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 X-Spam-Level: X-Spam-Status: No, score=-19.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_GIT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 914ADC433DB for ; Wed, 6 Jan 2021 01:19:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5F24922D01 for ; Wed, 6 Jan 2021 01:19:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726603AbhAFBTO (ORCPT ); Tue, 5 Jan 2021 20:19:14 -0500 Received: from mail.kernel.org ([198.145.29.99]:60412 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725768AbhAFBTN (ORCPT ); Tue, 5 Jan 2021 20:19:13 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 7DED8230FD; Wed, 6 Jan 2021 01:17:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1609895872; bh=JZ/A0Ej4DAs1pgQNQqM4FCqjxD7UEtQkZzWwKn4VsH0=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=hpZfLkbTX/cwRuUA535Txxwqp3jKj2UGCk47A6JOieqgFc0C03FDtut6txIvNx0Ts wHYqAN3rEEz2sdRjaz2deuyZn2og00LJ0xe8haAgs2jr04EXTRNhS1AFkTOek5PBMC U8s/EyykT+Zn5RuXGNIcppzXnqYv2l3IjXXINpM3t7zPEbXIJLzezw/vEhs4v394zG llIiCBQt63QL0nGZxPxssMw7KUebJEjOJFdqrYeAdbzF6GvfMEHnGyDkjT0GTee9Sm nVxtv0HJT8wNAjkMHq7Jipzrec/vXRSI+sJdHott0mJaA31KUcP0KGhAisNjcDLRGp M///Q36MEEj3w== From: paulmck@kernel.org To: linux-kernel@vger.kernel.org, rcu@vger.kernel.org, linux-mm@kvack.org Cc: cl@linux.com, penberg@kernel.org, rientjes@google.com, iamjoonsoo.kim@lge.com, akpm@linux-foundation.org, ming.lei@redhat.com, axboe@kernel.dk, kernel-team@fb.com, "Paul E. McKenney" Subject: [PATCH mm,percpu_ref,rcu 6/6] percpu_ref: Dump mem_dump_obj() info upon reference-count underflow Date: Tue, 5 Jan 2021 17:17:50 -0800 Message-Id: <20210106011750.13709-6-paulmck@kernel.org> X-Mailer: git-send-email 2.9.5 In-Reply-To: <20210106011603.GA13180@paulmck-ThinkPad-P72> References: <20210106011603.GA13180@paulmck-ThinkPad-P72> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: "Paul E. McKenney" Reference-count underflow for percpu_ref is detected in the RCU callback percpu_ref_switch_to_atomic_rcu(), and the resulting warning does not print anything allowing easy identification of which percpu_ref use case is underflowing. This is of course not normally a problem when developing a new percpu_ref use case because it is most likely that the problem resides in this new use case. However, when deploying a new kernel to a large set of servers, the underflow might well be a new corner case in any of the old percpu_ref use cases. This commit therefore calls mem_dump_obj() to dump out any additional available information on the underflowing percpu_ref instance. Cc: Ming Lei Cc: Jens Axboe Cc: Joonsoo Kim Reported-by: Andrii Nakryiko Signed-off-by: Paul E. McKenney --- lib/percpu-refcount.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-) diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c index e59eda0..a1071cd 100644 --- a/lib/percpu-refcount.c +++ b/lib/percpu-refcount.c @@ -5,6 +5,7 @@ #include #include #include +#include #include /* @@ -168,6 +169,7 @@ static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu) struct percpu_ref_data, rcu); struct percpu_ref *ref = data->ref; unsigned long __percpu *percpu_count = percpu_count_ptr(ref); + static atomic_t underflows; unsigned long count = 0; int cpu; @@ -191,9 +193,13 @@ static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu) */ atomic_long_add((long)count - PERCPU_COUNT_BIAS, &data->count); - WARN_ONCE(atomic_long_read(&data->count) <= 0, - "percpu ref (%ps) <= 0 (%ld) after switching to atomic", - data->release, atomic_long_read(&data->count)); + if (WARN_ONCE(atomic_long_read(&data->count) <= 0, + "percpu ref (%ps) <= 0 (%ld) after switching to atomic", + data->release, atomic_long_read(&data->count)) && + atomic_inc_return(&underflows) < 4) { + pr_err("%s(): percpu_ref underflow", __func__); + mem_dump_obj(data); + } /* @ref is viewed as dead on all CPUs, send out switch confirmation */ percpu_ref_call_confirm_rcu(rcu); -- 2.9.5