Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive spinning

[Avi Kivity <avi@redhat.com>]

On 04/06/2010 06:28 PM, Darren Hart wrote:
> Alan Cox wrote:
>> On Tue, 06 Apr 2010 15:35:31 +0200
>> Peter Zijlstra <peterz@infradead.org> wrote:
>>
>>> On Tue, 2010-04-06 at 16:28 +0300, Avi Kivity wrote:
>>>> Yes, but that's the best case for spinning.  You could simply use a 
>>>> userspace spinlock in this case. 
>>> Userspace spinlocks are evil.. they should _never_ be used.
>>
>> Thats a gross and inaccurate simplification. For the case Avi is talking
>> about spinning in userspace makes sense in a lot of environments. Once
>> you've got one thread pinned per cpu (or gang scheduling >-) ) there are
>> various environments where it makes complete and utter sense.
>
> Hi Alan,
>
> Do you feel some of these situations would also benefit from some 
> kernel assistance to stop spinning when the owner schedules out? Or 
> are you saying that there are situations where pure userspace 
> spinlocks will always be the best option?
>
> If the latter, I'd think that they would also be situations where 
> sched_yield() is not used as part of the spin loop. If so, then these 
> are not our target situations for FUTEX_LOCK_ADAPTIVE, which hopes to 
> provide a better informed mechanism for making spin or sleep 
> decisions. If sleeping isn't part of the locking construct 
> implementation, then FUTEX_LOCK_ADAPTIVE doesn't have much to offer.

IMO the best solution is to spin in userspace while the lock holder is 
running, fall into the kernel when it is scheduled out.

-- 
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