linux-kernel - Re: [RFC PATCH 1/2] sched: Fix exit

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Date:   Tue, 4 Aug 2020 18:51:18 +0200
From:   peterz@...radead.org
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     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 1/2] sched: Fix exit_mm vs membarrier

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.