On Sun, Aug 05, 2012 at 12:47:05AM +0200, Andrea Arcangeli wrote: > On Sat, Aug 04, 2012 at 03:02:45PM -0700, Paul E. McKenney wrote: > > OK, I'll bite. ;-) > > :)) > > > The most sane way for this to happen is with feedback-driven techniques > > involving profiling, similar to what is done for basic-block reordering > > or branch prediction. The idea is that you compile the kernel in an > > as-yet (and thankfully) mythical pointer-profiling mode, which records > > the values of pointer loads and also measures the pointer-load latency. > > If a situation is found where a given pointer almost always has the > > same value but has high load latency (for example, is almost always a > > high-latency cache miss), this fact is recorded and fed back into a > > subsequent kernel build. This subsequent kernel build might choose to > > speculate the value of the pointer concurrently with the pointer load. > > > > And of course, when interpreting the phrase "most sane way" at the > > beginning of the prior paragraph, it would probably be wise to keep > > in mind who wrote it. And that "most sane way" might have little or > > no resemblance to anything that typical kernel hackers would consider > > anywhere near sanity. ;-) > > I see. The above scenario is sure fair enough assumption. We're > clearly stretching the constraints to see what is theoretically > possible and this is a very clear explanation of how gcc could have an > hardcoded "guessed" address in the .text. > > Next step to clearify now, is how gcc can safely dereference such a > "guessed" address without the kernel knowing about it. > > If gcc would really dereference a guessed address coming from a > profiling run without kernel being aware of it, it would eventually > crash the kernel with an oops. gcc cannot know what another CPU will > do with the kernel pagetables. It'd be perfectly legitimate to > temporarily move the data at the "guessed address" to another page and > to update the pointer through stop_cpu during some weird "cpu > offlining scenario" or anything you can imagine. I mean gcc must > behave in all cases so it's not allowed to deference the guessed > address at any given time. > > The only way gcc could do the alpha thing and dereference the guessed > address before the real pointer, is with cooperation with the kernel. > The kernel should provide gcc "safe ranges" that won't crash the > kernel, and/or gcc could provide a .fixup section similar to the > current .fixup and the kernel should look it up during the page fault > handler in case the kernel is ok with temporarily getting faults in > that range. And in turn it can't happen unless we explicitly decide to > allow gcc to do it. And these are indeed some good reasons why I am not a fan of pointer-value speculation. ;-) > > > Furthermore the ACCESS_ONCE that Peter's patch added to gup_fast > > > pud/pgd can't prevent the compiler to read a guessed pmdp address as a > > > volatile variable, before reading the pmdp pointer and compare it with > > > the guessed address! So if it's 5 you worry about, when adding > > > ACCESS_ONCE in pudp/pgdp/pmdp is useless and won't fix it. You should > > > have added a barrier() instead. > > > > Most compiler writers I have discussed this with agreed that a volatile > > cast would suppress value speculation. The "volatile" keyword is not > > all that well specified in the C and C++ standards, but as "nix" said > > at http://lwn.net/Articles/509731/: > > > > volatile's meaning as 'minimize optimizations applied to things > > manipulating anything of volatile type, do not duplicate, elide, > > move, fold, spindle or mutilate' is of long standing. > > Ok, so if the above optimization would be possible, volatile would > stop it too, thanks for the quote and the explanation. > > On a side note I believe there's a few barrier()s that may be worth > converting to ACCESS_ONCE, that would take care of case 6) too in > addition to avoid clobbering more CPU registers than strictly > necessary. Not very important but a possible microoptimization. Agreed on both points. > > That said, value speculation as a compiler optimization makes me a bit > > nervous, so my current feeling is that is should be suppressed entirely. > > > > Hey, you asked, even if only implicitly! ;-) > > You're reading my mind! :) Or succesfully carrying out value speculation on it. ;-) Thanx, Paul -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>