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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id AE6A7C77B61 for ; Mon, 10 Apr 2023 17:38:13 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 27109280033; Mon, 10 Apr 2023 13:38:13 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 1FA4D280031; Mon, 10 Apr 2023 13:38:13 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 07396280033; Mon, 10 Apr 2023 13:38:13 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id EB520280031 for ; Mon, 10 Apr 2023 13:38:12 -0400 (EDT) Received: from smtpin12.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay09.hostedemail.com (Postfix) with ESMTP id B56E58047E for ; Mon, 10 Apr 2023 17:38:12 +0000 (UTC) X-FDA: 80666190024.12.912D0A6 Received: from mail-pl1-f182.google.com (mail-pl1-f182.google.com [209.85.214.182]) by imf24.hostedemail.com (Postfix) with ESMTP id E48AF180003 for ; Mon, 10 Apr 2023 17:38:09 +0000 (UTC) Authentication-Results: imf24.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=P0Yu8Rfc; spf=pass (imf24.hostedemail.com: domain of yury.norov@gmail.com designates 209.85.214.182 as permitted sender) smtp.mailfrom=yury.norov@gmail.com; dmarc=pass (policy=none) header.from=gmail.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1681148290; a=rsa-sha256; cv=none; b=NIlk4GgVmxvm+nZFuLyYGS+sIV1YEWqEODCBKS+3eziwYSLd+h2hLJojKFQvfa+GSQKzyy sZwMv3L7/NYCvvLy6cqnOBCzVI/HrMI6UmqDNOBi4+Afm3INzSJVjlii01Nsk94ptVjpZI Enq4xxdGKOI/fipHRjwbud0Q6yKVo4I= ARC-Authentication-Results: i=1; imf24.hostedemail.com; dkim=pass header.d=gmail.com header.s=20210112 header.b=P0Yu8Rfc; spf=pass (imf24.hostedemail.com: domain of yury.norov@gmail.com designates 209.85.214.182 as permitted sender) smtp.mailfrom=yury.norov@gmail.com; dmarc=pass (policy=none) header.from=gmail.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1681148290; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=Ao3YUfQTjR7qBJcO8EApW9sgj4iOJlnQmNbTg6seOCM=; b=Zyj8WL+h1srlIQ4IfORWEGeKiabZ+czfNxiLiNF/dLsEJF0jrzFhq6hzVG/5w/BiJ1kwJE 0nxHElb6oUIns30RNVjNWf5JDoXFrRdlW9K3dblQfBhmRROuZMZ4hKydXfjA6oLJHu/TyV Nhq+XIrZpTT91GEbwSt8CKNsgcrhi/M= Received: by mail-pl1-f182.google.com with SMTP id d9443c01a7336-1a516fb6523so5928485ad.3 for ; Mon, 10 Apr 2023 10:38:09 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; t=1681148289; x=1683740289; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:from:to:cc:subject:date:message-id:reply-to; bh=Ao3YUfQTjR7qBJcO8EApW9sgj4iOJlnQmNbTg6seOCM=; b=P0Yu8RfcWaztZwB5gMnan5ZL3+NGTMOr/SI9waTbhOWQ6rdGTe2U7QtFIaG39SXz+t UC2mJML+QxLBqgdyaG/o5khtJRjTHVoxXD7l5f86cFJ+TuEJwOCyQpmvYpSaD/BPhjDS GZm1gQ99xRFLixv29I0WdNwT74lDsw6E304guJokVTyUt/foZXzrI1UY5XS4maW4sLxu hLWi8rql/vtaA6kNxcKNBE+jmlfOmHqWPnZ10EvmSXMoAfhe7c0G+YixwJQiudhk3ziB enk1/JXewJXreHBvuuNXaAlNb2W3yIap9z6XJ4d9AQXbOJEcUPpbuoUUWitzo/QBcNKj eoxA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; t=1681148289; x=1683740289; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=Ao3YUfQTjR7qBJcO8EApW9sgj4iOJlnQmNbTg6seOCM=; b=BeTMd809d42OqfoozLl/5zfSDdcXjSFovptm/C0FLjO65QdW1ZHfqIKULBKmAuk8ZS o1TzdRlgCXajvBOWBOPziETd6SI6XNkpHB4pAZaagc4IFu06hNZxXIsu33oS9z1ApFjr ojTdwZJlrywuOnXSxq8fTAVYCmK2HY+pcYgDO+bt4NYOprzZsjCZRbEvJQxVqfo2PRpx Rl4WT2pS1gdrcDl/uAwQ72giMSzCUG+l9ztq1aStL9QffMAYPmKhvSTKcUbUwmXwyeho J6JnWGX/qATYSbR81+QXm/X00S2hSPscgVJ+/naY3QXl09gi3VyTLNdwfNEiLEaB4VDU PihQ== X-Gm-Message-State: AAQBX9dw1GttlZ9IiQQwFhEJ/JebDvBwntXCYSFfHwt5qsfzeptbrxdT NdYZp5jo2sqgkuFD3dw0whQ= X-Google-Smtp-Source: AKy350ayGqJbfMRsaUxLIZRJrldYOq2Bv00aa7GT286U/TmrEHGz6hGKmp5TiIHzbv/nBIer5z2MOg== X-Received: by 2002:a62:5fc7:0:b0:633:88de:7272 with SMTP id t190-20020a625fc7000000b0063388de7272mr8114909pfb.2.1681148288627; Mon, 