Xen devel - Dec 2009 - disabling irqs in a non-preemptive OS

I''ve been working on a tpm backend driver in mini-os and I''m
trying to
eliminate all possible race conditions.

My question is what exactly do you gain from disabling irqs with

local_irq_save()
local_irq_restore()

 From what I''ve gathered,
When you disable interupts it sets vcpu->evtchn_upcall_mask = 1, which I 
assume means mask all event channels.
Also, in sched.c, if anything tries to call schedule while irqs are 
disabled the OS will crash and print and error.

mini-os is a uniprocessor and non-preemptive OS so unless your thread 
blocks, an irq handler (in this case event channel handler) will never 
be run and thus won''t be able to touch your critical sections.

So really what do you get from disabling interrupts? Is it just a nice 
way of alerting you if something in your code block calls schedule()? Is 
it just a matter of good form and possibly securing your code in case 
someone adds preemption to mini os in the future? Or am I completely 
missing something here?


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On 31/12/2009 23:16, "Matthew Fioravante"
<Matthew.Fioravante@jhuapl.edu>
wrote:
> mini-os is a uniprocessor and non-preemptive OS so unless your thread
> blocks, an irq handler (in this case event channel handler) will never
> be run and thus won''t be able to touch your critical sections.
What makes you think that? ''Non-preemptive'' doesn''t
mean that you can''t be
interrupted by irq context. That''s why they''re called
interrupts!

 -- Keir



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That I didn''t realize. Now it makes sense, thanks!

Keir Fraser wrote:
> On 31/12/2009 23:16, "Matthew Fioravante"
<Matthew.Fioravante@jhuapl.edu>
> wrote:
>
>   
>> mini-os is a uniprocessor and non-preemptive OS so unless your thread
>> blocks, an irq handler (in this case event channel handler) will never
>> be run and thus won''t be able to touch your critical sections.
>>     
>
> What makes you think that? ''Non-preemptive''
doesn''t mean that you can''t be
> interrupted by irq context. That''s why they''re called
interrupts!
>
>  -- Keir
>
>
>   


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