Messages in this thread | | | Date | Fri, 27 Mar 2020 02:42:27 +0000 | From | Al Viro <> | Subject | Re: [RFC][PATCH 5/7] x86: convert arch_futex_atomic_op_inuser() to user_access_begin/user_access_end() |
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On Tue, Mar 24, 2020 at 01:57:19PM -0700, Linus Torvalds wrote: > On Tue, Mar 24, 2020 at 1:45 PM Al Viro <viro@zeniv.linux.org.uk> wrote: > > > > OK... BTW, I'd been trying to recall the reasons for the way > > __futex_atomic_op2() loop is done; ISTR some discussion along > > the lines of cacheline ping-pong prevention, but I'd been unable > > to reconstruct enough details to find it and I'm not sure it > > hadn't been about some other code ;-/ > > No, that doesn't look like any cacheline advantage I can think of - > quite the reverse. > > I suspect it's just lazy code, with the reload being unnecessary. Or > it might be written that way because you avoid an extra variable. > > In fact, from a cacheline optimization standpoint, there are > advantages to not doing the load even on the initial run: if you know > a certain value is particularly likely, there are advantages to just > _assuming_ that value, rather than loading it. So no initial load at > all, and just initialize the first value to the likely case. > > That can avoid an unnecessary "load for shared ownership" cacheline > state transition (since the cmpxchg will want to turn it into an > exclusive modified cacheline anyway). > > But I don't think that optimization is likely the case here, and > you're right, the loop would be better written with the initial load > outside the loop.
OK, updated branch is in the same place; changes: __futex_atomic_op{1,2} turned into unsafe_atomic_op{1,2}, with "goto on error" folded into those. And pointless reload removed from cmpxchg loop in unsafe_atomic_op2(). Diffstat: arch/alpha/include/asm/futex.h | 5 +- arch/arc/include/asm/futex.h | 5 +- arch/arm/include/asm/futex.h | 5 +- arch/arm64/include/asm/futex.h | 5 +- arch/hexagon/include/asm/futex.h | 5 +- arch/ia64/include/asm/futex.h | 5 +- arch/microblaze/include/asm/futex.h | 5 +- arch/mips/include/asm/futex.h | 5 +- arch/nds32/include/asm/futex.h | 6 +-- arch/openrisc/include/asm/futex.h | 5 +- arch/parisc/include/asm/futex.h | 2 - arch/powerpc/include/asm/futex.h | 5 +- arch/riscv/include/asm/futex.h | 5 +- arch/s390/include/asm/futex.h | 2 - arch/sh/include/asm/futex.h | 4 -- arch/sparc/include/asm/futex_64.h | 4 -- arch/x86/include/asm/futex.h | 97 ++++++++++++++++++++++++------------- arch/x86/include/asm/uaccess.h | 93 ----------------------------------- arch/xtensa/include/asm/futex.h | 5 +- include/asm-generic/futex.h | 2 - kernel/futex.c | 5 +- tools/objtool/check.c | 1 + 22 files changed, 93 insertions(+), 183 deletions(-)
Sorry about the fuckup when sending that patchset ;-/ It ended up cc'd to x86 list instead of the futex one; Message-Id of the beginning of the thread is <20200327022836.881203-1-viro@ZenIV.linux.org.uk>.
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