Messages in this thread | | | Subject | Re: [PATCH 3/3] ring-buffer: add design document | From | Huang Ying <> | Date | Thu, 11 Jun 2009 10:33:01 +0800 |
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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.
Best Regards, Huang Ying
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