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

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Date:   Wed, 7 Oct 2020 16:29:47 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org,
        Will Deacon <will@...nel.org>,
        "Paul E . McKenney" <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@...ck.org
Subject: Re: [RFC PATCH 1/3] sched: fix exit_mm vs membarrier (v3)

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.

Something like so to match kthread_unuse_mm().

--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -486,11 +486,13 @@ static void exit_mm(void)
 	 * rq->membarrier_state.
 	 */
 	smp_mb__after_spinlock();
+	local_irq_disable()
 	current->mm = NULL;
 	membarrier_update_current_mm(NULL);
-	mmap_read_unlock(mm);
 	enter_lazy_tlb(mm, current);
+	local_irq_enable();
 	task_unlock(current);
+	mmap_read_unlock(mm);
 	mm_update_next_owner(mm);
 	mmput(mm);
 	if (test_thread_flag(TIF_MEMDIE))