linux-kernel - Re: [PATCH 4/4] locking: Introduce smp_cond

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Date:	Wed, 11 Nov 2015 20:53:54 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Boqun Feng <boqun.feng@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
	linux-kernel@...r.kernel.org, paulmck@...ux.vnet.ibm.com,
	corbet@....net, mhocko@...nel.org, dhowells@...hat.com,
	torvalds@...ux-foundation.org, will.deacon@....com
Subject: Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()

He Boqun,

Let me first state that I can't answer authoritatively when it comes to
barriers. That said,

On 11/11, Boqun Feng wrote:
>
> But still, there is one suspicious use of smp_mb() in do_exit():
>
> 	/*
> 	 * The setting of TASK_RUNNING by try_to_wake_up() may be delayed
> 	 * when the following two conditions become true.
> 	 *   - There is race condition of mmap_sem (It is acquired by
> 	 *     exit_mm()), and
> 	 *   - SMI occurs before setting TASK_RUNINNG.
> 	 *     (or hypervisor of virtual machine switches to other guest)
> 	 *  As a result, we may become TASK_RUNNING after becoming TASK_DEAD
> 	 *
> 	 * To avoid it, we have to wait for releasing tsk->pi_lock which
> 	 * is held by try_to_wake_up()
> 	 */
> 	smp_mb();
> 	raw_spin_unlock_wait(&tsk->pi_lock);
>
> 	/* causes final put_task_struct in finish_task_switch(). */
> 	tsk->state = TASK_DEAD;
> 	tsk->flags |= PF_NOFREEZE;	/* tell freezer to ignore us */
> 	schedule();
>
> Seems like smp_mb() doesn't need here?

Please see my reply to peterz's email.

AFAICS, we need the barries on both sides. But, since we only need to
STORE into tsk->state after unlock_wait(), we can rely on the control
dependency and avoid the 2nd mb().

Oleg.

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