* Daniel Walker ([EMAIL PROTECTED]) wrote: > On Tue, 2007-02-27 at 02:38 -0500, Mathieu Desnoyers wrote: > > > > > I am concerned about the automatic fallback to the PIT when no other > > clock source is available. A clocksource read would be atomic when TSC > > or HPET are available, but would fall back on PIT otherwise. There > > should be some way to specify that a caller is only interested in atomic > > clock sources (if none are available, the call should simply return an > > error, or 0). > > > I'm not sure what you mean by using the RCU
The original proposal of this thread uses a RCU (read-copy-update) style update of the previous 64 bits counter : it swaps a pointer (atomically) upon update by incrementing a word-sized counter that is used, by the reader, to get the offest in the array (with a modulo operation) for the current readable data and as a way to detect incorrect reads of overwritten information (we re-read the word-sized counter after having read the data structure to make sure is has not been incremented. If we detect an increment, we redo the whole operation). > > I still think that an RCU style update mechanism would be a good way to > > fix the current clocksource read issue. Another, slower and non NMI > > safe way to do this would be with a read seqlock and with IRQ disabling. > > , but the pit clocksource > does disable interrupts with a spin_lock_irqsave(). > When I say "clocksource read issue", I am talking about race between the function you proposed earlier, which you say is used in -rt kernels for latency tracing (get_monotonic_cycles), and HPET and TSC "last cycles" updates. Mathieu -- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/