linux-kernel - Re: [PATCH] KVM: x86: Kick VCPU outside PIC lock again

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Date:	Wed, 24 Feb 2010 11:13:47 +0100
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Thomas Gleixner <tglx@...utronix.de>, KVM <kvm@...r.kernel.org>,
	Gleb Natapov <gleb@...hat.com>,
	RT <linux-rt-users@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86: Kick VCPU outside PIC lock again

Avi Kivity wrote:
> On 02/24/2010 11:54 AM, Jan Kiszka wrote:
>> Avi Kivity wrote:
>>    
>>> On 02/24/2010 11:41 AM, Jan Kiszka wrote:
>>>      
>>>> Thomas Gleixner wrote:
>>>>
>>>>        
>>>>> On Tue, 23 Feb 2010, Jan Kiszka wrote:
>>>>>
>>>>>
>>>>>          
>>>>>> Thomas Gleixner wrote:
>>>>>>
>>>>>>            
>>>>>>> The i8254/i8259 locks need to be real spinlocks on preempt-rt. Convert
>>>>>>> them to raw_spinlock. No change for !RT kernels.
>>>>>>>
>>>>>>>              
>>>>>> Doesn't fly for -rt anymore: pic_irq_update runs under this raw lock and
>>>>>> calls kvm_vcpu_kick which tries to wake_up some thread ->   scheduling
>>>>>> while atomic.
>>>>>>
>>>>>>            
>>>>> Hmm, a wakeup itself is fine. Is that code waking a wake queue ?
>>>>>
>>>>>          
>>>> Yes, it's a wake queue.
>>>>
>>>>        
>>> So what's the core issue?  Is the lock_t in the wait_queue a sleeping mutex?
>>>      
>> Yep.
>>    
> 
> I see.  Won't we hit the same issue when we call pic functions from 
> atomic context during the guest entry sequence?
> 

If there are such call paths, for sure. What concrete path(s) do you
have in mind?

Jan

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Corporate Competence Center Embedded Linux
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