Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <8735v3ex3h.ffs@nanos.tec.linutronix.de> <3C41339D-29A2-4AB1-958F-19DB0A92D8D7@amacapital.net>
 <CAHk-=wh0KoEZXPYMGkfkeVEerSCEF1AiCZSvz9TRrx=Kj74D+Q@mail.gmail.com>
 <CALCETrV9bCenqzzaW6Ra18tCvNP-my09decTjmLDVZZAQxR6VA@mail.gmail.com>
 <CAHk-=wgo6XEz3VQ9ntqzWLR3-hm1YXrXUz4_heDs4wcLe9NYvA@mail.gmail.com>
 <d26e3a82-8a2c-7354-d36b-cac945c208c7@kernel.dk> <CALCETrWmhquicE2C=G2Hmwfj4VNypXVxY-K3CWOkyMe9Edv88A@mail.gmail.com>
 <CAHk-=wgqK0qUskrzeWXmChErEm32UiOaUmynWdyrjAwNzkDKaw@mail.gmail.com>
 <8735v3jujv.ffs@nanos.tec.linutronix.de> <CAHk-=wi4Dyg_Z70J_hJbtFLPQDG+Zx3dP2jB5QrOdZC6W6j4Gw@mail.gmail.com>
 <12710fda-1732-ee55-9ac1-0df9882aa71b@samba.org> <CAHk-=wiR7c-UHh_3Rj-EU8=AbURKchnMFJWW7=5EH=qEUDT8wg@mail.gmail.com>
 <59ea3b5a-d7b3-b62e-cc83-1f32a83c4ac2@kernel.dk> <17471c9fec18765449ef3a5a4cddc23561b97f52.camel@trillion01.com>
 <CAHk-=whoJCocFsQ7+Sqq=dkuzHE+RXxvRdd4ZvyYqnsKBqsKAA@mail.gmail.com>
 <3df541c3-728c-c63d-eaeb-a4c382e01f0b@kernel.dk> <b360ed542526da0a510988ce30545f429a7da000.camel@trillion01.com>
 <4390e9fb839ebc0581083fc4fa7a82606432c0c0.camel@trillion01.com> <d9a12052074ba194fe0764c932603a5f85c5445a.camel@trillion01.com>
In-Reply-To: <d9a12052074ba194fe0764c932603a5f85c5445a.camel@trillion01.com>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 25 May 2021 10:23:39 -1000
Message-ID: <CAHk-=whZZ-g6qJ1Epox6c-_U-jSs+syA_7a8QKQkxZaudXXR_Q@mail.gmail.com>
Subject: Re: [PATCH] io_thread/x86: don't reset 'cs', 'ss', 'ds' and 'es'
 registers for io_threads
To:     Olivier Langlois <olivier@trillion01.com>
Cc:     Jens Axboe <axboe@kernel.dk>, Stefan Metzmacher <metze@samba.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        io-uring <io-uring@vger.kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Tue, May 25, 2021 at 9:40 AM Olivier Langlois <olivier@trillion01.com> wrote:
>
> If I look in the create_io_thread() function, I can see that CLONE_VM
> isn't set...

That would indeed be horribly buggy, but I'm not seeing it:

                .flags          = ((lower_32_bits(flags) | CLONE_VM |
                                    CLONE_UNTRACED) & ~CSIGNAL),

Yeah, it has that odd "first create 'flags' without the CLONE_VM, but
that is only used for that

        lower_32_bits(flags)

thing, and then we explicitly add CLONE_VM in there in the actual
kernel_clone_args.

It's because of how 'kernel_thread()' is written (that has historical
reasons), and the oddity is copied from there.

           Linus