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Date: Thu, 2 May 2024 09:37:17 -0700
From: Boqun Feng <boqun.feng@gmail.com>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Marco Elver <elver@google.com>,
	Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
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	Nathan Chancellor <nathan@kernel.org>,
	Arnd Bergmann <arnd@kernel.org>, Al Viro <viro@zeniv.linux.org.uk>,
	Jiri Slaby <jirislaby@kernel.org>
Subject: Re: [PATCH v3] tty: tty_io: remove hung_up_tty_fops
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On Wed, May 01, 2024 at 03:32:34PM -0700, Paul E. McKenney wrote:
> 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@linux-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.
> 

Yeah, I wonder the same, from what I read, "const volatile" seems to
be just a (non-volatile) relaxed atomic load.

Regards,
Boqun

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