[PATCH 6.19 290/781] mm/slab: fix false lockdep warning in __kfree_rcu_sheaf()

Greg Kroah-Hartman
2026-02-25T01:32:13Z

From: Harry Yoo <harry.yoo@oracle.com>

[ Upstream commit f8b4cd2dad097e4ea5aed3511f42b9eb771e7b19 ]

kvfree_call_rcu() can be called while holding a raw_spinlock_t.
Since __kfree_rcu_sheaf() may acquire a spinlock_t (which becomes a
sleeping lock on PREEMPT_RT) and violate lock nesting rules,
kvfree_call_rcu() bypasses the sheaves layer entirely on PREEMPT_RT.

However, lockdep still complains about acquiring spinlock_t while holding
raw_spinlock_t, even on !PREEMPT_RT where spinlock_t is a spinning lock.
This causes a false lockdep warning [1]:

=============================
[ BUG: Invalid wait context ]
6.19.0-rc6-next-20260120 #21508 Not tainted
-----------------------------
migration/1/23 is trying to lock:
ffff8afd01054e98 (&barn->lock){..-.}-{3:3}, at: barn_get_empty_sheaf+0x1d/0xb0
other info that might help us debug this:
context-{5:5}
3 locks held by migration/1/23:
#0: ffff8afd01fd89a8 (&p->pi_lock){-.-.}-{2:2}, at: __balance_push_cpu_stop+0x3f/0x200
#1: ffffffff9f15c5c8 (rcu_read_lock){....}-{1:3}, at: cpuset_cpus_allowed_fallback+0x27/0x250
#2: ffff8afd1f470be0 ((local_lock_t *)&pcs->lock){+.+.}-{3:3}, at: __kfree_rcu_sheaf+0x52/0x3d0
stack backtrace:
CPU: 1 UID: 0 PID: 23 Comm: migration/1 Not tainted 6.19.0-rc6-next-20260120 #21508 PREEMPTLAZY
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
Stopper: __balance_push_cpu_stop+0x0/0x200 <- balance_push+0x118/0x170
Call Trace:
<TASK>
__dump_stack+0x22/0x30
dump_stack_lvl+0x60/0x80
dump_stack+0x19/0x24
__lock_acquire+0xd3a/0x28e0
? __lock_acquire+0x5a9/0x28e0
? __lock_acquire+0x5a9/0x28e0
? barn_get_empty_sheaf+0x1d/0xb0
lock_acquire+0xc3/0x270
? barn_get_empty_sheaf+0x1d/0xb0
? __kfree_rcu_sheaf+0x52/0x3d0
_raw_spin_lock_irqsave+0x47/0x70
? barn_get_empty_sheaf+0x1d/0xb0
barn_get_empty_sheaf+0x1d/0xb0
? __kfree_rcu_sheaf+0x52/0x3d0
__kfree_rcu_sheaf+0x19f/0x3d0
kvfree_call_rcu+0xaf/0x390
set_cpus_allowed_force+0xc8/0xf0
[...]
</TASK>

This wasn't triggered until sheaves were enabled for all slab caches,
since kfree_rcu() wasn't being called with a raw spinlock held for
caches with sheaves (vma, maple node).

As suggested by Vlastimil Babka, fix this by using a lockdep map with
LD_WAIT_CONFIG wait type to tell lockdep that acquiring spinlock_t is valid
in this case, as those spinlocks won't be used on PREEMPT_RT.

Note that kfree_rcu_sheaf_map should be acquired using _try() variant,
otherwise the acquisition of the lockdep map itself will trigger an invalid
wait context warning.

Reported-by: Paul E. McKenney <paulmck@kernel.org>
Closes: https://lore.kernel.org/linux-mm/c858b9af-2510-448b-9ab3-058f7b80dd42@paulmck-laptop [1]
Fixes: ec66e0d59952 ("slab: add sheaf support for batching kfree_rcu() operations")
Suggested-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
mm/slub.c | 20 ++++++++++++++++++++
1 file changed, 20 insertions(+)

diff --git a/mm/slub.c b/mm/slub.c
index cdc1e652ec52f..e1583757331e7 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -6265,11 +6265,29 @@ static void rcu_free_sheaf(struct rcu_head *head)
free_empty_sheaf(s, sheaf);
}

+/*
+ * kvfree_call_rcu() can be called while holding a raw_spinlock_t. Since
+ * __kfree_rcu_sheaf() may acquire a spinlock_t (sleeping lock on PREEMPT_RT),
+ * this would violate lock nesting rules. Therefore, kvfree_call_rcu() avoids
+ * this problem by bypassing the sheaves layer entirely on PREEMPT_RT.
+ *
+ * However, lockdep still complains that it is invalid to acquire spinlock_t
+ * while holding raw_spinlock_t, even on !PREEMPT_RT where spinlock_t is a
+ * spinning lock. Tell lockdep that acquiring spinlock_t is valid here
+ * by temporarily raising the wait-type to LD_WAIT_CONFIG.
+ */
+static DEFINE_WAIT_OVERRIDE_MAP(kfree_rcu_sheaf_map, LD_WAIT_CONFIG);
+
bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj)
{
struct slub_percpu_sheaves *pcs;
struct slab_sheaf *rcu_sheaf;

+ if (WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_RT)))
+ return false;
+
+ lock_map_acquire_try(&kfree_rcu_sheaf_map);
+
if (!local_trylock(&s->cpu_sheaves->lock))
goto fail;

@@ -6346,10 +6364,12 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj)
local_unlock(&s->cpu_sheaves->lock);

stat(s, FREE_RCU_SHEAF);
+ lock_map_release(&kfree_rcu_sheaf_map);
return true;

fail:
stat(s, FREE_RCU_SHEAF_FAIL);
+ lock_map_release(&kfree_rcu_sheaf_map);
return false;
}

--
2.51.0