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

[Shrikanth Hegde <sshegde@linux.ibm.com>]

Hi Paul.

On 4/30/26 9:40 PM, Paul E. McKenney wrote:
> 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@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
>>
>> 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
> 

Nit: I was suggesting *not* is probably not needed in that changelog.
I agree with explanation.

>>>       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>


Alright. With those two explanations, this LGTM.

Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>