From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:39150 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752434AbdK0Ol0 (ORCPT ); Mon, 27 Nov 2017 09:41:26 -0500 Received: from pps.filterd (m0098393.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.21/8.16.0.21) with SMTP id vAREfCro112165 for ; Mon, 27 Nov 2017 09:41:25 -0500 Received: from e12.ny.us.ibm.com (e12.ny.us.ibm.com [129.33.205.202]) by mx0a-001b2d01.pphosted.com with ESMTP id 2egguj43p1-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Mon, 27 Nov 2017 09:41:23 -0500 Received: from localhost by e12.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 27 Nov 2017 09:41:22 -0500 Date: Mon, 27 Nov 2017 06:41:25 -0800 From: "Paul E. McKenney" To: Florian Weimer Cc: NeilBrown , Alexander Viro , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Josh Triplett Subject: Re: [PATCH] VFS: use synchronize_rcu_expedited() in namespace_unlock() Reply-To: paulmck@linux.vnet.ibm.com References: <87y3nyd4pu.fsf@notabene.neil.brown.name> <20171026122743.GX3659@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Message-Id: <20171127144125.GF3624@linux.vnet.ibm.com> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Mon, Nov 27, 2017 at 12:27:04PM +0100, Florian Weimer wrote: > On 10/26/2017 02:27 PM, Paul E. McKenney wrote: > >But just for completeness, one way to make this work across the board > >might be to instead use call_rcu(), with the callback function kicking > >off a workqueue handler to do the rest of the unmount. Of course, > >in saying that, I am ignoring any mutexes that you might be holding > >across this whole thing, and also ignoring any problems that might arise > >when returning to userspace with some portion of the unmount operation > >still pending. (For example, someone unmounting a filesystem and then > >immediately remounting that same filesystem.) > > You really need to complete all side effects of deallocating a > resource before returning to user space. Otherwise, it will never > be possible to allocate and deallocate resources in a tight loop > because you either get spurious failures because too many > unaccounted deallocations are stuck somewhere in the system (and the > user can't tell that this is due to a race), or you get an OOM > because the user manages to queue up too much state. > > We already have this problem with RLIMIT_NPROC, where waitpid etc. > return before the process is completely gone. On some > kernels/configurations, the resulting race is so wide that parallel > make no longer works reliable because it runs into fork failures. Or alternatively, use rcu_barrier() occasionally to wait for all preceding deferred deallocations. And there are quite a few other ways to take on this problem. Thanx, Paul