Re: [PATCH v2] introduce atomic_pointer to fix a race condition in cancelable mcs spinlocks

Linus Torvalds
2014-06-02T21:12:33Z

On Mon, Jun 2, 2014 at 2:02 PM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
>
> In the ->qlen case, interrupts are disabled and the current CPU is
> the only one who can write, so the read need not be volatile. In the
> ->n_barrier_done, modifications are done holding ->barrier_mutex, so again
> the read need not be volatile. In the sync_rcu_preempt_exp_count case,
> modifications are done holding sync_rcu_preempt_exp_mutex, so once again,
> the read need not be volatile. So I could do something like:
>
> ACCESS_ONCE(rdp->qlen) = rdp->qlen + 1;
>
> But that still makes gcc generate bad code
>
> The reason I was not all that worried about this is that these are not
> in fastpaths, and the last two are especially not in fastpaths.
>
> Suggestions?

So I think it probably *works*, but even so splitting it up to use
ACCESS_ONCE() on just the write is probably a better option, if only
because it would then make it much easier to change if we do end up
splitting reads and writes.

Because from a gcc code generation standpoint, using "volatile" will
always be horrible, because gcc will never be able to turn it into a
read-modify-write cycle. Arguable gcc _should_ be able to do that (it
is certainly allowable within the virtual machine definition), but I
understand why it doesn't ("volatile? Let's not optimize anything at
all, because it's special").

So "ACCESS_ONCE() + R-M-W" operation is actually pretty much
guaranteed to be "ACCESS_TWICE()", which may well be ok (performance
may not matter, and even when it does most architectures don't
actually have r-m-w instructions and when they do they aren't always
even faster), but I think it's just horribly horribly bad from a
conceptual and readability standpoint because it's so misleading.

So I'd actually rather see two explicit ACCESS_ONCE() calls - once to
read, once to write. Because that at least describes what is
happening, unlike the current situation.

Put another way: I can understand why you do it, and I can even agree
that it is "correct" from a functionality standpoint. But even despite
that all, I really don't like the construct very much..

Linus