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.
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.
Nigel Gamble nigel@nrg.org
Mountain View, CA, USA. http://www.nrg.org/
MontaVista Software nigel@mvista.com
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This archive was generated by hypermail 2b29 : Sun Apr 15 2001 - 21:00:14 EST