Re: Atomic accesses on ARM microcontrollers

[David Brown <david.brown@hesbynett.no>]

On 10/10/2020 22:18, Jonathan Wakely wrote:
> On Sat, 10 Oct 2020 at 20:43, David Brown <david.brown@hesbynett.no> wrote:
>> Is this strategy guaranteed to work in gcc, or is it a case of "it works
>> in a simple test, but might fail in a complicated program or with
>> different flags" ?
> 
> I think it works by design. My understanding is that users providing
> their own implementation of those calls is fully supported. I think
> that's partly why libatomic.so is a distinct library, and not just
> part of libgcc_s.so. The docs aren't entirely clear about this, they
> just say that if the compiler can't emit lock-free instructions for
> the atomic operation "a call is made to an external routine with the
> same parameters to be resolved at run time." But I think that "to be
> resolved at run time" means that you can choose how those calls will
> be resolved.
> 

Thank you for that - this sounds like the way to go for my code.

And I suppose I should file a bug report for the current libatomic that
comes with gcc.  It would be much better to have no support by default
than to have the current spinlocks - as it stands, on a microcontroller
like mine, it will all appear to work during development and most
testing.  But if in normal processing the code takes the spinlock and
then there is an interrupt whose handler tries to access the same atomic
(or at least one that shares the same lock), the interrupt handler will
be stalled forever.

David