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

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Message-ID: <20200816152330.GA87259@debian-boqun.qqnc3lrjykvubdpftowmye0fmh.lx.internal.cloudapp.net>
Date:   Sun, 16 Aug 2020 23:23:30 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Will Deacon <will@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Nicholas Piggin <npiggin@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm@...ck.org
Subject: Re: [RFC PATCH 1/3] sched: fix exit_mm vs membarrier (v2)

Hi Mathieu,

On Fri, Aug 14, 2020 at 12:43:56PM -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.
> 
> 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)
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
> Cc: Will Deacon <will@...nel.org>
> Cc: Paul E. McKenney <paulmck@...nel.org>
> Cc: Nicholas Piggin <npiggin@...il.com>
> Cc: Andy Lutomirski <luto@...capital.net>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Alan Stern <stern@...land.harvard.edu>
> Cc: linux-mm@...ck.org
> ---
> Changes since v1:
> - Use smp_mb__after_spinlock rather than smp_mb.
> - Document race scenario in commit message.
> ---
>  kernel/exit.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 733e80f334e7..fe64e6e28dd5 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -475,6 +475,14 @@ 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,global}_expedited() may not observe

Is it accurate to say that the correctness of
membarrier_global_expedited() relies on the observation of ->mm? Because
IIUC membarrier_global_expedited() loop doesn't check ->mm.

Regards,
Boqun

> +	 * that tsk->mm, and not issue an IPI. Membarrier requires a
> +	 * memory barrier after accessing user-space memory, before
> +	 * clearing tsk->mm.
> +	 */
> +	smp_mb__after_spinlock();
>  	current->mm = NULL;
>  	mmap_read_unlock(mm);
>  	enter_lazy_tlb(mm, current);
> -- 
> 2.11.0
>