Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()

[Boqun Feng]

On Wed, Nov 11, 2015 at 05:39:40PM +0800, Boqun Feng wrote:
> Hi Oleg,
> 
> On Tue, Nov 03, 2015 at 06:59:58PM +0100, Oleg Nesterov wrote:
> [snip]
> > 
> > Unfortunately this doesn't look exactly right...
> > 
> > spin_unlock_wait() is not equal to "while (locked) relax", the latter
> > is live-lockable or at least sub-optimal: we do not really need to spin
> 
> Just be curious, should spin_unlock_wait() semantically be an ACQUIRE?

Hmm.. I guess I was wrong, it doesn't need to be an ACQUIRE, it needs
only to use the control dependency to order the load of lock state and
stores following it.

> Because spin_unlock_wait() is used for us to wait for a certain lock to
> RELEASE so that we can do something *after* we observe the RELEASE.
> Considering the follow example:
> 
> 	CPU 0 				CPU 1
> 	============================	===========================
> 	{ X = 0 }
> 					WRITE_ONCE(X, 1);
> 					spin_unlock(&lock);
> 	spin_unlock_wait(&lock)
> 	r1 = READ_ONCE(X);
> 
> If spin_unlock_wait() is not an ACQUIRE, r1 can be 0 in this case,
> right? Am I missing something subtle here? Or spin_unlock_wait() itself
> doesn't have the ACQUIRE semantics, but it should always come with a
> smp_mb() *following* it to achieve the ACQUIRE semantics? However in
> do_exit(), an smp_mb() is preceding raw_spin_unlock_wait() rather than
> following, which makes me confused... could you explain that? Thank you
> ;-)
> 

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? Because the control dependency
already orders load of tsk->pi_lock and store of tsk->state, and this
control dependency order guarantee pairs with the spin_unlock(->pi_lock)
in try_to_wake_up() to avoid data race on ->state.

Regards,
Boqun

> Regards,
> Boqun
> 
> > until we observe !spin_is_locked(), we only need to synchronize with the
> > current owner of this lock. Once it drops the lock we can proceed, we
> > do not care if another thread takes the same lock right after that.
> > 
> > Oleg.
> >

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