On Tue, 21 Nov 2006, Paul E. McKenney wrote:
> On Tue, Nov 21, 2006 at 12:56:21PM -0500, Alan Stern wrote:
> > Here's another potential problem with the fast path approach. It's not
> > very serious, but you might want to keep it in mind.
> >
> > The idea is that a reader can start up on one CPU and finish on another,
> > and a writer might see the finish event but not the start event. For
> > example:
...
> > This requires two context switches to take place while the cpu loop in
> > srcu_readers_active_idx() runs, so perhaps it isn't realistic. Is it
> > worth worrying about?
>
> Thank you -very- -much- for finding the basis behind my paranoia!
> I guess my intuition is still in good working order. ;-)
Are you sure _this_ was the basis behind your paranoia? Maybe it had
something else in mind... :-)
> It might be unlikely, but that makes it even worse -- a strange memory
> corruption problem that happens only under heavy load, and even then only
> sometimes. No thank you!!!
>
> I suspect that this affects Jens as well, though I don't claim to
> completely understand his usage.
>
> One approach to get around this would be for the the "idx" returned from
> srcu_read_lock() to keep track of the CPU as well as the index within
> the CPU. This would require atomic_inc()/atomic_dec() on the fast path,
> but would not add much to the overhead on x86 because the smp_mb() imposes
> an atomic operation anyway. There would be little cache thrashing in the
> case where there is no preemption -- but if the readers almost always sleep,
> and where it is common for the srcu_read_unlock() to run on a different CPU
> than the srcu_read_lock(), then the additional cache thrashing could add
> significant overhead.
>
> Thoughts?
I don't like the thought of extra overhead from cache thrashing. Also it
seems silly to allocate per-cpu data and then write to another CPU's
element.
How about making srcu_readers_active_idx() so fast that there isn't time
for 2 context switches? Disabling interrupts ought to be good enough
(except in virtualized environments perhaps).
Alan
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