>>> On Thu, Feb 21, 2008 at 11:36 AM, in message <200802211736.26719.ak@xxxxxxx>,
Andi Kleen <ak@xxxxxxx> wrote:
> On Thursday 21 February 2008 16:27:22 Gregory Haskins wrote:
>
>> @@ -660,12 +660,12 @@ rt_spin_lock_fastlock(struct rt_mutex *lock,
>> void fastcall (*slowfn)(struct rt_mutex *lock))
>> {
>> /* Temporary HACK! */
>> - if (!current->in_printk)
>> - might_sleep();
>> - else if (in_atomic() || irqs_disabled())
>> + if (unlikely(current->in_printk) && (in_atomic() || irqs_disabled()))
>
> I have my doubts that gcc will honor unlikelies that don't affect
> the complete condition of an if.
>
> Also conditions guarding returns are by default predicted unlikely
> anyways AFAIK.
>
> The patch is likely a nop.
>
Yeah, you are probably right. We have found that the system is *extremely* touchy on how much overhead we have in the lock-acquisition path. For instance, using a non-inline version of adaptive_wait() can cost 5-10% in disk-io throughput. So we were trying to find places to shave anywhere we could. That being said, I didn't record any difference from this patch, so you are probably exactly right. It just seemed like "the right thing to do" so I left it in.
-Greg
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