Re: ARMv5 and atomic operations

[Andrew Haley <aph@xxxxxxxxxx>]

On 04/24/2012 05:18 PM, Jon Masters wrote:
> On 04/23/2012 04:31 PM, Nicolas Pitre wrote:
>> On Mon, 23 Apr 2012, Andrew Haley wrote:
>>
>>> On 04/23/2012 06:36 PM, Thomas Meyer wrote:
>>>> 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.
>>
>> Kernel helpers don't disappear with time.  You therefore can probe for 
>> their availability (see the documentation) in case the kernel support 
>> could be backported, or just refuse to run if the kernel version isn't 
>> recent enough.  This is not much different from relying on a new 
>> syscall.
> 
> I wasn't going to reply yet just due to lack of time and because Nico
> covered the kernel helper stuff so well in his earlier posts. But just
> to add, these kernel helpers have grown a little over time in higher
> kernel revisions, but there is a mechanism to discover the revision
> available (note: these are not VDSO but if it helps folks understand
> them, think of them like that). They are the right way to solve the
> atomics problem as best as we can for older devices. I believe the
> correct thing to do is to get broken upstream projects to adopt generic
> non-reimplemented-of-their-own routines that will transparently use the
> kernel helpers if needed.

But is that going to help me?  Let's say I'm on an ARMv5 and need to
do a longword operation.  I can either use a lightweight spinlock or
take the hit of a system call to do the cmpxchg8 operation.  Am I
really better off going into the kernel to do that?

Andrew.
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