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SubjectRe: [PATCH RFC 02/12] x86/ticketlock: convert spin loop to C
  On 08/02/2010 08:07 AM, Peter Zijlstra wrote:
> On Fri, 2010-07-16 at 18:03 -0700, Jeremy Fitzhardinge wrote:
>> + register union {
>> + struct __raw_tickets tickets;
>> + unsigned short slock;
>> + } inc = { .slock = 1<< TICKET_SHIFT };
> register arch_spinlock_t inc = { .tickets = { .head = 1, .tail = 0 } };
>
> > From a quick look you can basically replace all TICKET_SHIFT usage (1<<
> TICKET_SHIFT) with such a constant.

Mostly. In the later patch to convert trylock in to C, you need it to
construct an argument for cmpxchg (which can only take a scalar, even if
it does have a struct packed into it).

> [ Also, does gcc really listen to the register hint these days? ]

It doesn't make much different in this case. I think the only real
effect is that its illegal to take the address of a register variable.

>> + asm volatile (LOCK_PREFIX "xaddw %w0, %1\n"
>> + : "+Q" (inc), "+m" (lock->slock) : : "memory", "cc");
> "+Q" (inc->slock)
>
>> + for (;;) {
>> + if (inc.tickets.head == inc.tickets.tail)
>> + return;
>> + cpu_relax();
>> + inc.tickets.head = ACCESS_ONCE(lock->tickets.head);
>> + }
>> + barrier(); /* make sure nothing creeps before the lock is taken */
>> }
> How will it ever get to that barrier() ?

The compiler treats this as being:

for (;;) {
if (inc.tickets.head == inc.tickets.tail)
goto out;
...
}
out: barrier();
}
(Which would probably be a reasonable way to clarify the code.)

Without the barrier there's a risk of locked-region code being scheduled
before the for(;;) loop.

J


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