Hi Nick, On Mon, Sep 30, 2019 at 02:50:10PM -0700, Nick Desaulniers wrote: > On Mon, Sep 30, 2019 at 5:18 AM Will Deacon <will@xxxxxxxxxx> wrote: > > On Mon, Sep 30, 2019 at 09:05:11PM +0900, Masahiro Yamada wrote: > > > On Mon, Sep 30, 2019 at 8:26 PM Will Deacon <will@xxxxxxxxxx> wrote: > > > > FWIW, we've run into issues with CONFIG_OPTIMIZE_INLINING and local > > > > variables marked as 'register' where GCC would do crazy things and end > > > > up corrupting data, so I suspect the use of fixed registers in the arm > > > > uaccess functions is hitting something similar: > > > > > > > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91111 > > > > > > No. Not similar at all. > > > > They're similar in that enabling CONFIG_OPTIMIZE_INLINING causes register > > variables to go wrong. I agree that the ARM code looks dodgy with > > that call to uaccess_save_and_enable(), but there are __asmeq macros > > in there to try to catch that, so it's still very fishy. > > > > > I fixed it already. See > > > https://lore.kernel.org/patchwork/patch/1132459/ > > > > You fixed the specific case above for 32-bit ARM, but the arm64 case > > is due to a compiler bug. As it happens, we've reworked our atomics > > in 5.4 so that particular issue no longer triggers, but the fact remains > > that GCC has been shown to screw up explicit register allocation for > > perfectly legitimate code when giving the flexibility to move code out > > of line. > > So __attribute__((always_inline)) doesn't guarantee that code will be > inlined. For instance in LLVM's inliner, it asks/answers "should I > inline" and "can I inline." "Should" has to do with a cost model, and > is very heuristic-y. "Can" has more to do with the transforms, and > whether they're all implemented and safe. If you if you say > __attribute__((always_inline)), the answer to "can I inline this" can > still be *no*. The only way to guarantee inlining is via the C > preprocessor. The only way to prevent inlining is via > __attribute__((no_inline)). inline and __attribute__((always_inline)) > are a heuristic laden mess and should not be relied upon. I would > also look closely at code that *requires* inlining or the lack there > of to be correct. That the kernel no longer compiles at -O0 is not a > good thing IMO, and hurts developers that want a short > compile/execute/debug cycle. > > In this case, if there's a known codegen bug in a particular compiler > or certain versions of it, I recommend the use of either the C > preprocessor or __attribute__((no_inline)) to get the desired behavior > localized to the function in question, and for us to proceed with > Masahiro's cleanup. Hmm, I don't see how that would help. The problem occurs when things are moved out of line by the compiler (see below). > The comment above the use of CONFIG_OPTIMIZE_INLINING in > include/linux/compiler_types.h says: > * Force always-inline if the user requests it so via the .config. > Which makes me grimace (__attribute__((always_inline)) doesn't *force* > anything as per above), and the idea that forcing things marked inline > to also be __attribute__((always_inline)) is an "optimization" (re: > the name of the config; CONFIG_OPTIMIZE_INLINING) is also highly > suspect. Aggressive inlining leads to image size bloat, instruction > cache and register pressure; it is not exclusively an optimization. Agreed on all of this, but the fact remains that GCC has been shown to *miscompile* the arm64 kernel with CONFIG_OPTIMIZE_INLINING=y. Please, look at this thread: https://www.spinics.net/lists/arm-kernel/msg730329.html https://www.spinics.net/lists/arm-kernel/msg730512.html GCC decides to pull an atomic operation out-of-line and, in doing so, gets the register allocations subtly wrong when passing a 'register' variable into an inline asm. I would like to avoid this sort of thing happening, since it can result in really nasty bugs that manifest at runtime and are extremely difficult to debug, which is why I would much prefer not to have this option on by default for arm64. I sent a patch already: https://lkml.kernel.org/r/20190930114540.27498-1-will@xxxxxxxxxx and I'm happy to spin a v2 which depends on !CC_IS_CLANG as well. Reducing the instruction cache footprint is great, but not if the resulting code is broken! Will