10 Apr 2023 10:38:08 -0700 (PDT) Received: from localhost ([216.228.127.130]) by smtp.gmail.com with ESMTPSA id w11-20020a63d74b000000b00513cc8c9597sm7196540pgi.10.2023.04.10.10.38.07 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 10 Apr 2023 10:38:08 -0700 (PDT) Date: Mon, 10 Apr 2023 10:38:06 -0700 From: Yury Norov To: "yebin (H)" Cc: Ye Bin , dennis@kernel.org, tj@kernel.org, cl@linux.com, linux-mm@kvack.org, andriy.shevchenko@linux.intel.com, linux@rasmusvillemoes.dk, linux-kernel@vger.kernel.org, dchinner@redhat.com Subject: Re: [PATCH 2/2] lib/percpu_counter: fix dying cpu compare race Message-ID: References: <20230404014206.3752945-1-yebin@huaweicloud.com> <20230404014206.3752945-3-yebin@huaweicloud.com> <642BC9A1.4040802@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <642BC9A1.4040802@huawei.com> X-Rspam-User: X-Rspamd-Queue-Id: E48AF180003 X-Rspamd-Server: rspam01 X-Stat-Signature: jgfebhgptng5sg37eu355fcj9gb3sahr X-HE-Tag: 1681148289-367631 X-HE-Meta: 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 k33bC76S 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000005, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Apr 04, 2023 at 02:54:25PM +0800, yebin (H) wrote: > > > On 2023/4/4 10:50, Yury Norov wrote: > > On Tue, Apr 04, 2023 at 09:42:06AM +0800, Ye Bin wrote: > > > From: Ye Bin > > > > > > In commit 8b57b11cca88 ("pcpcntrs: fix dying cpu summation race") a race > > > condition between a cpu dying and percpu_counter_sum() iterating online CPUs > > > was identified. > > > Acctually, there's the same race condition between a cpu dying and > > > __percpu_counter_compare(). Here, use 'num_online_cpus()' for quick judgment. > > > But 'num_online_cpus()' will be decreased before call 'percpu_counter_cpu_dead()', > > > then maybe return incorrect result. > > > To solve above issue, also need to add dying CPUs count when do quick judgment > > > in __percpu_counter_compare(). > > Not sure I completely understood the race you are describing. All CPU > > accounting is protected with percpu_counters_lock. Is it a real race > > that you've faced, or hypothetical? If it's real, can you share stack > > traces? > > > Signed-off-by: Ye Bin > > > --- > > > lib/percpu_counter.c | 11 ++++++++++- > > > 1 file changed, 10 insertions(+), 1 deletion(-) > > > > > > diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c > > > index 5004463c4f9f..399840cb0012 100644 > > > --- a/lib/percpu_counter.c > > > +++ b/lib/percpu_counter.c > > > @@ -227,6 +227,15 @@ static int percpu_counter_cpu_dead(unsigned int cpu) > > > return 0; > > > } > > > +static __always_inline unsigned int num_count_cpus(void) > > This doesn't look like a good name. Maybe num_offline_cpus? > > > > > +{ > > > +#ifdef CONFIG_HOTPLUG_CPU > > > + return (num_online_cpus() + num_dying_cpus()); > > ^ ^ > > 'return' is not a function. Braces are not needed > > > > Generally speaking, a sequence of atomic operations is not an atomic > > operation, so the above doesn't look correct. I don't think that it > > would be possible to implement raceless accounting based on 2 separate > > counters. > Yes, there is indeed a concurrency issue with doing so here. But I saw that > the process was first > set up dying_mask and then reduce the number of online CPUs. The total > quantity maybe is larger > than the actual value and may fall back to a slow path.But this won't cause > any problems. This sounds like an implementation detail. If it will change in future, your accounting will get broken. If you think it's a consistent behavior and will be preserved in future, then it must be properly commented in your patch. Thanks, Yury