Messages in this thread | | | Subject | Re: data dependency naming inconsistency | From | Akira Yokosawa <> | Date | Fri, 15 Oct 2021 07:48:09 +0900 |
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On Thu, 14 Oct 2021 23:29:43 +0900, Akira Yokosawa wrote: > [-CC akys: my 2nd address] > On Thu, 14 Oct 2021 01:37:17 -0400, Michael S. Tsirkin wrote: >> On Thu, Oct 14, 2021 at 01:43:24PM +0900, Akira Yokosawa wrote: >>> On Mon, 11 Oct 2021 07:07:08 -0400, Michael S. Tsirkin wrote: >>>> Hello Paul, all! >>> >>> Hello Michael, >>> >>> I thought Paul would respond soon, but looks like he has not >>> done so.
This is because Michael used Paul's old email address.
Forwarding to his current address.
Paul, you can see the thread at the lore archive: https://lore.kernel.org/lkml/20211011064233-mutt-send-email-mst@kernel.org/T/
Thanks, Akira
>>> So, I'm trying to give some hint to your findings. >>> >>>> I've been reading with interest Paul's posts about Rust interactions with LKMM >>>> https://paulmck.livejournal.com/63316.html >>>> and in particular it states: >>>> A data dependency involves a load whose return value directly or >>>> indirectly determine the value stored by a later store, which results in >>>> the load being ordered before the store. >>>> >>>> This matches the perf book: >>>> A data dependency occurs when the value returned by >>>> a load instruction is used to compute the data stored by >>>> a later store instruction. >>> >>> You might likely be aware, but these concern "data dependency", >>> not a _barrier_. >>> >>>> >>>> however, memory-barriers.txt states: >>>> >>>> A data dependency barrier is a partial ordering on interdependent loads >>>> only; it is not required to have any effect on stores, independent loads >>>> or overlapping loads. >>>> >>>> It also says: >>>> A data-dependency barrier is not required to order dependent writes >>>> because the CPUs that the Linux kernel supports don't do writes >>>> until they are certain (1) that the write will actually happen, (2) >>>> of the location of the write, and (3) of the value to be written. >>> >>> These concern the historic "data-dependency barrier", or >>> [smp_]read_barrier_depends(), which existed until Linux kernel v4.14. > > Ah... I should have said ", which existed prior to Linux kernel v4.15". > This invited off-by-one error below... > >>> >>>> >>>> so the result it the same: writes are ordered without a barrier, >>>> reads are ordered by a barrier. >>>> >>>> However, it would seem that a bit more consistency in naming won't >>>> hurt. >>> >>> So, I don't think the historic term of "data-dependency barrier" >>> can be changed. >>> >>> I guess the right approach would be to further de-emphasize >>> "data-dependency barrier"/"data dependency barrier" in >>> memory-barriers.txt. >>> >>> Rewrite by commit 8ca924aeb4f2 ("Documentation/barriers: Remove >>> references to [smp_]read_barrier_depends()") did some of such >>> changes, but it failed to update the introductory section of >>> "VARIETIES OF MEMORY BARRIER". >>> The part Michael quoted above belongs to it. >>> I don't think it has any merit keeping it around. >>> >>> Also, there remain a couple of ascii-art diagrams concerning >>> <data dependency barrier> in the first part of "EXAMPLES OF MEMORY >>> BARRIER SEQUENCES" section, which, I think, can be removed as well. >>> >>> Hope this helps clarify the circumstances. >> >> It does, thanks! It might be worth adding a sentence along the lines of >> >> "NB: a data dependency barrier is distinct from a data dependency: it's >> a barrier that used to be required in the presence of a data dependency. >> Since v4.14 Linux no longer offers an API for a data dependency barrier. > > Since v4.15 > >> Instead, using READ_ONCE is sufficient for ordering in the presence of a >> data dependency". > > > Maybe. > > But I'm more inclined to get rid of remaining contents related to the > "data dependency barrier". > > Thanks, Akira > >> >> >>> Paul, what is your take on the naming of "data dependency"/ >>> "data dependency barrier"? >>> >>> Thanks, Akira >>> >>>> >>>> Thanks, >>>> >>>> -- >>>> MST >>
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