Re: [PATCH] mm/slab: fix false lockdep warning in __kfree_rcu_sheaf()

[Vlastimil Babka <vbabka@suse.cz>]

On 1/21/26 14:16, Harry Yoo wrote:
> 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>

Thanks a lot! Will add to the sheaves-for-all series.