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From: peterz@infradead.org
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
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 18:51:18 +0200	[thread overview]
Message-ID: <20200804165118.GN2657@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <709073430.39864.1596552521779.JavaMail.zimbra@efficios.com>

On Tue, Aug 04, 2020 at 10:48:41AM -0400, Mathieu Desnoyers wrote:
> Here is the scenario I have in mind:

> 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)
> 

Ah, yes of course.

We really should have a bunch of these scenarios in membarrier.c.



Now, the above cannot happen because we have an unconditional
atomic_dec_and_test() in do_exit() before exit_mm(), but I'm sure
relying on that is a wee bit dodgy.


  reply	other threads:[~2020-08-04 16:51 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
2020-08-04 16:51     ` peterz [this message]
2020-08-04 17:25       ` Mathieu Desnoyers

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