Messages in this thread |  | | | Date | Mon, 02 Aug 2010 08:17:53 -0700 | | From | Jeremy Fitzhardinge <> | | Subject | Re: [PATCH RFC 02/12] x86/ticketlock: convert spin loop to C |
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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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