linux-kernel - Re: [PATCH] Fix preemptible lazy mode bug

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Date:	Wed, 05 Sep 2007 21:10:44 +0100
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	Zachary Amsden <zach@...are.com>,
	Linus Torvalds <torvalds@...l.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>,
	Chris Wright <chrisw@...s-sol.org>, stable@...nel.org,
	Virtualization Mailing List <virtualization@...ts.osdl.org>,
	Andi Kleen <ak@...e.de>,
	Anthony Liguori <anthony@...emonkey.ws>
Subject: Re: [PATCH] Fix preemptible lazy mode bug

Rusty Russell wrote:
> On Tue, 2007-09-04 at 14:42 +0100, Jeremy Fitzhardinge wrote:
>   
>> Rusty Russell wrote:
>>     
>>>  static inline void arch_flush_lazy_mmu_mode(void)
>>>  {
>>> -	PVOP_VCALL1(set_lazy_mode, PARAVIRT_LAZY_FLUSH);
>>> +	if (unlikely(__get_cpu_var(paravirt_lazy_mode) == PARAVIRT_LAZY_MMU))
>>> +		arch_leave_lazy_mmu_mode();
>>>  }
>>>   
>>>       
>> This changes the semantics a bit; previously "flush" would flush
>> anything pending but leave us in lazy mode.  This just drops lazymode
>> altogether?
>>
>> I guess if we assume that flushing is a rare event then its OK, but I
>> think the name's a bit misleading.  How does it differ from plain
>> arch_leave_lazy_mmu_mode()?
>>     
>
> Whether it's likely or unlikely to be in lazy mode, basically.  But
> you're right, this should be folded, since we don't want to "leave" lazy
> mode twice.
>   

Hm, I think there's still a problem here.  In the current code, you can
legitimately flush lazy mode with preemption enabled (ie, there's no
lazy mode currently active), but it's always a bug to enable/disable
lazy mode with preemption enabled.   Certainly enabling lazy mode with
preemption enabled is always a bug, but you could make disable
preempt-safe (and the bug checking should be in the common code).

    J
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