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mm/slab: fix false lockdep warning in __kfree_rcu_sheaf()

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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: ec66e0d599 ("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>
This commit is contained in:
Harry Yoo
2026-01-23 07:52:40 +01:00
committed by Vlastimil Babka
parent b55b423e85
commit f8b4cd2dad
1 changed files with 20 additions and 0 deletions
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20
mm/slub.c
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@@ -6265,11 +6265,29 @@ empty:
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 @@ do_free:
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;
}