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 15:13:38 +0200 [thread overview]
Message-ID: <20260422131338.GI3102924@noisy.programming.kicks-ass.net> (raw)
In-Reply-To: <20260422125647.GP3126523@noisy.programming.kicks-ass.net>
On Wed, Apr 22, 2026 at 02:56:47PM +0200, Peter Zijlstra wrote:
> 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.
Also: https://lore.kernel.org/all/874io5andc.ffs@tglx/
next prev parent reply other threads:[~2026-04-22 13:13 UTC|newest]
Thread overview: 59+ 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
2026-04-22 13:13 ` Peter Zijlstra [this message]
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
2026-04-27 7:06 ` Florian Weimer
2026-04-27 16:12 ` 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
2026-04-24 7:56 ` Dmitry Vyukov
2026-04-24 8:32 ` Mathias Stearn
2026-04-24 9:30 ` Dmitry Vyukov
2026-04-24 14:16 ` Thomas Gleixner
2026-04-24 15:03 ` Peter Zijlstra
2026-04-24 19:44 ` Thomas Gleixner
2026-04-26 22:04 ` Thomas Gleixner
2026-04-27 7:40 ` Florian Weimer
2026-04-27 11:03 ` Thomas Gleixner
2026-04-27 18:35 ` Mathieu Desnoyers
2026-04-27 21:06 ` Thomas Gleixner
2026-04-28 6:11 ` Dmitry Vyukov
2026-04-28 8:07 ` Thomas Gleixner
2026-04-28 8:18 ` Thomas Gleixner
2026-04-28 10:53 ` Dmitry Vyukov
2026-04-28 13:31 ` Mathias Stearn
2026-04-28 15:46 ` Thomas Gleixner
2026-04-28 7:39 ` Peter Zijlstra
2026-04-28 8:13 ` Peter Zijlstra
2026-04-28 8:51 ` Thomas Gleixner
2026-04-28 8:03 ` Peter Zijlstra
2026-04-28 8:36 ` 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
2026-04-24 16:45 ` [PATCH] arm64/entry: Fix arm64-specific rseq brokenness (was: Re: [REGRESSION] rseq: refactoring in v6.19 broke everyone on arm64) " Mark Rutland
2026-04-28 1:39 ` [PATCH] arm64/entry: Fix arm64-specific rseq brokenness Jinjie Ruan
2026-04-28 13:40 ` Mark Rutland
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