linux-kernel - Re: [PATCH] documentation: Fix two-CPU control-dependency example

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Date:   Thu, 20 Jul 2017 14:14:34 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Akira Yokosawa <akiyks@...il.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] documentation: Fix two-CPU control-dependency example

On Wed, Jul 19, 2017 at 10:47:04PM -0700, Paul E. McKenney wrote:
[...]
> > Hi Paul,
> > 
> > I know the compiler could optimize atomics in very interesting ways, but
> > this case is about volatile, so I guess our case is still fine? ;-)
> 
> Hello, Boqun,
> 
> When I asked that question, the answer I got was "the compiler must
> emit the load instruction, but is under no obligation to actually use the
> value loaded".
> 
> I don't happen to like that answer, by the way.  ;-)
> 

Me neither, seems to me the kernel happens to work well at
compiler-optimization's mercy ;-/

With claim like that, compiler could do optimization as turning:

	struct task_struct *owner;

	for (;;) {
		owner = READ_ONCE(lock->owner);
		if (owner && !mutex_spin_on_owner(lock, owner))
			break;
		/* ... */

into:

	struct task_struct *owner;

	owner = READ_ONCE(lock->owner);

	for (;;READ_ONCE(lock->owner)) {
		if (owner && !mutex_spin_on_owner(lock, owner))
			break;
		/* ... */

Because the load executed in every loop, and they just happen to choose
not to use the values. And this is within their rights!

Regards,
Boqun

> 							Thanx, Paul
> 
> > > http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0062r1.html
> > > 
> > 
> > Great material to wake up mind in the morning! Thanks.
> > 
> > Regards,
> > Boqun
> > 
> > > What are your thoughts on this?
> > > 
> > > 							Thanx, Paul
> > > 
> > > >             Thanks, Akira
> > > > 
> > > > > That said, I very much welcome critical reviews of memory-barriers.txt,
> > > > > so please do feel free to continue doing that!
> > > > > 
> > > > > 							Thanx, Paul
> > > > > 
> > > > > 
> > > > 
> > > 
> > 
>