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 53605C433F5 for ; Wed, 29 Sep 2021 07:31:55 +0000 (UTC) Received: from lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id B38D2613D3 for ; Wed, 29 Sep 2021 07:31:54 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org B38D2613D3 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.vnet.ibm.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=lists.ozlabs.org Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4HK7Nn0zrlz306l for ; Wed, 29 Sep 2021 17:31:53 +1000 (AEST) Authentication-Results: lists.ozlabs.org; dkim=fail reason="signature verification failed" (2048-bit key; unprotected) header.d=ibm.com header.i=@ibm.com header.a=rsa-sha256 header.s=pp1 header.b=a0bct6qS; dkim-atps=neutral Authentication-Results: lists.ozlabs.org; spf=none (no SPF record) smtp.mailfrom=linux.vnet.ibm.com (client-ip=148.163.156.1; helo=mx0a-001b2d01.pphosted.com; envelope-from=srikar@linux.vnet.ibm.com; receiver=) Authentication-Results: lists.ozlabs.org; dkim=pass (2048-bit key; unprotected) header.d=ibm.com header.i=@ibm.com header.a=rsa-sha256 header.s=pp1 header.b=a0bct6qS; dkim-atps=neutral Received: from mx0a-001b2d01.pphosted.com (mx0a-001b2d01.pphosted.com [148.163.156.1]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4HK7MX5YM0z30Hj for ; Wed, 29 Sep 2021 17:30:48 +1000 (AEST) Received: from pps.filterd (m0098393.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.1.2/8.16.1.2) with SMTP id 18T70RRm024894; Wed, 29 Sep 2021 03:30:44 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ibm.com; h=date : from : to : cc : subject : message-id : reply-to : references : mime-version : content-type : in-reply-to; s=pp1; bh=JqhpbbHOmdnqkip5sOdgoe5dcpB1zw/9oXqEPqvJ/Xc=; b=a0bct6qSm/9dlksFC6bJwWu+19VnpB0+IgoHPJf9GsBrwCh4sjyr5tcRkPg7fnm/PwUQ 990Fk2Hxbh5VYcbbCwolofwPV30GWCJKH5BXQfPZmcSAzCnK8/kEEPE2/FtvBMpvUBZv 56KYOLzbj6vct2jji2ldcD8yn8TD6O1s9rxiq9OCvV1jeifSDZcb+0OFaZYeswf5iwEl 3SWicDRE5qytUfO3RCWYXxnP+y+L1rp8oMaLPx0Uzfg8m85gN2WxiZg6TVRrh8bONWl6 RSDkWXuC+ZFroocBwoY7jcJlUDRJK/VwdQkc1n5xlzULHWatE/vOIU3KKWLu2qhgFE4U fQ== Received: from pps.reinject (localhost [127.0.0.1]) by mx0a-001b2d01.pphosted.com with ESMTP id 3bckcj0naa-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Wed, 29 Sep 2021 03:30:44 -0400 Received: from m0098393.ppops.net (m0098393.ppops.net [127.0.0.1]) by pps.reinject (8.16.0.43/8.16.0.43) with SMTP id 18T71G9m030625; Wed, 29 Sep 2021 03:30:44 -0400 Received: from ppma03ams.nl.ibm.com (62.31.33a9.ip4.static.sl-reverse.com [169.51.49.98]) by mx0a-001b2d01.pphosted.com with ESMTP id 3bckcj0n9d-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Wed, 29 Sep 2021 03:30:44 -0400 Received: from pps.filterd (ppma03ams.nl.ibm.com [127.0.0.1]) by ppma03ams.nl.ibm.com (8.16.1.2/8.16.1.2) with SMTP id 18T7C1tK003404; Wed, 29 Sep 2021 07:30:41 GMT Received: from b06avi18626390.portsmouth.uk.ibm.com (b06avi18626390.portsmouth.uk.ibm.com [9.149.26.192]) by ppma03ams.nl.ibm.com with ESMTP id 3b9uda4v3x-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Wed, 29 Sep 2021 07:30:41 +0000 Received: from b06wcsmtp001.portsmouth.uk.ibm.com (b06wcsmtp001.portsmouth.uk.ibm.com [9.149.105.160]) by b06avi18626390.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id 18T7Pa9t54919628 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Wed, 29 Sep 2021 07:25:36 GMT Received: from b06wcsmtp001.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id E7321A4062; Wed, 29 Sep 2021 07:30:38 +0000 (GMT) Received: from b06wcsmtp001.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 7A1E9A4072; Wed, 29 Sep 2021 07:30:37 +0000 (GMT) Received: from linux.vnet.ibm.com (unknown [9.126.150.29]) by b06wcsmtp001.portsmouth.uk.ibm.com (Postfix) with SMTP; Wed, 29 Sep 2021 07:30:37 +0000 (GMT) Date: Wed, 29 Sep 2021 13:00:36 +0530 From: Srikar Dronamraju To: Nathan Lynch Subject: Re: [PATCH v2 2/2] powerpc/paravirt: correct preempt debug splat in vcpu_is_preempted() Message-ID: <20210929073036.GG2004@linux.vnet.ibm.com> References: <20210928214147.312412-1-nathanl@linux.ibm.com> <20210928214147.312412-3-nathanl@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <20210928214147.312412-3-nathanl@linux.ibm.com> X-TM-AS-GCONF: 00 X-Proofpoint-GUID: wObYIvn84P737sd0B9c164X1Mibqe6yJ X-Proofpoint-ORIG-GUID: KePodetO-n8PMeKevSnQ2ghtg58EcLY7 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.182.1,Aquarius:18.0.790,Hydra:6.0.391,FMLib:17.0.607.475 definitions=2021-09-29_02,2021-09-28_01,2020-04-07_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 mlxlogscore=999 suspectscore=0 impostorscore=0 lowpriorityscore=0 spamscore=0 phishscore=0 malwarescore=0 bulkscore=0 clxscore=1015 mlxscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2109230001 definitions=main-2109290041 X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Srikar Dronamraju Cc: linuxppc-dev@lists.ozlabs.org, npiggin@gmail.com Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" * Nathan Lynch [2021-09-28 16:41:47]: > 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. > > Signed-off-by: Nathan Lynch > Fixes: ca3f969dcb11 ("powerpc/paravirt: Use is_kvm_guest() in vcpu_is_preempted()") Looks good to me. Reviewed-by: Srikar Dronamraju > --- > 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 39f173961f6a..eb7df559ae74 100644 > --- a/arch/powerpc/include/asm/paravirt.h > +++ b/arch/powerpc/include/asm/paravirt.h > @@ -110,7 +110,23 @@ 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()); > > /* > * The PowerVM hypervisor dispatches VMs on a whole core > -- > 2.31.1 > -- Thanks and Regards Srikar Dronamraju