Subject: [arm arch] local_irq_restore() does not play well with local_fiq_enable()

From: [email-masked] (Russell King - ARM Linux)
Date: Thu, 20 Mar 2014 21:09:41 +0000

On Thu, Mar 20, 2014 at 08:49:24PM +0000, Jonathan Bell wrote:
> On Thu, 20 Mar 2014 20:02:34 -0000, Russell King - ARM Linux  
> <[email-masked]> wrote:
>
>> On Thu, Mar 20, 2014 at 07:54:02PM +0000, Jonathan Bell wrote:
>>> hcd_init:
>>> 	setup the FIQ state
>>> 	install the fiq handler
>>> 	enable_fiq(USB)
>>> 	local_fiq_enable()
>>> 	usb_add_hcd()
>>
>> You're not supposed to use local_fiq_enable() to enable FIQs in a device
>> driver - they should already be enabled by this point.
>>
>> There has been a hole in that where FIQs haven't been enabled in the
>> idle thread, but that's a bug which needs fixing.  Otherwise, you should
>> assume that FIQs are always unmasked everywhere except for any short
>> code sequences that are contained within a short local_fiq_disable()..
>> local_fiq_enable() block.
>>
>
> Relying on FIQs being enabled elsewhere is in fact what we used to do  
> until ~3.10, whereupon some change (most likely the bug you describe)  
> broke this behaviour.
>
> Our use of local_fiq_disable() and local_fiq_enable() are indeed  
> constrained to minimally small critical sections (mostly reading/writing 
> a single hardware register or state variable) within the dwc_otg driver 
> that "handles" the results from the FIQ.

Realise that the state of the CPSR is per-process.  So if you're enabling
it in PID 1 when your driver initialises and it isn't enabled in PID 0,
then it will still remain not enabled in PID 0 - and that's a big problem
because that means whenever the system is idle, FIQs will be masked.

Now, what I'm reading in 3.14-rc7 is that:

- there is a local_fiq_enable() in arch_cpu_idle_prepare() which ensures
  that FIQs will be enabled for the idle loop for PID0.

- secondary CPUs have a local_fiq_enable() in secondary_start_kernel()
  which ensures that the have FIQs enabled too.

- when a kernel thread is created, the initial register set is created
  by copy_thread() which sets the CPSR to just 'SVC_MODE', thus clearing
  the IRQ and FIQ mask bits in any spawned thread.

So that all appears to be correct.

> But even if FIQs are enabled before entering the idle thread, any kernel  
> thread that sleeps (and yields the CPU) with an irqflags variable that 
> was saved before the call to local_fiq_enable has the potential to 
> corrupt the F bit when the thread subsequently wakes up.

No, because local_fiq_enable() only ever affects the thread on the CPU
which executed it.

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