linux-kernel - Re: [RFC PATCH 1/3] sched: fix exit

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Date:   Wed, 7 Oct 2020 10:57:52 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Boqun Feng <boqun.feng@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Will Deacon <will@...nel.org>, paulmck <paulmck@...nel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alan Stern <stern@...land.harvard.edu>,
        linux-mm <linux-mm@...ck.org>
Subject: Re: [RFC PATCH 1/3] sched: fix exit_mm vs membarrier (v3)

----- On Oct 7, 2020, at 10:29 AM, Peter Zijlstra peterz@...radead.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