linux-kernel - Re: [PATCH v1 2/2] epoll: Use __user_write_access_begin() and unsafe_put_user() in epoll_put

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Message-ID: <CAHk-=wiTN=1Sm5DVnXDtQ-tSiPuz-7QVpgP2vt6LgaEnhbubfw@mail.gmail.com>
Date: Tue, 28 Oct 2025 09:58:59 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Kuniyuki Iwashima <kuniyu@...gle.com>
Cc: David Laight <david.laight.linux@...il.com>, dave.hansen@...el.com, alex@...ti.fr, 
	aou@...s.berkeley.edu, axboe@...nel.dk, bp@...en8.de, brauner@...nel.org, 
	catalin.marinas@....com, christophe.leroy@...roup.eu, 
	dave.hansen@...ux.intel.com, edumazet@...gle.com, hpa@...or.com, 
	kuni1840@...il.com, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org, 
	linuxppc-dev@...ts.ozlabs.org, maddy@...ux.ibm.com, mingo@...hat.com, 
	mpe@...erman.id.au, npiggin@...il.com, palmer@...belt.com, pjw@...nel.org, 
	tglx@...utronix.de, will@...nel.org, x86@...nel.org
Subject: Re: [PATCH v1 2/2] epoll: Use __user_write_access_begin() and
 unsafe_put_user() in epoll_put_uevent().

On Tue, 28 Oct 2025 at 09:42, Kuniyuki Iwashima <kuniyu@...gle.com> wrote:
>
> This is the Zen 2 platform, so probably the stac/clac cost will be
> more expensive than you expect on Zen 5.

Yeah, clac/stac are horrenously expensive on Zen 2. I think they are
microcoded - and not the fast kind - so effectively serializing.

They got enormously faster and pipelined in later Zen microarchitectures.

Sadly, Agner Fog doesn't do timings on those instructions, and I
haven't found any other place that has some overview of the
performance on different microarchitectures.

                Linus