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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 In-Reply-To: (Linus Torvalds's message of "Thu, 23 Apr 2026 14:28:35 -0700") References: <20260422125647.GP3126523@noisy.programming.kicks-ass.net> <20260422131338.GI3102924@noisy.programming.kicks-ass.net> <87fr4l28zn.ffs@tglx> <87cxzp1tn6.ffs@tglx> <874il11jac.ffs@tglx> Date: Mon, 27 Apr 2026 09:06:13 +0200 Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 X-Mimecast-MFC-PROC-ID: 00VoddaEIAn7PHlSwVuzaXb_VEtj07Pks9OmbclbFk8_1777273583 X-Mimecast-Originator: redhat.com Content-Type: text/plain X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260427_080644_234426_E68F525F X-CRM114-Status: GOOD ( 12.47 ) 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 * Linus Torvalds: >> // Note: this makes __rseq_abi.cpu_id_start unusable for its original purpose. >> >> Do you seriously want to proliferate that? > > Absolutely. > > That's how clever hacks work - they take advantage of things past > their design parameters. "If it works, it's not stupid". > > We don't then turn around and say "you were clever, and we did > something stupid, so now we'll hurt you". > > This is all 100% on the RSEQ kernel code, not on users who took > advantage of it. RSEQ was intended to be modular, with more than one library using it within a process, without coordination (beyond sticking to the RSEQ protocol). The tcmalloc approach is incompatible with that. Once tcmalloc starts using RSEQ in its peculiar way, nothing else in the process can, and vice versa. This is far from ideal because the particular descheduling notification that tcmalloc uses could be implemented in a much simpler way than full RSEQ, given its non-modular nature. Thanks, Florian