From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Rafael J. Wysocki" Subject: Re: Async resume patch (was: Re: [GIT PULL] PM updates for 2.6.33) Date: Tue, 8 Dec 2009 21:30:14 +0100 Message-ID: <200912082130.14634.rjw@sisk.pl> References: Mime-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Return-path: Received: from ogre.sisk.pl ([217.79.144.158]:49074 "EHLO ogre.sisk.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965757AbZLHU3s (ORCPT ); Tue, 8 Dec 2009 15:29:48 -0500 In-Reply-To: Sender: linux-acpi-owner@vger.kernel.org List-Id: linux-acpi@vger.kernel.org To: Alan Stern Cc: Linus Torvalds , Zhang Rui , LKML , ACPI Devel Maling List , pm list On Tuesday 08 December 2009, Alan Stern wrote: > On Tue, 8 Dec 2009, Rafael J. Wysocki wrote: > > > Suppose we use rwsem and during suspend each child uses a down_read() on a > > parent and then the parent uses down_write() on itself. What if, whatever the > > reason, the parent is a bit early and does the down_write() before one of the > > children has a chance to do the down_read()? Aren't we toast? > > > > Do we need any direct protection against that or does it just work itself out > > in a way I just don't see right now? > > That's not the way it should be done. Linus had children taking their > parents' locks during suspend, which is simple but leads to > difficulties. > > Instead, the PM core should do a down_write() on each device before > starting the device's async suspend routine, and an up_write() when the > routine finishes. Parents should, at the start of their async routine, > do down_read() on each of their children plus whatever other devices > they need to wait for. The core can do the waiting for children part > and the driver's suspend routine can handle any other waiting. > > This is a little more awkward because it requires the parent to iterate > through its children. I can live with that. > But it does solve the off-tree dependency problem for suspends. That's a plus, but I still think we're trying to create a barrier-alike mechanism using lock. There's one more possibility to consider, though. What if we use a completion instead of the flag + wait queue? It surely is a standard synchronization mechanism and it seems it might work here. Rafael