[Xenomai] arm & 64bit atomics

[Xenomai] arm & 64bit atomics

[Michael Haberler]

we found this: http://stackoverflow.com/questions/9857760/can-an-arm-interrupt-occur-in-mid-instruction and are considering which impact that has on machinekit, which uses doubles in memory shared between threads, relying on updates being atomic

I wonder if this topic has come up in the Xenomai context?

specifically - can it happen that an RT thread is rescheduled in the middle of a 64bit store?

also, I found that for older ARM's there are kernel mode helpers to support 64bit atomics (https://www.kernel.org/doc/Documentation/arm/kernel_user_helpers.txt) - I assume those work fine with Xenomai?

sorry about the fuzzy question - still trying to figure if/how this affects us

- Michael

Re: [Xenomai] arm & 64bit atomics

[Gilles Chanteperdrix]

On 07/03/2014 04:56 PM, Michael Haberler wrote:
> we found this: http://stackoverflow.com/questions/9857760/can-an-arm-interrupt-occur-in-mid-instruction and are considering which impact that has on machinekit, which uses doubles in memory shared between threads, relying on updates being atomic
> 
> I wonder if this topic has come up in the Xenomai context?
> 
> specifically - can it happen that an RT thread is rescheduled in the middle of a 64bit store?

Obviously, yes.

> 
> also, I found that for older ARM's there are kernel mode helpers to support 64bit atomics (https://www.kernel.org/doc/Documentation/arm/kernel_user_helpers.txt) - I assume those work fine with Xenomai?

Yes, the kuser helpers work, but they are more made for implementers of
a user-space run-time environment than for applications. Applications
should use the __sync_* primitives gcc proposes, and which will do the
right thing.

-- 
                                                                Gilles.