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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Sun, 26 Nov 2023 08:51:35 -0800
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Subject: Re: lockless case of retain_dentry() (was Re: [PATCH 09/15] fold the
 call of retain_dentry() into fast_dput())
To: Guo Ren <guoren@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>, Peter Zijlstra <peterz@infradead.org>, 
	linux-fsdevel@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"

On Sun, 26 Nov 2023 at 08:39, Guo Ren <guoren@kernel.org> wrote:
>
> Here is my optimization advice:
>
> #define CMPXCHG_LOOP(CODE, SUCCESS) do {                                        \
>         int retry = 100;                                                        \
>         struct lockref old;                                                     \
>         BUILD_BUG_ON(sizeof(old) != 8);                                         \
> +       prefetchw(lockref);                                                     \\

No.

We're not adding software prefetches to generic code. Been there, done
that. They *never* improve performance on good hardware. They end up
helping on some random (usually particularly bad) microarchitecture,
and then they hurt everybody else.

And the real optimization advice is: "don't run on crap hardware".

It really is that simple. Good hardware does OoO and sees the future write.

> Micro-arch could give prefetchw more guarantee:

Well, in practice, they never do, and in fact they are often buggy and
cause problems because they weren't actually tested very much.

                 Linus