linux-kernel - Re: [PATCH v3] tty: tty_io: remove hung_up_tty

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Message-ID: <b3b81374-a19d-4bf5-abb3-15e48c72f01a@paulmck-laptop>
Date: Wed, 1 May 2024 15:32:34 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Marco Elver <elver@...gle.com>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	syzbot <syzbot+b7c3ba8cdc2f6cf83c21@...kaller.appspotmail.com>,
	linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
	Nathan Chancellor <nathan@...nel.org>,
	Arnd Bergmann <arnd@...nel.org>, Al Viro <viro@...iv.linux.org.uk>,
	Jiri Slaby <jirislaby@...nel.org>
Subject: Re: [PATCH v3] tty: tty_io: remove hung_up_tty_fops

On Wed, May 01, 2024 at 02:49:17PM -0700, Paul E. McKenney wrote:
> On Wed, May 01, 2024 at 02:20:35PM -0700, Linus Torvalds wrote:
> > On Wed, 1 May 2024 at 14:06, Linus Torvalds
> > <torvalds@...ux-foundation.org> wrote:

[ . . . ]

> > I'd love to see an extension where "const volatile" basically means
> > exactly that: the volatile tells the compiler that it can't
> > rematerialize by doing the load multiple times, but the "const" would
> > say that if the compiler sees two or more accesses, it can still CSE
> > them.

Except that "const volatile" already means "you cannot write to it,
and reads will not be fused".  :-/

> No promises, other than that if we don't ask, they won't say "yes".
> 
> Let me see what can be done.

>From a semantics viewpoint __atomic_load_n(&x, __ATOMIC_RELAXED) would
work for loading from x.  The compilers that I tried currently do not
fuse loads, but they are allowed to do so.

Or is there something I am missing that would make this not work?
Aside from compilers not yet optimizing this case.

							Thanx, Paul