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

[Paul E. McKenney]

On Thu, Apr 30, 2026 at 12:38:16PM +0530, Shrikanth Hegde wrote:
> Hi Paul.
> 
> On 4/29/26 11:31 PM, Paul E. McKenney wrote:

[ . . . ]

Sorry, missed one...

> > ------------------------------------------------------------------------
> > 
> > commit f8d5aaaf90f8294890802ce8dccbafd9850ac5f9
> > Author: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > 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
> 
> for CPUs that never will be online due to being present in the cpu_possible_mask?

Exactly.

Just because a CPU is in cpu_possible_mask doesn't mean that it will
ever actually come online.  For example, for single-threaded performance
reasons, a given system might choose to bring online only one CPU from
each hypertheaded core.  In that case, the other CPU in each hyperthreaded
core could be in the cpu_possible_mask, but would never come online.

							Thanx, Paul

> >      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@xxxxxxxxxxxxx>
> >      Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> >      Tested-by: Vasily Gorbik <gor@xxxxxxxxxxxxx>
> >      Cc: Tejun Heo <tj@xxxxxxxxxx>