Buildresult: linus/um-defconfig/um-x86_64-gcc12 built on Mar 8 2023, 11:30

kisskb

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Status:	OK
Date/Time:	Mar 8 2023, 11:30
Duration:	0:00:45.385415
Builder:	ka7
Revision:	cpumask: be more careful with 'cpumask_setall()' (`63355b9884b3d1677de6bd1517cd2b8a9bf53978)`
Target:	linus/um-defconfig/um-x86_64-gcc12
Branch:	linus
Compiler:	um-x86_64-gcc12 (x86_64-linux-gcc.br_real (Buildroot 2021.11-4428-g6b6741b) 12.2.0 / GNU ld (GNU Binutils) 2.39)
Config:	defconfig (download)
Log:	Download original No warnings found in log. Full Log # git rev-parse -q --verify 63355b9884b3d1677de6bd1517cd2b8a9bf53978^{commit} 63355b9884b3d1677de6bd1517cd2b8a9bf53978 already have revision, skipping fetch # git checkout -q -f -B kisskb 63355b9884b3d1677de6bd1517cd2b8a9bf53978 # git clean -qxdf # < git log -1 # commit 63355b9884b3d1677de6bd1517cd2b8a9bf53978 # Author: Linus Torvalds <torvalds@linux-foundation.org> # Date: Tue Mar 7 12:16:18 2023 -0800 # # cpumask: be more careful with 'cpumask_setall()' # # Commit 596ff4a09b89 ("cpumask: re-introduce constant-sized cpumask # optimizations") changed cpumask_setall() to use "bitmap_set()" instead # of "bitmap_fill()", because bitmap_fill() would explicitly set all the # bits of a constant sized small bitmap, and that's exactly what we don't # want: we want to only set bits up to 'nr_cpu_ids', which is what # "bitmap_set()" does. # # However, Yury correctly points out that while "bitmap_set()" does indeed # only set bits up to the required bitmap size, it doesn't _clear_ bits # above that size, so the upper bits would still not have well-defined # values. # # Now, none of this should really matter, since any bits set past # 'nr_cpu_ids' should always be ignored in the first place. Yes, the bit # scanning functions might return them as a result, but since users should # always consider the ">= nr_cpu_ids" condition to mean "no more bits", # that shouldn't have any actual effect (see previous commit 8ca09d5fa354 # "cpumask: fix incorrect cpumask scanning result checks"). # # But let's just do it right, the way the code was _intended_ to work. We # have had enough lazy code that works but bites us in the *rse later # (again, see previous commit) that there's no reason to not just do this # properly. # # It turns out that "bitmap_fill()" gets this all right for the complex # case, and really only fails for the inlined optimized case that just # fills the whole word. And while we could just fix bitmap_fill() to use # the proper last word mask, there's two issues with that: # # - the cpumask case wants to do the _optimization_ based on "NR_CPUS is # a small constant", but then wants to do the actual bit _fill_ based # on "nr_cpu_ids" that isn't necessarily that same constant # # - we have lots of non-cpumask users of bitmap_fill(), and while they # hopefully don't care, and probably would want the proper semantics # anyway ("only set bits up to the limit"), I do not want the cpumask # changes to impact other parts # # So this ends up just doing the single-word optimization by hand in the # cpumask code. If our cpumask is fundamentally limited to a single word, # just do the proper "fill in that word" exactly. And if it's the more # complex multi-word case, then the generic bitmap_fill() will DTRT. # # This is all an example of how our bitmap function optimizations really # are somewhat broken. They conflate the "this is size of the bitmap" # optimizations with the actual bit(s) we want to set. # # In many cases we really want to have the two be separate things: # sometimes we base our optimizations on the size of the whole bitmap ("I # know this whole bitmap fits in a single word, so I'll just use # single-word accesses"), and sometimes we base them on the bit we are # looking at ("this is just acting on bits that are in the first word, so # I'll use single-word accesses"). # # Notice how the end result of the two optimizations are the same, but the # way we get to them are quite different. # # And all our cpumask optimization games are really about that fundamental # distinction, and we'd often really want to pass in both the "this is the # bit I'm working on" (which _can_ be a small constant but might be # variable), and "I know it's in this range even if it's variable" (based # on CONFIG_NR_CPUS). # # So this cpumask_setall() implementation just makes that explicit. It # checks the "I statically know the size is small" using the known static # size of the cpumask (which is what that 'small_cpumask_bits' is all # about), but then sets the actual bits using the exact number of cpus we # have (ie 'nr_cpumask_bits') # # Of course, in a perfect world, the compiler would have done all the # range analysis (possibly with help from us just telling it that # "this value is always in this range"), and would do all of this for us. # But that is not the world we live in. # # While we dream of that perfect world, this does that manual logic to # make it all work out. And this was a very long explanation for a small # code change that shouldn't even matter. # # Reported-by: Yury Norov <yury.norov@gmail.com> # Link: https://lore.kernel.org/lkml/ZAV9nGG9e1%2FrV+L%2F@yury-laptop/ # Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> # < /opt/cross/kisskb/x86-64--glibc--bleeding-edge-2022.08-1/bin/x86_64-linux-gcc --version # < /opt/cross/kisskb/x86-64--glibc--bleeding-edge-2022.08-1/bin/x86_64-linux-ld --version # < git log --format=%s --max-count=1 63355b9884b3d1677de6bd1517cd2b8a9bf53978 # < make -s -j 32 ARCH=um O=/kisskb/build/linus_um-defconfig_um-x86_64-gcc12 CROSS_COMPILE=/opt/cross/kisskb/x86-64--glibc--bleeding-edge-2022.08-1/bin/x86_64-linux- SUBARCH=x86_64 defconfig # < make -s -j 32 ARCH=um O=/kisskb/build/linus_um-defconfig_um-x86_64-gcc12 CROSS_COMPILE=/opt/cross/kisskb/x86-64--glibc--bleeding-edge-2022.08-1/bin/x86_64-linux- SUBARCH=x86_64 help # make -s -j 32 ARCH=um O=/kisskb/build/linus_um-defconfig_um-x86_64-gcc12 CROSS_COMPILE=/opt/cross/kisskb/x86-64--glibc--bleeding-edge-2022.08-1/bin/x86_64-linux- SUBARCH=x86_64 olddefconfig # make -s -j 32 ARCH=um O=/kisskb/build/linus_um-defconfig_um-x86_64-gcc12 CROSS_COMPILE=/opt/cross/kisskb/x86-64--glibc--bleeding-edge-2022.08-1/bin/x86_64-linux- SUBARCH=x86_64 LINK linux Completed OK # rm -rf /kisskb/build/linus_um-defconfig_um-x86_64-gcc12 # Build took: 0:00:45.385415

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