Re: BUG: workqueue lockup - SRCU schedules work on not-online CPUs during size transition

[Paul E. McKenney]

On Thu, Apr 09, 2026 at 01:10:14PM -0700, Paul E. McKenney wrote:
> On Thu, Apr 09, 2026 at 09:15:50PM +0200, Vasily Gorbik wrote:

[ . . . ]

> > Yes, tested on s390 LPAR (76 online, 400 possible) as well as
> > on x86 KVM with --smp 16,maxcpus=255 and CONFIG_NR_CPUS=256
> > no more workqueue lockup in both cases.
> > 
> > Thank you!
> > 
> > Tested-by: Vasily Gorbik <gor@xxxxxxxxxxxxx>
> 
> Thank you for testing this!
> 
> Please see below for an updated patch.  Tejun's patch might obsolete
> this one, but just in case he balks at SRCU queueing handlers for CPUs
> that are not even in the cpu_possible_mask.  ;-)

And because we don't invoke SRCU callbacks on CPUs that are not yet fully
online, such CPUs had better not invoke call_srcu(), synchronize_srcu(),
or synchronize_srcu_expedited() on a CPU that is not yet fully online.
I am therefore adding the warning shown below.

Better paranoid late than paranoid not at all.  ;-)

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index a67af44fc0745..d62509efb52f5 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1431,6 +1431,7 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
 static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
 			rcu_callback_t func, bool do_norm)
 {
+	WARN_ON_ONCE(!rcu_cpu_beenfullyonline(raw_smp_processor_id()));
 	if (debug_rcu_head_queue(rhp)) {
 		/* Probable double call_srcu(), so leak the callback. */
 		WRITE_ONCE(rhp->func, srcu_leak_callback);