Re: [RFC PATCH 2/2] sched: membarrier: cover kthread_use_mm

[peterz]

On Tue, Jul 28, 2020 at 12:00:10PM -0400, Mathieu Desnoyers wrote:
> Add comments and memory barrier to kthread_use_mm and kthread_unuse_mm
> to allow the effect of membarrier(2) to apply to kthreads accessing
> user-space memory as well.
> 
> Given that no prior kthread use this guarantee and that it only affects
> kthreads, adding this guarantee does not affect user-space ABI.
> 
> Refine the check in membarrier_global_expedited to exclude runqueues
> running the idle thread rather than all kthreads from the IPI cpumask.
> 
> This patch applies on top of this patch from Peter Zijlstra:
> "mm: fix kthread_use_mm() vs TLB invalidate" currently in Andrew
> Morton's tree.
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> ---
>  kernel/kthread.c          | 19 +++++++++++++++++++
>  kernel/sched/membarrier.c |  8 ++------
>  2 files changed, 21 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index 48925b17920e..ef2435517f14 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -1258,8 +1258,19 @@ void kthread_use_mm(struct mm_struct *mm)
>  	finish_arch_post_lock_switch();
>  #endif
>  
> +	/*
> +	 * When a kthread starts operating on an address space, the loop
> +	 * in membarrier_{private,global}_expedited() may not observe
> +	 * that tsk->mm, and not issue an IPI. Membarrier requires a
> +	 * memory barrier after storing to tsk->mm, before accessing
> +	 * user-space memory. A full memory barrier for membarrier
> +	 * {PRIVATE,GLOBAL}_EXPEDITED is implicitly provided by
> +	 * mmdrop().
> +	 */
>  	if (active_mm != mm)
>  		mmdrop(active_mm);
> +	else
> +		smp_mb();
>  
>  	to_kthread(tsk)->oldfs = get_fs();
>  	set_fs(USER_DS);
> @@ -1280,6 +1291,14 @@ void kthread_unuse_mm(struct mm_struct *mm)
>  	set_fs(to_kthread(tsk)->oldfs);
>  
>  	task_lock(tsk);
> +	/*
> +	 * When a kthread stops operating on an address space, the loop
> +	 * in membarrier_{private,global}_expedited() may not observe
> +	 * that tsk->mm, and not issue an IPI. Membarrier requires a
> +	 * memory barrier after accessing user-space memory, before
> +	 * clearing tsk->mm.
> +	 */
> +	smp_mb();
>  	sync_mm_rss(mm);
>  	local_irq_disable();

Would it make sense to put the smp_mb() inside the IRQ disable region?

>  	tsk->mm = NULL;
> diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
> index 168479a7d61b..8a294483074d 100644
> --- a/kernel/sched/membarrier.c
> +++ b/kernel/sched/membarrier.c
> @@ -100,13 +100,9 @@ static int membarrier_global_expedited(void)
>  		    MEMBARRIER_STATE_GLOBAL_EXPEDITED))
>  			continue;
>  
> -		/*
> -		 * Skip the CPU if it runs a kernel thread. The scheduler
> -		 * leaves the prior task mm in place as an optimization when
> -		 * scheduling a kthread.
> -		 */
> +		/* Skip the CPU if it runs the idle thread. */
>  		p = rcu_dereference(cpu_rq(cpu)->curr);
> -		if (p->flags & PF_KTHREAD)
> +		if (is_idle_task(p))
>  			continue;

Do we want to add a:

	WARN_ON_ONCE(current->mm);

in play_idle_precise() ?

Because, if I read this right, we rely on the idle thread not having an
mm.