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SubjectRe: [RFC PATCH 2/3] sched: membarrier: cover kthread_use_mm (v2)
----- On Aug 16, 2020, at 11:29 AM, Boqun Feng boqun.feng@gmail.com 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@efficios.com>
>> Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Paul E. McKenney <paulmck@kernel.org>
>> Cc: Nicholas Piggin <npiggin@gmail.com>
>> Cc: Andy Lutomirski <luto@amacapital.net>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> ---
>> 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 ?

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

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