On Fri, 10 Feb 2017, Stephen Hemminger wrote:
> Since sequence count algorithm is done by hypervisor, better to not reuse
> seqcount.
> Still concerned that the code is racy.
That's a different question and can only be answered by the hypervisor
folks. Dunno, whether they have barrier requirements. The seqcount stuff
relies on:
do {
seq = READ_ONCE(s->sequence);
smp_rmb();
sample_data();
smp_rmb();
} while (s->sequence != seq);
which pairs with the writer side:
s->sequence++;
smp_wmb();
update_data();
smp_wmb();
s->sequence++;
That's important if the stuff happens cross CPU. If the update happens on
the same CPU then this is a different story and as there are VMexits
involved they might provide the required ordering already. But I can't tell
as I have no idea how that host side thing is done.
Thanks,
tglx
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