FAILED: patch "[PATCH] mm/slub: hold cpus_read_lock around" failed to apply to 6.18-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 6.18-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x 67ea9d353d0ba12bdbc9183ff568dead9e949b80
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026052843-flavoring-boat-ccba@gregkh' --subject-prefix 'PATCH 6.18.y' HEAD^..

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 67ea9d353d0ba12bdbc9183ff568dead9e949b80 Mon Sep 17 00:00:00 2001
From: Qing Wang <wangqing7171@gmail.com>
Date: Tue, 12 May 2026 11:50:35 +0800
Subject: [PATCH] mm/slub: hold cpus_read_lock around
 flush_rcu_sheaves_on_cache()

flush_rcu_sheaves_on_cache() calls queue_work_on() in a
for_each_online_cpu() loop, which requires the cpu to stay online.
But cpus_read_lock() is not held in kvfree_rcu_barrier_on_cache() and the
set of "online cpus" is subject to change.

There are two paths that call flush_rcu_sheaves_on_cache():

  // has cpus_read_lock()
  flush_all_rcu_sheaves()
    -> flush_rcu_sheaves_on_cache()

  // no cpus_read_lock()
  kvfree_rcu_barrier_on_cache()
    -> flush_rcu_sheaves_on_cache()

Fix this by holding cpus_read_lock() in kvfree_rcu_barrier_on_cache().

Why not move cpus_read_lock() from flush_all_rcu_sheaves() into
flush_rcu_sheaves_on_cache()? The reason is it would introduce a new lock
order (slab_mutex -> cpu_hotplug_lock). The reverse order
(cpu_hotplug_lock -> slab_mutex) is established by

- cpuhp_setup_state_nocalls(..., slub_cpu_setup, ...)
- kmem_cache_destroy()

The two orders together would form an AB-BA deadlock.

Finally, add lockdep_assert_cpus_held() in flush_rcu_sheaves_on_cache()
to catch the same problem in the future.

Fixes: 0f35040de593 ("mm/slab: introduce kvfree_rcu_barrier_on_cache() for cache destruction")
Cc: <stable@vger.kernel.org>
Signed-off-by: Qing Wang <wangqing7171@gmail.com>
Link: https://patch.msgid.link/20260512035035.762317-1-wangqing7171@gmail.com
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

diff --git a/mm/slab_common.c b/mm/slab_common.c
index d5a70a831a2a..8b661fff5eed 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -2110,7 +2110,9 @@ EXPORT_SYMBOL_GPL(kvfree_rcu_barrier);
 void kvfree_rcu_barrier_on_cache(struct kmem_cache *s)
 {
 	if (cache_has_sheaves(s)) {
+		cpus_read_lock();
 		flush_rcu_sheaves_on_cache(s);
+		cpus_read_unlock();
 		rcu_barrier();
 	}
 
diff --git a/mm/slub.c b/mm/slub.c
index 0baa906f39ab..9ad80b7f601a 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -4024,6 +4024,7 @@ void flush_rcu_sheaves_on_cache(struct kmem_cache *s)
 	struct slub_flush_work *sfw;
 	unsigned int cpu;
 
+	lockdep_assert_cpus_held();
 	mutex_lock(&flush_lock);
 
 	for_each_online_cpu(cpu) {