Re: [Xen-devel] [PATCH 00/10] [PATCH RFC V2] Paravirtualized ticketlocks

[Jeremy Fitzhardinge]

On 09/28/2011 09:10 AM, Linus Torvalds wrote:
> On Wed, Sep 28, 2011 at 8:55 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>> just use "lock xaddw" there too.
>> I'm afraid that's not possible, as that might carry from the low 8 bits
>> into the upper 8 ones, which must be avoided.
> Oh damn, you're right. So I guess the "right" way to do things is with
> cmpxchg, but some nasty mfence setup could do it too.

Could do something like:

	if (ticket->head >= 254)
		prev = xadd(&ticket->head_tail, 0xff02);
	else
		prev = xadd(&ticket->head_tail, 0x0002);

to compensate for the overflow.

    J
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