Re: [RFC PATCH 2/3] sched: membarrier: cover kthread_use_mm (v2)

[Boqun Feng]

On Fri, Aug 14, 2020 at 12:43:57PM -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.
> 
> 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>
> ---
> Changes since v1:
> - Add WARN_ON_ONCE(current->mm) in play_idle_precise (PeterZ),
> - Use smp_mb__after_spinlock rather than smp_mb after task_lock.
> ---
>  kernel/kthread.c          | 19 +++++++++++++++++++
>  kernel/sched/idle.c       |  1 +
>  kernel/sched/membarrier.c |  8 ++------
>  3 files changed, 22 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index 3edaa380dc7b..77aaaa7bc8d9 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -1255,8 +1255,19 @@ 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(), or explicitly with smp_mb().
> +	 */
>  	if (active_mm != mm)
>  		mmdrop(active_mm);
> +	else
> +		smp_mb();

Similar question here: could smp_mb() guarantee the correctness of
GLOBAL_EXPEDITED? Don't you need membarrier_switch_mm() here and in
kthread_unuse_mm(), too?

Am I miss something here?

Regards,
Boqun

>  
>  	to_kthread(tsk)->oldfs = force_uaccess_begin();
>  }
> @@ -1276,6 +1287,14 @@ void kthread_unuse_mm(struct mm_struct *mm)
>  	force_uaccess_end(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__after_spinlock();
>  	sync_mm_rss(mm);
>  	local_irq_disable();
>  	tsk->mm = NULL;
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index 6bf34986f45c..3443ee8335d0 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -341,6 +341,7 @@ void play_idle_precise(u64 duration_ns, u64 latency_ns)
>  	WARN_ON_ONCE(!(current->flags & PF_KTHREAD));
>  	WARN_ON_ONCE(!(current->flags & PF_NO_SETAFFINITY));
>  	WARN_ON_ONCE(!duration_ns);
> +	WARN_ON_ONCE(current->mm);
>  
>  	rcu_sleep_check();
>  	preempt_disable();
> 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;
>  
>  		__cpumask_set_cpu(cpu, tmpmask);
> -- 
> 2.11.0
>