On Tue, May 10, 2022 at 09:32:05PM -0700, John Hubbard wrote: > On 5/10/22 17:09, Minchan Kim wrote: > > On Tue, May 10, 2022 at 04:58:13PM -0700, John Hubbard wrote: > > > On 5/10/22 4:31 PM, Minchan Kim wrote: > > > > > > + int __mt = get_pageblock_migratetype(page); > > > > > > + int mt = __READ_ONCE(__mt); > > > > > > > > > > Although I saw the email discussion about this in v2, that discussion > > > > > didn't go far enough. It started with "don't use volatile", and went > > > > > on to "try __READ_ONCE() instead", but it should have continued on > > > > > to "you don't need this at all". > > > > > > > > That's really what I want to hear from experts so wanted to learn > > > > "Why". How could we prevent refetching of the mt if we don't use > > > > __READ_ONCE or volatile there? > > > > > > > > > > > > > > Because you don't. There is nothing you are racing with, and adding > > > > > __READ_ONCE() in order to avoid a completely not-going-to-happen > > > > > compiler re-invocation of a significant code block is just very wrong. > > > > > > > > > > So let's just let it go entirely. :) > > > > > > > > Yeah, once it's clear for everyone, I am happy to remove the > > > > unnecessary lines. > > > > > > > > > > > > > > > + > > > > > > + if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE) > > > > > > > > > > > With or without __READ_ONCE() or volatile or anything else, > > > this code will do what you want. Which is: loosely check > > > for either of the above. > > > > > > What functional problem do you think you are preventing > > > with __READ_ONCE()? Because I don't see one. > > > > I discussed the issue at v1 so please take a look. > > > > https://lore.kernel.org/all/YnFvmc+eMoXvLCWf@xxxxxxxxxx/ > > I read that, but there was never any real justification there for needing > to prevent a re-read of mt, just a preference: "I'd like to keep use the local > variable mt's value in folloing conditions checks instead of refetching > the value from get_pageblock_migratetype." > > But I don't believe that there is any combination of values of mt that > will cause a problem here. > > I also think that once we pull in experts, they will tell us that the > compiler is not going to re-run a non-trivial function to re-fetch a > value, but I'm not one of those experts, so that's still arguable. But > imagine what the kernel code would look like if every time we call > a large function, we have to consider if it actually gets called some > arbitrary number of times, due to (anti-) optimizations by the compiler. > This seems like something that is not really happening. Maybe, I might be paranoid since I have heard too subtle things about how compiler could changes high level language code so wanted be careful especially when we do lockless-stuff. Who cares when we change the large(?) function to small(?) function later on? I'd like to hear from experts to decide it.
