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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 1 May 2024 14:06:29 -0700
Message-ID: <CAHk-=whTakjVGgBC5OtoZ5Foo=hd4-g+NZ79nkMDVj6Ug7ARKQ@mail.gmail.com>
Subject: Re: [PATCH v3] tty: tty_io: remove hung_up_tty_fops
To: Marco Elver <elver@google.com>
Cc: paulmck@kernel.org, Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>, 
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Dmitry Vyukov <dvyukov@google.com>, 
	syzbot <syzbot+b7c3ba8cdc2f6cf83c21@syzkaller.appspotmail.com>, 
	linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com, 
	Nathan Chancellor <nathan@kernel.org>, Arnd Bergmann <arnd@kernel.org>, Al Viro <viro@zeniv.linux.org.uk>, 
	Jiri Slaby <jirislaby@kernel.org>
Content-Type: text/plain; charset="UTF-8"

On Wed, 1 May 2024 at 13:15, Marco Elver <elver@google.com> wrote:
>
> This is relatively trivial:
>
> #ifdef __SANITIZE_THREAD__
> #define __data_racy volatile
> #endif

I really wouldn't want to make a code generation difference, but I
guess when the sanitizer is on, the compiler generating crap code
isn't a huge deal.

> In some cases it might cause the compiler to complain if converting a
> volatile pointer to a non-volatile pointer

No. Note that it's not the *pointer* that is volatile, it's the
structure member.

So it would be something like

        const struct file_operations    * __data_racy f_op;

and only the load of f_op would be volatile - not the pointer itself.

Of course, if somebody then does "&file->f_op" to get a pointer to a
pointer, *that* would now be a volatile pointer, but I don't see
people doing that.

So I guess this might be a way forward. Anybody want to verify?

Now, the "hung_up_tty_fops" *do* need to be expanded to have hung up
ops for every op that is non-NULL in the normal tty ops. That was a
real bug. We'd also want to add a big comment to the tty fops to make
sure anybody who adds a new tty f_op member to make sure to populate
the hung up version too.

                Linus