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.