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

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Date: Sat, 4 May 2024 12:25:12 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: paulmck@...nel.org
Cc: Boqun Feng <boqun.feng@...il.com>, 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 Sat, 4 May 2024 at 12:11, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> Anyway, the above are all normal optimizations that compilers
> *already* do, and the only real issue is that with memory ordering,
> the "dominance" thing will need to take into account the ordering
> requirements of other memory operations with stricter memory ordering
> in between. So, for example, if you have
>
>     __atomic_store_n(ptr,1, RELAXED);
>     __atomic_load_n(otherptr,2, ACQUIRE);
>     __atomic_store_n(ptr,2, RELAXED);
>
> then the second store doesn't dominate the first store, because
> there's a stricter memory ordering instruction in between.

Actually, that was a bad example, because in that pattern, the second
store does actually dominate (the second store can not move up beyond
the ACQUIRE, but the first store can indeed move down after it, so
dominance analysis does actually allow the second store to strictly
dominate the first one).

So the ACQUIRE would need to be SEQ for my example to be valid.

Of course, usually the barrier that stops domination is something
entirely different. Even without an actual conditional (which is
almost certainly the most common reason why dominance goes away), it
might be a function call (which could do any arbitrary memory ordering
operations - there was a clang bug in this very case) or something
like an explicit full memory barrier.

Anyway, take that email as a "written in the MUA on the fly". There
might be other thinkos in there. But I think the big picture was
right.

                   Linus