Messages in this thread Patch in this message | | | Date | Fri, 21 Dec 2012 20:40:46 -0800 | Subject | Re: [RFC PATCH 1/3] x86,smp: move waiting on contended lock out of line | From | Michel Lespinasse <> |
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On Fri, Dec 21, 2012 at 3:50 PM, Rik van Riel <riel@redhat.com> wrote: > diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h > index 33692ea..2a45eb0 100644 > --- a/arch/x86/include/asm/spinlock.h > +++ b/arch/x86/include/asm/spinlock.h > @@ -34,6 +34,8 @@ > # define UNLOCK_LOCK_PREFIX > #endif > > +extern void ticket_spin_lock_wait(arch_spinlock_t *, struct __raw_tickets); > + > /* > * Ticket locks are conceptually two parts, one indicating the current head of > * the queue, and the other indicating the current tail. The lock is acquired > @@ -53,12 +55,11 @@ static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock) > > inc = xadd(&lock->tickets, inc); > > - for (;;) { > - if (inc.head == inc.tail) > - break; > - cpu_relax(); > - inc.head = ACCESS_ONCE(lock->tickets.head); > - } > + if (inc.head == inc.tail) > + goto out; > + > + ticket_spin_lock_wait(lock, inc); > + out:
why not just:
if (inc.head != inc.tail) ticket_spin_lock_wait(lock, inc)
> barrier(); /* make sure nothing creeps before the lock is taken */ > } > > diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c > index 48d2b7d..20da354 100644 > --- a/arch/x86/kernel/smp.c > +++ b/arch/x86/kernel/smp.c > @@ -113,6 +113,20 @@ static atomic_t stopping_cpu = ATOMIC_INIT(-1); > static bool smp_no_nmi_ipi = false; > > /* > + * Wait on a congested ticket spinlock. > + */ > +void ticket_spin_lock_wait(arch_spinlock_t *lock, struct __raw_tickets inc) > +{ > + for (;;) { > + cpu_relax(); > + inc.head = ACCESS_ONCE(lock->tickets.head); > + > + if (inc.head == inc.tail) > + break; > + }
Why not just:
do { cpu_relax() inc.head = ... } while (inc.head != inc.tail);
Other than that, no problems with the principle of it. -- Michel "Walken" Lespinasse A program is never fully debugged until the last user dies.
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