[PATCH] rtems: rtems_interrupt_enable/flash/disable()

[Sebastian Huber]

On 12/06/15 22:26, Pavel Pisa wrote:
> Hello Sebastian and others,
>
> On Friday 12 of June 2015 17:47:42 Sebastian Huber wrote:
>> ----- Gedare Bloom <gedare at gwu.edu> schrieb:
>>> I see. It provides the mutual exclusion for (SMP) applications that
>>> rely on interrupt_disable/enable locks?
>>>
>>> I guess we can never get rid of it as long as we allow for users to
>>> call isr_disable/enable?
>> Yes, an alternative is to remove the rtems_interrupt_disable/enable/flash()
>> on SMP configurations.  A pure interrupt disable is not really helpful.
> this is exactly same debate as has been solved in Linux kernel
> lists more years ago.

It seems that every problem was solved by Linux at some time.

> The Linux kernel community resolution was
> to forbid control global interrupts disable from drivers
> and all kernel code. Only option is to proceed with local
> (on singel CPU) disabel operations
>
>    local_irq_save(flags)
>    local_irq_disable()
>
>    local_irq_restore(flags);
>
> The original function names for global irq disable and enable have
> been kept on single CPU configurations for some transition period.
> Then they have been removed without replacement.
>
> Local IRQ disable is good mostly to build other synchronization
> primitives. At the level of drivers and higher level code
> they are not of much use, because there is no guarantee that code
> manipulating same data is not run on the other CPU.
> So at the end allmost all sites solely combine disable of interrupt
> together with spinlock
>
> macro to store flags
>    spin_lock_irqsave(lock, flags)
> function
>    spin_unlock_irqrestore(spinlock_t *lock, unsigned long flags)
>
> These lock do not deadlock even when called from interrupts,
> because any lock of this kind could be taken when IRQ
> is enabled.
>
> As for RTEMS, I would suggest to follow the same road.
> That is to define
>
>    rtems_local_interrupt_disable/enable/flash()
>
> and for compatibility define rtems_interrupt_disable/enable/flash()
> to use these on UP targets. Fail the build of applications directly
> manipulating by IRQ state on the SMP targets. There are not much SMP
> applications and targets yet runnuning on RTEMS. So the impact is not
> so wide for start and as the applications are transformed to use
> irqsave spinlocks when the are moved to SMP environment. If the global
> IRQ disable is supported then applications do not stop to use it
> and it would lead to need keep support of this feature for too long.
> Even if you decide that providing global IRQ disable is required
> for now it should be defined with deprecated attribute to be extremely
> noisy.

I am fine with this change to use 
rtems_local_interrupt_disable/enable/flash(). We should decide what to 
do before the release.

-- 
Sebastian Huber, embedded brains GmbH

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