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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2E4CFC3525B for ; Tue, 16 Nov 2021 00:36:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0EE9961507 for ; Tue, 16 Nov 2021 00:36:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1355740AbhKPAij (ORCPT ); Mon, 15 Nov 2021 19:38:39 -0500 Received: from mail.kernel.org ([198.145.29.99]:45394 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1344356AbhKOTYe (ORCPT ); Mon, 15 Nov 2021 14:24:34 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id D4A7D6366F; Mon, 15 Nov 2021 18:56:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1637002585; bh=l0aKp8ca26bq3qWZd9dQxTWCS4ZcATBIyEK80Ijq2g8=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=DmSRWsvZCTJxuRu8JrFtpyAI/vAdquAImCYNvos88Ri21ivgGKASwI66Nj87Ad07n zpJguGazZDruXWN8zSda8Pifxy4DG8cvQvyyZ7FrKHcd+Tz8MEEuO9ypAMqBEly7dw jQWeW+1AmOdqlVeVsCP9MO2V01BU2LjD6jvIzgsI= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Nathan Lynch , Srikar Dronamraju , Michael Ellerman , Sasha Levin Subject: [PATCH 5.15 622/917] powerpc/paravirt: correct preempt debug splat in vcpu_is_preempted() Date: Mon, 15 Nov 2021 18:01:57 +0100 Message-Id: <20211115165449.885533008@linuxfoundation.org> X-Mailer: git-send-email 2.33.1 In-Reply-To: <20211115165428.722074685@linuxfoundation.org> References: <20211115165428.722074685@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org From: Nathan Lynch [ Upstream commit fda0eb220021a97c1d656434b9340ebf3fc4704a ] vcpu_is_preempted() can be used outside of preempt-disabled critical sections, yielding warnings such as: BUG: using smp_processor_id() in preemptible [00000000] code: systemd-udevd/185 caller is rwsem_spin_on_owner+0x1cc/0x2d0 CPU: 1 PID: 185 Comm: systemd-udevd Not tainted 5.15.0-rc2+ #33 Call Trace: [c000000012907ac0] [c000000000aa30a8] dump_stack_lvl+0xac/0x108 (unreliable) [c000000012907b00] [c000000001371f70] check_preemption_disabled+0x150/0x160 [c000000012907b90] [c0000000001e0e8c] rwsem_spin_on_owner+0x1cc/0x2d0 [c000000012907be0] [c0000000001e1408] rwsem_down_write_slowpath+0x478/0x9a0 [c000000012907ca0] [c000000000576cf4] filename_create+0x94/0x1e0 [c000000012907d10] [c00000000057ac08] do_symlinkat+0x68/0x1a0 [c000000012907d70] [c00000000057ae18] sys_symlink+0x58/0x70 [c000000012907da0] [c00000000002e448] system_call_exception+0x198/0x3c0 [c000000012907e10] [c00000000000c54c] system_call_common+0xec/0x250 The result of vcpu_is_preempted() is always used speculatively, and the function does not access per-cpu resources in a (Linux) preempt-unsafe way. Use raw_smp_processor_id() to avoid such warnings, adding explanatory comments. Fixes: ca3f969dcb11 ("powerpc/paravirt: Use is_kvm_guest() in vcpu_is_preempted()") Signed-off-by: Nathan Lynch Reviewed-by: Srikar Dronamraju Signed-off-by: Michael Ellerman Link: https://lore.kernel.org/r/20210928214147.312412-3-nathanl@linux.ibm.com Signed-off-by: Sasha Levin --- arch/powerpc/include/asm/paravirt.h | 18 +++++++++++++++++- 1 file changed, 17 insertions(+), 1 deletion(-) diff --git a/arch/powerpc/include/asm/paravirt.h b/arch/powerpc/include/asm/paravirt.h index bcb7b5f917be6..b325022ffa2b0 100644 --- a/arch/powerpc/include/asm/paravirt.h +++ b/arch/powerpc/include/asm/paravirt.h @@ -97,7 +97,23 @@ static inline bool vcpu_is_preempted(int cpu) #ifdef CONFIG_PPC_SPLPAR if (!is_kvm_guest()) { - int first_cpu = cpu_first_thread_sibling(smp_processor_id()); + int first_cpu; + + /* + * The result of vcpu_is_preempted() is used in a + * speculative way, and is always subject to invalidation + * by events internal and external to Linux. While we can + * be called in preemptable context (in the Linux sense), + * we're not accessing per-cpu resources in a way that can + * race destructively with Linux scheduler preemption and + * migration, and callers can tolerate the potential for + * error introduced by sampling the CPU index without + * pinning the task to it. So it is permissible to use + * raw_smp_processor_id() here to defeat the preempt debug + * warnings that can arise from using smp_processor_id() + * in arbitrary contexts. + */ + first_cpu = cpu_first_thread_sibling(raw_smp_processor_id()); /* * Preemption can only happen at core granularity. This CPU -- 2.33.0