Re: port-arm/52666: ARM spinlocks are suboptimal

[Matt Thomas <matt%3am-software.com@localhost>]

> On Oct 28, 2017, at 11:40 AM, coypu%SDF.ORG@localhost wrote:
> 
>> Number:         52666
>> Category:       port-arm
>> Synopsis:       ARM spinlocks are suboptimal
>> Confidential:   no
>> Severity:       serious
>> Priority:       medium
>> Responsible:    port-arm-maintainer
>> State:          open
>> Class:          sw-bug
>> Submitter-Id:   net
>> Arrival-Date:   Sat Oct 28 18:40:00 +0000 2017
>> Originator:     coypu
>> Release:        NetBSD 8.99.5
>> Organization:
>> Environment:
> NetBSD 8.99.5
>> Description:
> newer ARM has an instruction 'wfe' essentially made for spinlocks. we should use it, but I don't see a relevant use of it, only an unused macro.
> 
> (In fact only x86 has a spinlock backoff function).
>> How-To-Repeat:
> 
>> Fix:
> 


Note that the sev 'send event' instruction needs to be used in parallel.  When I experimented earlier I found that wfe/sev would many spurious wakeups negating advantages.