxxxxxxxx/T/#m76d73ed6b03cd930778ac4d20a777f22a08d6824
    Reviewed-by: Jiri Pirko <jiri@xxxxxxxxxx>
    Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/net/core/dev.c b/net/core/dev.c
index 331848eca7d31..e8fb4ef8a85f8 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -10488,8 +10488,9 @@ static struct net_device *netdev_wait_allrefs_any(struct list_head *list)
 			rebroadcast_time = jiffies;
 		}
 
+		rcu_barrier();
+
 		if (!wait) {
-			rcu_barrier();
 			wait = WAIT_REFS_MIN_MSECS;
 		} else {
 			msleep(wait);