linux-kernel - Re: [RFC][PATCH 5/7] x86: convert arch_futex_atomic_op_inuser() to user_access_begin/user_access

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Date:   Tue, 24 Mar 2020 09:19:15 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 5/7] x86: convert arch_futex_atomic_op_inuser() to user_access_begin/user_access_end()

On Mon, Mar 23, 2020 at 7:08 PM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> > And wouldn't it be lovely to get rid of the error return thing, and
> > pass in a label instead, the way "usafe_get/put_user()" works too?
> > That might be a separate patch from the "reorg" thing, though.
>
> OK, ret wouldn't be in the list of outputs that way and
> *uaddr could become an input (we only care about the address,
> same as for put_user), but oldval is a genuine output -

Yes, initially we'd have to do the "jump to label" inside the macro,
because gcc doesn't support asm goto with outputs.

But that's no different from "unsafe_get_user()". We still pass it a
label, even though we can't use it in the inline asm.

Yet.

I have patches to make it work with newer versions of clang, and I
hope that gcc will eventually also accept the semantics of "asm goto
with outputs only has the output on the fallthrough".

So _currently_ it would be only syntactic sugar: moving the error
handling inside the macro, and making it syntactically match
unsafe_get_user().

But long term is would allow us to generate better code too.

                Linus