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SubjectRe: [RFC PATCH 1/3] sched: fix exit_mm vs membarrier (v3)
----- On Oct 7, 2020, at 10:29 AM, Peter Zijlstra peterz@infradead.org wrote:

> On Thu, Sep 24, 2020 at 01:25:06PM -0400, Mathieu Desnoyers wrote:
>> diff --git a/kernel/exit.c b/kernel/exit.c
>> index 733e80f334e7..0767a2dbf245 100644
>> --- a/kernel/exit.c
>> +++ b/kernel/exit.c
>> @@ -475,7 +475,19 @@ static void exit_mm(void)
>> BUG_ON(mm != current->active_mm);
>> /* more a memory barrier than a real lock */
>> task_lock(current);
>> + /*
>> + * When a thread stops operating on an address space, the loop
>> + * in membarrier_private_expedited() may not observe that
>> + * tsk->mm, and the loop in membarrier_global_expedited() may
>> + * not observe a MEMBARRIER_STATE_GLOBAL_EXPEDITED
>> + * rq->membarrier_state, so those would not issue an IPI.
>> + * Membarrier requires a memory barrier after accessing
>> + * user-space memory, before clearing tsk->mm or the
>> + * rq->membarrier_state.
>> + */
>> + smp_mb__after_spinlock();
>> current->mm = NULL;
>> + membarrier_update_current_mm(NULL);
>> mmap_read_unlock(mm);
>> enter_lazy_tlb(mm, current);
>> task_unlock(current);
>
> This site seems to be lacking in IRQ disabling. As proposed it will
> explode on RT.

Right, so irq off is needed for accessing this_rq()'s fields safely,
correct ?

I'll fold that fix in my patch for the next round, thanks!

Mathieu

>
> Something like so to match kthread_unuse_mm().
>
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -486,11 +486,13 @@ static void exit_mm(void)
> * rq->membarrier_state.
> */
> smp_mb__after_spinlock();
> + local_irq_disable()
> current->mm = NULL;
> membarrier_update_current_mm(NULL);
> - mmap_read_unlock(mm);
> enter_lazy_tlb(mm, current);
> + local_irq_enable();
> task_unlock(current);
> + mmap_read_unlock(mm);
> mm_update_next_owner(mm);
> mmput(mm);
> if (test_thread_flag(TIF_MEMDIE))

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

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