[fedora-arm] ARMv5 and atomic operations

[Andrew Haley]

On 04/24/2012 05:40 PM, Nicolas Pitre wrote:
> On Tue, 24 Apr 2012, Andrew Haley wrote:
> 
>> On 04/24/2012 05:18 PM, Jon Masters wrote:
>>> 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?
> 
> Absolutely!  Because the kernel cmpxchg8 routine is _not_ a system call.  
> and it is going to be lighter than your custom solution with a lock.

Okay, I think I see your point: you're going to use ldm/stm in the
handler and detect an interrupt, then restart if necessary.  Right,
that may well be worth doing.  I'll still need the locking fallback
for old kernels, though.

Andrew.