linux-kernel - Re: [RFC PATCH 1/3] x86,smp: move waiting on contended lock out of line

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Date:	Fri, 21 Dec 2012 23:48:24 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Michel Lespinasse <walken@...gle.com>
CC:	linux-kernel@...r.kernel.org, aquini@...hat.com,
	lwoodman@...hat.com, jeremy@...p.org,
	Jan Beulich <JBeulich@...ell.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH 1/3] x86,smp: move waiting on contended lock out of
 line

On 12/21/2012 11:40 PM, Michel Lespinasse wrote:
> On Fri, Dec 21, 2012 at 3:50 PM, Rik van Riel <riel@...hat.com> wrote:

>> @@ -53,12 +55,11 @@ static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
>>
>>          inc = xadd(&lock->tickets, inc);

>> +       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)

That makes the code nicer, thank you. Applied.

>> +++ 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.

In patch #3 I do something else inside the head == tail
conditional block, so this one is best left alone.

Thank you for the comments.

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