linux-kernel - Re: [PATCH 00/13] [PATCH RFC] Paravirtualized ticketlocks

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Date:	Fri, 2 Sep 2011 08:38:22 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...nel.dk>, Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	KVM <kvm@...r.kernel.org>, Andi Kleen <andi@...stfloor.org>,
	Xen Devel <xen-devel@...ts.xensource.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: Re: [PATCH 00/13] [PATCH RFC] Paravirtualized ticketlocks

On Thu, Sep 1, 2011 at 5:54 PM, Jeremy Fitzhardinge <jeremy@...p.org> wrote:
>
> The inner part of ticket lock code becomes:
>        inc = xadd(&lock->tickets, inc);
>        inc.tail &= ~TICKET_SLOWPATH_FLAG;
>
>        for (;;) {
>                unsigned count = SPIN_THRESHOLD;
>
>                do {
>                        if (inc.head == inc.tail)
>                                goto out;
>                        cpu_relax();
>                        inc.head = ACCESS_ONCE(lock->tickets.head);
>                } while (--count);
>                __ticket_lock_spinning(lock, inc.tail);
>        }

Hmm. It strikes me that I don't think you should touch the
TICKET_SLOWPATH_FLAG in the fastpath at all.

Can't you just do this:

   inc = xadd(&lock->tickets, inc);
   if (likely(inc.head == inc.tail))
     goto out;

   ### SLOWPATH ###
   inc.tail &= ~TICKET_SLOWPATH_FLAG;
   for (;;) {
      .. as before ..

which might alleviate the problem with the fastpath being polluted by
all those silly slowpath things.  Hmm?

(This assumes that TICKET_SLOWPATH_FLAG is never set in inc.head, so
if it's set that equality check will fail. I didn't actually check if
that assumption was correct)

                         Linus
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