linux-kernel - Re: [PATCH] pi-futex: set PF_EXITING without taking ->pi

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Date:	Sat, 21 Jul 2007 20:39:47 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	Eric Dumazet <dada1@...mosbay.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ulrich Drepper <drepper@...hat.com>,
	linux-kernel@...r.kernel.org,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH] pi-futex: set PF_EXITING without taking ->pi_lock

On 07/21, Ingo Molnar wrote:
> 
> * Oleg Nesterov <oleg@...sign.ru> wrote:
> 
> > 	static inline void ccids_read_lock(void)
> > 	{
> > 		atomic_inc(&ccids_lockct);
> > 		spin_unlock_wait(&ccids_lock);
> > 	}
> > 
> > This looks racy, in theory atomic_inc() and spin_unlock_wait() could 
> > be re-ordered. However, in this particular case we have an "optimized" 
> > smp_mb_after_atomic_inc(), perhaps it is good that the caller can 
> > choose the "right" barrier by hand.
> 
> _all_ default locking and atomic APIs should be barrier-safe i believe. 
> (and that includes atomic_inc() too) Most people dont have barriers on 
> their mind when their code. _If_ someone is barrier-conscious then we 
> should have barrier-less APIs too for that purpose of squeezing the last 
> half cycle out of the code, but it should be a non-default choice. The 
> reason: nobody notices an unnecessary barrier, but a missing barrier can 
> be nasty.

Personally, I agree (but I am not sure the idea to make atomic_inc()
barrier-safe would be very popular).

Question: should we make spinlock_t barrier-safe?

Suppose that the task "p" does

	current->state = TASK_INTERRUPIBLE;
	mb();

	if (CONDITION)
		break;

	schedule();

and another CPU does

	CONDITION = 1;
	try_to_wake_up(p);


This is commonly used, but not correct _in theory_. If wake_up() happens
when p->array != NULL, we have

	CONDITION = 1;			// [1]
	spin_lock(rq->lock);
	task->state = TASK_RUNNING;	// [2]

and we can miss an event. Because in theory [1] may leak into the critical
section, and could be re-ordered with [2].

Another problem is that try_to_wake_up() first checks task->state and does
nothing if it is TASK_RUNNING, so we need a full mb(), not just wmb().

Should we change spin_lock(), or introduce smp_mb_before_spinlock(), or I
missed something?

NOTE: I do not pretend to know what kind of barrier spin_lock() provides
in practice, but according to the documentation lock() is only a one-way
barrier.

(I am glad I have an opportunity to raise this issue again :)

Oleg.

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