Re: [PATCH net-next v2 2/2] ptr_ring: make __ptr_ring_empty() checking more reliable

On 2021/6/27 14:03, Michael S. Tsirkin wrote:

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So if now we need this to be reliable then
we also need smp_wmb before writing r->queue[consumer_head],
there could be other gotchas.

Yes, This patch does not make it strictly reliable.
T think I could mention that in the commit log?

OK so it's not that it makes it more reliable - this patch simply makes
a possible false positive less likely while making  a false negative
more likely. Our assumption is that a false negative is cheaper then?

How do we know that it is?

And even if we prove the ptr_ring itself is faster now,
how do we know what affects callers in a better way a
false positive or a false negative?

I would rather we worked on actually making it reliable
e.g. if we can guarantee no false positives, that would be
a net win.

I thought deeper about the case you mentioned above, it
seems for the above to happen, the consumer_head need to
be rolled back to zero and incremented to the point when
caller of __ptr_ring_empty() is still *not* able to see the
r->queue[] which has been set to NULL in __ptr_ring_discard_one().

It seems smp_wmb() only need to be done once when consumer_head
is rolled back to zero, and maybe that is enough to make sure the
case you mentioned is fixed too?

And the smp_wmb() is only done once in a round of producing/
consuming, so the performance impact should be minimized?(of
course we need to test it too).

Sorry I don't really understand the question here.
I think I agree it's enough to do one smp_wmb between
the write of r->queue and write of consumer_head
to help guarantee no false positives.
What other code changes are necessary I can't yet say
without more a deeper code review.

Ok, thanks for the reviewing.
Will add handling the case you mentioned above in V3 if there
is no noticable performanc impact for handling the above case.