From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1751820AbbCGSS2 (ORCPT <rfc822;w@1wt.eu>);
	Sat, 7 Mar 2015 13:18:28 -0500
Received: from userp1040.oracle.com ([156.151.31.81]:35703 "EHLO
	userp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1750802AbbCGSSZ (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Sat, 7 Mar 2015 13:18:25 -0500
Message-ID: <54FB40BE.1070309@oracle.com>
Date: Sat, 07 Mar 2015 13:17:34 -0500
From: Sasha Levin <sasha.levin@oracle.com>
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0
MIME-Version: 1.0
To: Jason Low <jason.low2@hp.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>, Peter Zijlstra <peterz@infradead.org>
CC: Davidlohr Bueso <dave@stgolabs.net>, Tim Chen <tim.c.chen@linux.intel.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Michel Lespinasse <walken@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Dave Jones <davej@codemonkey.org.uk>,
        Ming Lei <ming.lei@canonical.com>
Subject: Re: [PATCH] locking/rwsem: Fix lock optimistic spinning when owner
 is not running
References: <1425714331.2475.388.camel@j-VirtualBox>
In-Reply-To: <1425714331.2475.388.camel@j-VirtualBox>
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
X-Source-IP: acsinet21.oracle.com [141.146.126.237]
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 03/07/2015 02:45 AM, Jason Low wrote:
> Fixes tip commit b3fd4f03ca0b (locking/rwsem: Avoid deceiving lock spinners).
> 
> Ming reported soft lockups occurring when running xfstest due to
> commit b3fd4f03ca0b.
> 
> When doing optimistic spinning in rwsem, threads should stop spinning when
> the lock owner is not running. While a thread is spinning on owner, if
> the owner reschedules, owner->on_cpu returns false and we stop spinning.
> 
> However, commit b3fd4f03ca0b essentially caused the check to get ignored
> because when we break out of the spin loop due to !on_cpu, we continue
> spinning if sem->owner != NULL.
> 
> This patch fixes this by making sure we stop spinning if the owner is not
> running. Furthermore, just like with mutexes, refactor the code such that
> we don't have separate checks for owner_running(). This makes it more
> straightforward in terms of why we exit the spin on owner loop and we
> would also avoid needing to "guess" why we broke out of the loop to make
> this more readable.

That seems to solve the hangs I'm seeing as well.


Thanks,
Sasha