Re: [PATCH 3/3] ring-buffer: add design document

[Mathieu Desnoyers]

* Huang Ying (ying.huang@xxxxxxxxx) wrote:
> On Thu, 2009-06-11 at 09:58 +0800, Steven Rostedt wrote:
> > On Thu, 11 Jun 2009, Huang Ying wrote:
> > 
> > > On Thu, 2009-06-11 at 03:53 +0800, Steven Rostedt wrote:
> > > > +
> > > > +cmpxchg - hardware assisted atomic transaction that performs the following:
> > > > +
> > > > +   A = B iff previous A == C
> > > > +
> > > > +   R = cmpxchg(A, C, B) is saying that we replace A with B if and only if
> > > > +      current A is equal to C, and we put the old (current) A into R
> > > > +
> > > > +   R gets the previous A regardless if A is updated with B or not.
> > > > +
> > > > +   To see if the update was successful a compare of R == C may be used.
> > > 
> > > As far as I know, some architectures have no hardware assisted (NMI
> > > safe) cmpxchg. Is it OK to use cmpxchg in architecture-independent code?
> > 
> > I can fall back to the lock solution for those archs without cmpxchg. It 
> > is NMI safe, because we do spin_trylock() in NMI context. If we fail to 
> > acquire the lock in NMI context, we simply drop the packet.
> 
> Yes. For users do not care about packet drop, it is acceptable. But
> please select the implementation at run-time instead of build time.
> Because on some architecture such as ARM, whether CPU has cmpxchg
> support is determined at run-time.
> 
> > Are these archs without cmpxchg and NMIs, a concern for you?
> 
> ARM has no cmpxchg until ARM v6, but it has NMI like mechanism named
> FIQ.
> 

One could probably adapt the cmpxchg for earlier ARM so it disables
FIQs. Note that the current limitation is that there is only a
fiq disable, not a fiq save/restore.

Mathieu

> Best Regards,
> Huang Ying
> 
> 

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Mathieu Desnoyers
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