From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: 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 1/2] sched: Fix exit_mm vs membarrier
Date: Tue, 4 Aug 2020 10:48:41 -0400 (EDT) [thread overview]
Message-ID: <709073430.39864.1596552521779.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <20200804143419.GL2657@hirez.programming.kicks-ass.net>
----- On Aug 4, 2020, at 10:34 AM, Peter Zijlstra peterz@infradead.org wrote:
> On Tue, Jul 28, 2020 at 12:00:09PM -0400, Mathieu Desnoyers wrote:
>> exit_mm should issue memory barriers after user-space memory accesses,
>> before clearing current->mm, to order user-space memory accesses
>> performed prior to exit_mm before clearing tsk->mm, which has the
>> effect of skipping the membarrier private expedited IPIs.
>>
>> The membarrier system call can be issued concurrently with do_exit
>> if we have thread groups created with CLONE_VM but not CLONE_THREAD.
>
> I'm still wonder what the exact failure case is though; exit_mm() is on
> the exit path (as the name very much implies) and the thread is about to
> die. The context switch that follows guarantees a full barrier before we
> run anything else again.
Here is the scenario I have in mind:
Two thread groups are created, A and B. Thread group B is created by
issuing clone from group A with flag CLONE_VM set, but not CLONE_THREAD.
Let's assume we have a single thread within each thread group (Thread A
and Thread B).
The AFAIU we can have:
Userspace variables:
int x = 0, y = 0;
CPU 0 CPU 1
Thread A Thread B
(in thread group A) (in thread group B)
x = 1
barrier()
y = 1
exit()
exit_mm()
current->mm = NULL;
r1 = load y
membarrier()
skips CPU 0 (no IPI) because its current mm is NULL
r2 = load x
BUG_ON(r1 == 1 && r2 == 0)
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
next prev parent reply other threads:[~2020-08-04 14:48 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-28 16:00 [RFC PATCH 1/2] sched: Fix exit_mm vs membarrier Mathieu Desnoyers
2020-08-04 14:34 ` peterz
2020-08-04 14:48 ` Mathieu Desnoyers [this message]
2020-08-04 16:51 ` peterz
2020-08-04 17:25 ` Mathieu Desnoyers
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