From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 5A03EF9EDCD for ; Wed, 22 Apr 2026 13:13:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=LVW29VgTBcSEwz3Up8blF0S6ObWPFVHT6wdQNGV4Ppk=; b=adFQAUGbnlppVZN6ukDMbY1mg2 EkcUr9MDzmaLqThlxTgH8HDMHie2NdYEZv+74wyWaOBgGeKcRUyExrEio8gfSQ44aStfv/lO428zV CJKn2Odnp9xKnzMAm5ksg4dsO5v2rrT+Hgz5+eVkzVG3d8wtajvA3LhCv6zgGwhoXzLLrPWLaD0Bu aE8r7Fd5mndnIzGJLKEESipygtRwRHGBs9tRZ+YNAWE7Z1rBFGlH/bf3tr1kJ6dwd2hg0/itrP52N hNetpq0XOyu3TP1VwgIuRFXu7T2+0kxB84YrS2Df+z0QPyv8d3n7r16OuZRQl1LmE+64uWRdC41Bv MZ0zHQbQ==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1wFXOi-0000000AGrG-0oHu; Wed, 22 Apr 2026 13:13:44 +0000 Received: from casper.infradead.org ([2001:8b0:10b:1236::1]) by bombadil.infradead.org with esmtps (Exim 4.98.2 #2 (Red Hat Linux)) id 1wFXOg-0000000AGqj-3TOx for linux-arm-kernel@bombadil.infradead.org; Wed, 22 Apr 2026 13:13:42 +0000 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260422125647.GP3126523@noisy.programming.kicks-ass.net> X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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/