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=-6.8 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,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 24EBEC43331 for ; Mon, 11 Nov 2019 18:34:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E49BD2190F for ; Mon, 11 Nov 2019 18:34:17 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1573497258; bh=8TCRa5j57pe7+Mx5RkLfWcn+fZoJqmv6pWa/XoPllB8=; h=From:To:Cc:Subject:Date:In-Reply-To:References:List-ID:From; b=0KwUqeTM7VUmRP+dHG0dfzw6Wki/Sz8S2D5/UxRVbHq875R2PXJCnvorGk/u9Qhog 8RFX0o358nWfwanmt7E4XkfInBfVS1raO1Ir9csx+HHEx4SmApYbrBRspHNsXcavvd pY78CbV5q3B7qG0mBrEO72iEdsYc8H+BJeReAQ/I= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727137AbfKKSeQ (ORCPT ); Mon, 11 Nov 2019 13:34:16 -0500 Received: from mail.kernel.org ([198.145.29.99]:51762 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728294AbfKKSeL (ORCPT ); Mon, 11 Nov 2019 13:34:11 -0500 Received: from localhost (83-86-89-107.cable.dynamic.v4.ziggo.nl [83.86.89.107]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 94A3E21925; Mon, 11 Nov 2019 18:34:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1573497251; bh=8TCRa5j57pe7+Mx5RkLfWcn+fZoJqmv6pWa/XoPllB8=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=or6BUi9M2laiJGqMdwAy02nDLABFmbVG2ih6r6G1pFiFrYk/l/poQb/LSYZ0qXYRP ET+zchhChuH4kKifmBFen7AYHi/SKNKHAdF4mgkgBVJ9K0pegr3ONlK30Bp/hc/gD0 TWyNJJYJ2ApgAATUsQCel3POYAfjhc0WBqBeNn4w= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Kevin Hao , Linus Torvalds , Andrew Morton Subject: [PATCH 4.9 15/65] dump_stack: avoid the livelock of the dump_lock Date: Mon, 11 Nov 2019 19:28:15 +0100 Message-Id: <20191111181343.934460230@linuxfoundation.org> X-Mailer: git-send-email 2.24.0 In-Reply-To: <20191111181331.917659011@linuxfoundation.org> References: <20191111181331.917659011@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Kevin Hao commit 5cbf2fff3bba8d3c6a4d47c1754de1cf57e2b01f upstream. In the current code, we use the atomic_cmpxchg() to serialize the output of the dump_stack(), but this implementation suffers the thundering herd problem. We have observed such kind of livelock on a Marvell cn96xx board(24 cpus) when heavily using the dump_stack() in a kprobe handler. Actually we can let the competitors to wait for the releasing of the lock before jumping to atomic_cmpxchg(). This will definitely mitigate the thundering herd problem. Thanks Linus for the suggestion. [akpm@linux-foundation.org: fix comment] Link: http://lkml.kernel.org/r/20191030031637.6025-1-haokexin@gmail.com Fixes: b58d977432c8 ("dump_stack: serialize the output from dump_stack()") Signed-off-by: Kevin Hao Suggested-by: Linus Torvalds Cc: Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman --- lib/dump_stack.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) --- a/lib/dump_stack.c +++ b/lib/dump_stack.c @@ -44,7 +44,12 @@ retry: was_locked = 1; } else { local_irq_restore(flags); - cpu_relax(); + /* + * Wait for the lock to release before jumping to + * atomic_cmpxchg() in order to mitigate the thundering herd + * problem. + */ + do { cpu_relax(); } while (atomic_read(&dump_lock) != -1); goto retry; }