From: Peter Zijlstra <peterz@infradead.org>
To: Mathias Stearn <mathias@mongodb.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>, Boqun Feng <boqun.feng@gmail.com>,
"Paul E. McKenney" <paulmck@kernel.org>,
Chris Kennelly <ckennelly@google.com>,
Dmitry Vyukov <dvyukov@google.com>,
regressions@lists.linux.dev, linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org,
Ingo Molnar <mingo@kernel.org>,
Mark Rutland <mark.rutland@arm.com>,
Jinjie Ruan <ruanjinjie@huawei.com>,
Blake Oler <blake.oler@mongodb.com>
Subject: Re: [REGRESSION] rseq: refactoring in v6.19 broke everyone on arm64 and tcmalloc everywhere
Date: Wed, 22 Apr 2026 14:56:47 +0200 [thread overview]
Message-ID: <20260422125647.GP3126523@noisy.programming.kicks-ass.net> (raw)
In-Reply-To: <CAHnCjA25b+nO2n5CeifknSKHssJpPrjnf+dtr7UgzRw4Zgu=oA@mail.gmail.com>
On Wed, Apr 22, 2026 at 11:50:26AM +0200, Mathias Stearn wrote:
> Additionally, it breaks tcmalloc specifically by failing to overwrite
> the cpu_id_start field at points where it was relied on for
> correctness.
This specific behaviour was documented as being wrong and running with
DEBUG_RSEQ would have flagged it.
The tcmalloc issue has been contentious for a long time. The tcmalloc
folks relied on something that was documented to be wrong. It has been
reported to the tcmalloc people many years ago and if you were to run
tcmalloc on most any kernel (very much including 6.19) with
DEBUG_RSEQ=y, it would have yelled.
The tcmalloc people didn't care. There was a proposal for an RSEQ
extension for what they need, and they didn't care. All this should be
in their bugzilla or whatever.
The RSEQ rework improved performance significantly for everyone, and
kept all the documented behaviour (+- arm64 bug). Tcmalloc got screwed
over because they relied on implementation behaviour that was
specifically documented to be broken. And they didn't care. Google was
very much aware of this. And hasn't lifted a finger to remedy it.
next prev parent reply other threads:[~2026-04-22 12:56 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-04-22 9:50 [REGRESSION] rseq: refactoring in v6.19 broke everyone on arm64 and tcmalloc everywhere Mathias Stearn
2026-04-22 12:56 ` Peter Zijlstra [this message]
2026-04-22 13:13 ` Peter Zijlstra
2026-04-23 10:38 ` Mathias Stearn
[not found] ` <CAHnCjA2fa+dP1+yCYNQrTXQaW-JdtfMj7wMikwMeeCRg-3NhiA@mail.gmail.com>
2026-04-23 11:48 ` Thomas Gleixner
2026-04-23 12:11 ` Mathias Stearn
2026-04-23 17:19 ` Thomas Gleixner
2026-04-23 17:38 ` Chris Kennelly
2026-04-23 17:47 ` Mathieu Desnoyers
2026-04-23 19:39 ` Thomas Gleixner
2026-04-23 17:41 ` Linus Torvalds
2026-04-23 18:35 ` Mathias Stearn
2026-04-23 18:53 ` Mark Rutland
2026-04-23 21:03 ` Thomas Gleixner
2026-04-23 21:28 ` Linus Torvalds
2026-04-23 23:08 ` Linus Torvalds
2026-04-22 13:09 ` Mark Rutland
2026-04-22 17:49 ` Thomas Gleixner
2026-04-22 18:11 ` Mark Rutland
2026-04-22 19:47 ` Thomas Gleixner
2026-04-23 1:48 ` Jinjie Ruan
2026-04-23 5:53 ` Dmitry Vyukov
2026-04-23 10:39 ` Thomas Gleixner
2026-04-23 10:51 ` Mathias Stearn
2026-04-23 12:24 ` David Laight
2026-04-23 19:31 ` Thomas Gleixner
2026-04-23 12:11 ` Alejandro Colomar
2026-04-23 12:54 ` Mathieu Desnoyers
2026-04-23 12:29 ` Mathieu Desnoyers
2026-04-23 12:36 ` Dmitry Vyukov
2026-04-23 12:53 ` Mathieu Desnoyers
2026-04-23 12:58 ` Dmitry Vyukov
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20260422125647.GP3126523@noisy.programming.kicks-ass.net \
--to=peterz@infradead.org \
--cc=blake.oler@mongodb.com \
--cc=boqun.feng@gmail.com \
--cc=catalin.marinas@arm.com \
--cc=ckennelly@google.com \
--cc=dvyukov@google.com \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mark.rutland@arm.com \
--cc=mathias@mongodb.com \
--cc=mathieu.desnoyers@efficios.com \
--cc=mingo@kernel.org \
--cc=paulmck@kernel.org \
--cc=regressions@lists.linux.dev \
--cc=ruanjinjie@huawei.com \
--cc=tglx@linutronix.de \
--cc=will@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox