Re: [PATCH 04/33] mm: vmstat: Prepare to protect against concurrent isolated cpuset change

[David Lechner <dlechner@baylibre.com>]

On 1/1/26 4:13 PM, Frederic Weisbecker wrote:
> The HK_TYPE_DOMAIN housekeeping cpumask will soon be made modifiable at
> runtime. In order to synchronize against vmstat workqueue to make sure
> that no asynchronous vmstat work is pending or executing on a newly made
> isolated CPU, target and queue a vmstat work under the same RCU read
> side critical section.
> 
> Whenever housekeeping will update the HK_TYPE_DOMAIN cpumask, a vmstat
> workqueue flush will also be issued in a further change to make sure
> that no work remains pending after a CPU has been made isolated.
> 
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> ---
>  mm/vmstat.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 65de88cdf40e..ed19c0d42de6 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -2144,11 +2144,13 @@ static void vmstat_shepherd(struct work_struct *w)
>  		 * infrastructure ever noticing. Skip regular flushing from vmstat_shepherd
>  		 * for all isolated CPUs to avoid interference with the isolated workload.
>  		 */
> -		if (cpu_is_isolated(cpu))
> -			continue;
> +		scoped_guard(rcu) {
> +			if (cpu_is_isolated(cpu))
> +				continue;

I think this might have introduced a bug - or at least an unintentional change
in the program flow.

scoped_guard() is implemented using a for loop. Now this continue statement will
only exit the scoped_guard() scope rather than continuing the outer for loop. This
means that cond_resched() will be called when it previously was not.

>  
> -		if (!delayed_work_pending(dw) && need_update(cpu))
> -			queue_delayed_work_on(cpu, mm_percpu_wq, dw, 0);
> +			if (!delayed_work_pending(dw) && need_update(cpu))
> +				queue_delayed_work_on(cpu, mm_percpu_wq, dw, 0);
> +		}
>  
>  		cond_resched();
>  	}