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

[Vasily Gorbik <gor@linux.ibm.com>]

On Tue, Apr 14, 2026 at 12:24:12PM -0700, Paul E. McKenney wrote:
> On Thu, Apr 09, 2026 at 09:03:26PM -0700, Paul E. McKenney wrote:
> Please see below for the full patch, including refraining from queueing
> workqueue handlers on not-yet-online CPUs and diverting SRCU callbacks
> from not-yet-fully-online CPUs to the boot CPU's callback queue.
...
> commit ce533a60b2ef29a9b516cc717e77c6b679bc09c0
> Author: Paul E. McKenney <paulmck@kernel.org>
> Date:   Thu Apr 9 11:16:02 2026 -0700
> 
>     srcu: Don't queue workqueue handlers to never-online CPUs
>     
>     While an srcu_struct structure is in the midst of switching from CPU-0
>     to all-CPUs state, it can attempt to invoke callbacks for CPUs that
>     have never been online.  Worse yet, it can attempt in invoke callbacks
>     for CPUs that never will be online due to not being present in the
>     cpu_possible_mask.  This can cause hangs on s390, which is not set up to
>     deal with workqueue handlers being scheduled on such CPUs.  This commit
>     therefore causes Tree SRCU to refrain from queueing workqueue handlers
>     on CPUs that have not yet (and might never) come online.
>     
>     Because callbacks are not invoked on CPUs that have not been
>     online, it is an error to invoke call_srcu(), synchronize_srcu(), or
>     synchronize_srcu_expedited() on a CPU that is not yet fully online.
>     However, it turns out to be less code to redirect the callbacks
>     from too-early invocations of call_srcu() than to warn about such
>     invocations.  This commit therefore also redirects callbacks queued on
>     not-yet-fully-online CPUs to the boot CPU.
>     
>     Reported-by: Vasily Gorbik <gor@linux.ibm.com>
>     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>     Tested-by: Vasily Gorbik <gor@linux.ibm.com>
>     Cc: Tejun Heo <tj@kernel.org>

I retested it on s390 and on x86 KVM with --smp 16,maxcpus=255, all
looks good to me.

FWIW, again:

Tested-by: Vasily Gorbik <gor@linux.ibm.com>

Would you mind adding Cc: stable so it gets picked up for v7.0?
61bbcfb50514 ("srcu: Push srcu_node allocation to GP when
non-preemptible") is what made it reproducible for us.

Thank you!