Re: [Xen-devel] [PATCH v2 2/6] xen: sched: fix locking for insert_vcpu() in credit1 and RTDS

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On Wed, 2015-10-14 at 17:54 +0200, Dario Faggioli wrote:

So, about this:

> Note that it is safe to get rid of the locking in
> schedule_cpu_switch() as the pCPU being switched is, at
> the time of the switch, not a valid member of any cpupool,
> so no scheduling event should be expected on it, locking
> or not.
> 
I changed my mind about removing this locking.

In fact...
 
> @@ -1509,8 +1507,6 @@ int schedule_cpu_switch(unsigned int cpu,
> struct cpupool *c)
>          return -ENOMEM;
>      }
>  
> -    lock = pcpu_schedule_lock_irqsave(cpu, &flags);
> -
>      SCHED_OP(old_ops, tick_suspend, cpu);
>      vpriv_old = idle->sched_priv;
>      idle->sched_priv = vpriv;
> @@ -1520,8 +1516,6 @@ int schedule_cpu_switch(unsigned int cpu,
> struct cpupool *c)
>      SCHED_OP(new_ops, tick_resume, cpu);
>      SCHED_OP(new_ops, insert_vcpu, idle);
>  
> -    pcpu_schedule_unlock_irqrestore(lock, flags, cpu);
> -
>      SCHED_OP(old_ops, free_vdata, vpriv_old);
>      SCHED_OP(old_ops, free_pdata, ppriv_old, cpu);
>  
... while reworking and testing my other series (the one about Credit2
runqueues) I'm seeing some issues (even without the patches from that
series applied) that may be related to this lock not being taken any
longer.

Also (less strong an argument, but still), when splitting allocation and 
initialization of per-pCPU data (which is what that series does) I'd probably 
have to reinstate some similar locking again. :-/

I'll investigate more and respin this series.

Thanks and Regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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