Re: ARMv5 and atomic operations

[Andrew Haley <aph@xxxxxxxxxx>]

On 04/23/2012 06:36 PM, Thomas Meyer wrote:
> Am Sonntag, den 22.04.2012, 22:34 -0400 schrieb Chris Tyler:
>> On Sun, 2012-04-22 at 18:16 -0700, Brendan Conoboy wrote:
>>> As we get closer to having 100% package coverage in F17-ARM we're 
>>> running into harder build failures due to the limitations of the chips 
>>> we're building for.  The problem I've noticed on many of the recent 
>>> failures is due to the lack of atomic operations (These didn't arrive 
>>> until ARMv6).  
> 
>> 6. There is the kernel's "user space atomic helper" (kuser_cmpxchg64) at
>> 0xffff0f60, see Documentation/arm/kernel_user_helpers.txt. The kernel
>> puts an instruction sequence here tuned for the current arch that can be
>> called by userland to provide an atomic compare/exchange -- if it can be
>> done natively, the instruction sequence does that, otherwise it does a
>> syscall (with IRQ protection etc). Would this solve the problems you've
>> identified?
> 
> I'm running the Ubuntun 2.6.38 Tegra2 kernel (because of their fbdev
> support) on top of Fedora 17 armv5el on an Toshiba AC100 Laptop. The
> rsyslog package crashed everytime because of the missing kernel support
> of cmpxchg64. So when relying on the kernel helpers make sure that the
> resp. kernel support exists.

Indeed.  I had to write a workaround in IcedTea (i.e. java) on ARM for
just this reason.  If you can't depend on a kernel helper being there I
can't see it's of any use.

Andrew.
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