[Xen-devel] disabling irqs in a non-preemptive OS

[Matthew Fioravante <Matthew.Fioravante@xxxxxxxxxx>]

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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