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