Re: Possible KCSAN/lockdep issue in -next barns

[Harry Yoo <harry.yoo@oracle.com>]

On Wed, Jan 21, 2026 at 07:58:29AM -0800, Paul E. McKenney wrote:
> On Wed, Jan 21, 2026 at 04:17:24PM +0900, Harry Yoo wrote:
> > On Wed, Jan 21, 2026 at 04:06:25PM +0900, Harry Yoo wrote:
> > > On Tue, Jan 20, 2026 at 09:41:14PM -0800, Paul E. McKenney wrote:
> > > > Hello, Vlastimil,
> > > > 
> > > > On the off-chance that this is useful new news.  This happened in
> > > > rcutorture's TREE04 scenario running with KCSAN and lockdep enabled.
> > > > 
> > > > Thoughts?
> > > > 
> > > > 						Thanx, Paul
> > > > 
> > > > ------------------------------------------------------------------------
> > > > 
> > > > [  253.872153] =============================
> > > > [  253.872271] [ BUG: Invalid wait context ]
> > > > [  253.872271] 6.19.0-rc6-next-20260120 #21508 Not tainted
> > > > [  253.875774] -----------------------------
> > > > [  253.877983] migration/1/23 is trying to lock:
> > > > [  253.877983] ffff8afd01054e98 (&barn->lock){..-.}-{3:3}, at: barn_get_empty_sheaf+0x1d/0xb0
> > > > [  253.877983] other info that might help us debug this:
> > > > [  253.877983] context-{5:5}
> > > > [  253.877983] 3 locks held by migration/1/23:
> > > > [  253.877983]  #0: ffff8afd01fd89a8 (&p->pi_lock){-.-.}-{2:2}, at: __balance_push_cpu_stop+0x3f/0x200
> > > > [  253.877983]  #1: ffffffff9f15c5c8 (rcu_read_lock){....}-{1:3}, at: cpuset_cpus_allowed_fallback+0x27/0x250
> > > > [  253.877983]  #2: ffff8afd1f470be0 ((local_lock_t *)&pcs->lock){+.+.}-{3:3}, at: __kfree_rcu_sheaf+0x52/0x3d0
> > > > [  253.877983] stack backtrace:
> > > > [  253.877983] CPU: 1 UID: 0 PID: 23 Comm: migration/1 Not tainted 6.19.0-rc6-next-20260120 #21508 PREEMPTLAZY
> > > > [  253.877983] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
> > > > [  253.877983] Stopper: __balance_push_cpu_stop+0x0/0x200 <- balance_push+0x118/0x170
> > > > [  253.877983] Call Trace:
> > > > [  253.877983]  <TASK>
> > > > [  253.877983]  __dump_stack+0x22/0x30
> > > > [  253.877983]  dump_stack_lvl+0x60/0x80
> > > > [  253.877983]  dump_stack+0x19/0x24
> > > > [  253.877983]  __lock_acquire+0xd3a/0x28e0
> > > > [  253.877983]  ? __lock_acquire+0x5a9/0x28e0
> > > > [  253.877983]  ? __lock_acquire+0x5a9/0x28e0
> > > > [  253.877983]  ? barn_get_empty_sheaf+0x1d/0xb0
> > > > [  253.877983]  lock_acquire+0xc3/0x270
> > > > [  253.877983]  ? barn_get_empty_sheaf+0x1d/0xb0
> > > > [  253.877983]  ? __kfree_rcu_sheaf+0x52/0x3d0
> > > > [  253.877983]  _raw_spin_lock_irqsave+0x47/0x70
> > > > [  253.877983]  ? barn_get_empty_sheaf+0x1d/0xb0
> > > > [  253.877983]  barn_get_empty_sheaf+0x1d/0xb0
> > > > [  253.877983]  ? __kfree_rcu_sheaf+0x52/0x3d0
> > > > [  253.877983]  __kfree_rcu_sheaf+0x19f/0x3d0
> > > > [  253.877983]  kvfree_call_rcu+0xaf/0x390
> > > > [  253.877983]  set_cpus_allowed_force+0xc8/0xf0
> > > 
> > > set_cpus_allowed_force() is calling kfree_rcu() with a raw spinlock
> > > (pi_lock) held, but barn->lock is normal spinlock, lockdep prints an
> > > error because on PREEMPT_RT normal spinlocks are converted to
> > > sleeping locks but raw spinlocks are not.
> > > 
> > > Hmm... but it isn't great to convert barn->lock to
> > > a raw spinlock just because of this?
> > 
> > No, on PREEMPT_RT kfree_call_rcu() doesn't use sheaves.
> > Is it benign and it's simply due to PROVE_RAW_LOCK_NESTING?
> 
> Unless your kernel is built with CONFIG_PREEMPT_RT=y, in which case it
> is a real bug that could call schedule() with preemption disabled.

Right, but it's fine because this path is never called with
CONFIG_PREEMPT_RT=y. Yesterday sent a fix to suppress this warning.

Thanks for reporting it, Paul!

-- 
Cheers,
Harry / Hyeonggon