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

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Message-ID: <20200327035005.GU23230@ZenIV.linux.org.uk>
Date:   Fri, 27 Mar 2020 03:50:05 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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 08:23:59PM -0700, Linus Torvalds wrote:
> 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.

You are not - I'd missed that one while renumbering the labels.  Fixed and
pushed.