Re: [patch V2 0/4] sched/mmcid: Cure mode transition woes

[Peter Zijlstra]

On Mon, Feb 02, 2026 at 10:39:35AM +0100, Thomas Gleixner wrote:

> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -10445,6 +10445,12 @@ static bool mm_update_max_cids(struct mm
>  
>  	/* Flip the mode and set the transition flag to bridge the transfer */
>  	WRITE_ONCE(mc->mode, mc->mode ^ (MM_CID_TRANSIT | MM_CID_ONCPU));
> +	/*
> +	 * Order the store against the subsequent fixups so that
> +	 * acquire(rq::lock) cannot be reordered by the CPU before the
> +	 * store.
> +	 */
> +	smp_mb();
>  	return true;
>  }
>  
> @@ -10487,6 +10493,16 @@ static inline void mm_update_cpus_allowe
>  	irq_work_queue(&mc->irq_work);
>  }
>  
> +static inline void mm_cid_complete_transit(struct mm_struct *mm, unsigned int mode)
> +{
> +	/*
> +	 * Ensure that the store removing the TRANSIT bit cannot be
> +	 * reordered by the CPU before the fixups have been completed.
> +	 */
> +	smp_mb();
> +	WRITE_ONCE(mm->mm_cid.mode, mode);
> +}

I think this could've been smp_store_release(), but this is the slow
path so nobody cares and this is nicely symmetric.