linux-kernel - Re: [RFC][PATCH v2 6/8] x86: don't reload after cmpxchg in unsafe_atomic

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Message-ID: <CAHk-=wga5Z9qk0Wa-Jpwb7x+4BG6C17cHfqX4KKqWm9jATpQUw@mail.gmail.com>
Date:   Thu, 26 Mar 2020 20:23:59 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH v2 6/8] x86: don't reload after cmpxchg in
 unsafe_atomic_op2() loop

On Thu, Mar 26, 2020 at 7:28 PM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> From: Al Viro <viro@...iv.linux.org.uk>
>
> lock cmpxchg leaves the current value in eax; no need to reload it.

I think this one is buggy.

Patch edited to remove the "-" lines, so that you see the end result:

>         int oldval = 0, ret, tem;                               \
>         asm volatile("1:\tmovl  %2, %0\n"                       \
> +                    "2:\tmovl\t%0, %3\n"                       \
>                      "\t" insn "\n"                             \
> +                    "3:\t" LOCK_PREFIX "cmpxchgl %3, %2\n"     \
> +                    "\tjnz\t2b\n"                              \
> +                    "4:\n"                                     \
>                      "\t.section .fixup,\"ax\"\n"               \
> +                    "5:\tmov\t%5, %1\n"                        \
>                      "\tjmp\t3b\n"                              \
>                      "\t.previous\n"                            \
> +                    _ASM_EXTABLE_UA(1b, 5b)                    \
> +                    _ASM_EXTABLE_UA(3b, 5b)                    \
>                      : "=&a" (oldval), "=&r" (ret),             \
>                        "+m" (*uaddr), "=&r" (tem)               \
>                      : "r" (oparg), "i" (-EFAULT), "1" (0));    \

I think that

                       "\tjmp\t3b\n"

line in the fixup section should be

                       "\tjmp\t4b\n"

because you don't want to jump to the cmpxchg instruction.

Maybe I'm misreading it.

               Linus