> On Tue, 10 Apr 2001, Paul McKenney wrote:
> > The algorithms we have been looking at need to have absolute guarantees
> > that earlier activity has completed. The most straightforward way to
> > guarantee this is to have the critical-section activity run with
preemption
> > disabled. Most of these code segments either take out locks or run
> > with interrupts disabled anyway, so there is little or no degradation
of
> > latency in this case. In fact, in many cases, latency would actually
be
> > improved due to removal of explicit locking primitives.
> >
> > I believe that one of the issues that pushes in this direction is the
> > discovery that "synchronize_kernel()" could not be a nop in a UP kernel
> > unless the read-side critical sections disable preemption (either in
> > the natural course of events, or artificially if need be). Andi or
> > Rusty can correct me if I missed something in the previous exchange...
> >
> > The read-side code segments are almost always quite short, and, again,
> > they would almost always otherwise need to be protected by a lock of
> > some sort, which would disable preemption in any event.
> >
> > Thoughts?
>
> Disabling preemption is a possible solution if the critical section is
short
> - less than 100us - otherwise preemption latencies become a problem.
Seems like a reasonable restriction. Of course, this same limit applies
to locks and interrupt disabling, right?
> The implementation of synchronize_kernel() that Rusty and I discussed
> earlier in this thread would work in other cases, such as module
> unloading, where there was a concern that it was not practical to have
> any sort of lock in the read-side code path and the write side was not
> time critical.
True, but only if the synchronize_kernel() implementation is applied to UP
kernels, also.
Thanx, Paul
> Nigel Gamble [EMAIL PROTECTED]
> Mountain View, CA, USA. http://www.nrg.org/
>
> MontaVista Software [EMAIL PROTECTED]
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