From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4E67B3D1CAA; Wed, 22 Apr 2026 13:13:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.50.34 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776863628; cv=none; b=MP+NNTzEA2mqIkid/1flehMB+ZsvT/kkQ8a5Bdi9J8JrGQmxpQvJSsT7/pDFcoxUmHGIrAQCdSjGvW7S+BvqycnIAyzvkqDX0cFf5obpzRd3DJzgE32B+Q8185/cgpYQKcSKOmjYWjg8eoBOcqvSC0y6QB8GWdN9KiRFzCAr4gY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1776863628; c=relaxed/simple; bh=kxIVk+2lF1IFEhTBCo9akungjiYvqR+2SYK2EwxtkmU=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=cHLpgfYwjp1EhG12/xZ/MChUQJFnmk6pb8Ywn3sCIYTf7Iz5IuZsinqpDnW5CU0Tka2ORGpgQTnn/bl4W4Ucg8eOLRvZN0b4ecl6cnTe/8zS4QuBFDAupDfCQ+v7z2653MxqAmn/026EVEbQ+hto3HqEQG9s5Guiy4R0r6oNQAI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=tX/+ArC+; arc=none smtp.client-ip=90.155.50.34 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="tX/+ArC+" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=LVW29VgTBcSEwz3Up8blF0S6ObWPFVHT6wdQNGV4Ppk=; b=tX/+ArC+dmBJ6gpIMcIYmbvE+A jhYyel6wJNfb+MJR+kK3WOCw7ZNeKOlhAGIcKkLUMM89y85oTDHlF2nxnLtxR4TUIAzw2am5ABfPx 3WkGAsz2BIIbNjXqiihnqldwDQFS9bKqNfjO0wBefgzxYL2QvIVy8PXI3+GilcYW27NU1pnZCK5KZ fbdBWi9e5pRJjHehmA3JtJwrmcTaw4Dk2AriRrqDXEAa+n4ietTA625ZzWgXA+C8xx9h90JiyKpHS 4TVPAMKdCeGSyANp7YOpkz7SprOauIUcKRikeDXvk5RgnFpSMPjS6ZuUU548gtCgNqhJhUbCrdW4Z 2zL5+/Rg==; Received: from 2001-1c00-8d85-4b00-266e-96ff-fe07-7dcc.cable.dynamic.v6.ziggo.nl ([2001:1c00:8d85:4b00:266e:96ff:fe07:7dcc] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1wFXOd-0000000Bn56-26nM; Wed, 22 Apr 2026 13:13:39 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 2301B3008E2; Wed, 22 Apr 2026 15:13:38 +0200 (CEST) Date: Wed, 22 Apr 2026 15:13:38 +0200 From: Peter Zijlstra To: Mathias Stearn Cc: Thomas Gleixner , Mathieu Desnoyers , Catalin Marinas , Will Deacon , Boqun Feng , "Paul E. McKenney" , Chris Kennelly , Dmitry Vyukov , regressions@lists.linux.dev, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Ingo Molnar , Mark Rutland , Jinjie Ruan , Blake Oler Subject: Re: [REGRESSION] rseq: refactoring in v6.19 broke everyone on arm64 and tcmalloc everywhere Message-ID: <20260422131338.GI3102924@noisy.programming.kicks-ass.net> References: <20260422125647.GP3126523@noisy.programming.kicks-ass.net> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline 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/