From: Al Viro <viro@zeniv.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [RFC][PATCH 5/7] x86: convert arch_futex_atomic_op_inuser() to user_access_begin/user_access_end()
Date: Fri, 27 Mar 2020 02:42:27 +0000 [thread overview]
Message-ID: <20200327024227.GT23230@ZenIV.linux.org.uk> (raw)
In-Reply-To: <CAHk-=whnTRF5yA2MrPGcmMm=hXaGHfC2HEDtNzA=_1=szhJ4-w@mail.gmail.com>
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>.
next prev parent reply other threads:[~2020-03-27 2:42 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-23 18:50 [RFC][PATCHSET] futex uaccess cleanups Al Viro
2020-03-23 18:51 ` [RFC][PATCH 1/7] futex: arch_futex_atomic_op_inuser() calling conventions change Al Viro
2020-03-23 18:51 ` [RFC][PATCH 2/7] sh: no need of access_ok() in arch_futex_atomic_op_inuser() Al Viro
2020-03-23 18:51 ` [RFC][PATCH 3/7] [parisc, s390, sparc64] no need for access_ok() in futex handling Al Viro
2020-03-23 18:51 ` [RFC][PATCH 4/7] objtool: whitelist __sanitizer_cov_trace_switch() Al Viro
2020-03-23 18:51 ` [RFC][PATCH 5/7] x86: convert arch_futex_atomic_op_inuser() to user_access_begin/user_access_end() Al Viro
2020-03-23 19:06 ` Linus Torvalds
2020-03-24 2:08 ` Al Viro
2020-03-24 16:19 ` Linus Torvalds
2020-03-24 20:42 ` Al Viro
2020-03-24 20:57 ` Linus Torvalds
2020-03-27 2:42 ` Al Viro [this message]
2020-03-27 3:42 ` Linus Torvalds
2020-03-27 3:49 ` Linus Torvalds
2020-03-27 4:03 ` Linus Torvalds
2020-03-27 4:35 ` Josh Poimboeuf
2020-03-23 18:51 ` [RFC][PATCH 6/7] generic arch_futex_atomic_op_inuser() doesn't need access_ok() Al Viro
2020-03-23 18:51 ` [RFC][PATCH 7/7] x86: get rid of user_atomic_cmpxchg_inatomic() Al Viro
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