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

[Boqun Feng]

On Mon, Aug 24, 2020 at 11:27:49AM -0400, Mathieu Desnoyers wrote:
> ----- On Aug 16, 2020, at 11:29 AM, Boqun Feng boqun.feng@xxxxxxxxx wrote:
> 
> > 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?
> 
> I think you have a good point there. Which brings me to wonder why we
> don't have membarrier_switch_mm() when entering/leaving lazy TLB mode.
> This means an IPI can be sent to a kthread even if it does not use an
> mm, just because the membarrier state in the runqueue is that of the
> active_mm.
> 
> Thoughts ?
> 

Right, I think we should also handle the percpu membarrier_state. The
basic rule is whenever we change current->mm or current (i.e. rq->curr)
itself, we need to update the percpu membarrier_state accordingly.

Regards,
Boqun

> Thanks,
> 
> Mathieu
> 
> > 
> > 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
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com