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From: Peter Zijlstra <peterz@infradead.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Boqun Feng <boqun.feng@gmail.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Will Deacon <will@kernel.org>, paulmck <paulmck@kernel.org>,
	Nicholas Piggin <npiggin@gmail.com>,
	Andy Lutomirski <luto@amacapital.net>,
	Thomas Gleixner <tglx@linutronix.de>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Alan Stern <stern@rowland.harvard.edu>,
	linux-mm <linux-mm@kvack.org>
Subject: Re: [RFC PATCH 1/3] sched: fix exit_mm vs membarrier (v3)
Date: Wed, 7 Oct 2020 17:09:08 +0200	[thread overview]
Message-ID: <20201007150908.GI2628@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <1929468067.11124.1602082672412.JavaMail.zimbra@efficios.com>

On Wed, Oct 07, 2020 at 10:57:52AM -0400, Mathieu Desnoyers wrote:
> ----- 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 ?

Yes, but also we're having IRQs disabled on ever other site that mucks
with ->mm these days.


      reply	other threads:[~2020-10-07 15:09 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20200924172508.8724-1-mathieu.desnoyers@efficios.com>
2020-09-24 17:25 ` Mathieu Desnoyers
2020-10-07 14:29   ` Peter Zijlstra
2020-10-07 14:57     ` Mathieu Desnoyers
2020-10-07 15:09       ` Peter Zijlstra [this message]

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