From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757872Ab2CSLUs (ORCPT ); Mon, 19 Mar 2012 07:20:48 -0400 Received: from ogre.sisk.pl ([217.79.144.158]:52353 "EHLO ogre.sisk.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752075Ab2CSLUq (ORCPT ); Mon, 19 Mar 2012 07:20:46 -0400 From: "Rafael J. Wysocki" To: Stephen Boyd Subject: Re: [PATCH] firmware loader: don't cancel _nowait requests when helper is not yet available Date: Mon, 19 Mar 2012 12:24:59 +0100 User-Agent: KMail/1.13.6 (Linux/3.3.0-rc7+; KDE/4.6.0; x86_64; ; ) Cc: Linus Torvalds , Saravana Kannan , Kay Sievers , Greg KH , Christian Lamparter , linux-kernel@vger.kernel.org, "Srivatsa S. Bhat" , alan@lxorguk.ukuu.org.uk, Linux PM mailing list References: <201203032122.36745.chunkeey@googlemail.com> <201203181301.19048.rjw@sisk.pl> <4F66D2E8.2090906@codeaurora.org> In-Reply-To: <4F66D2E8.2090906@codeaurora.org> MIME-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Message-Id: <201203191225.00199.rjw@sisk.pl> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Monday, March 19, 2012, Stephen Boyd wrote: > On 3/18/2012 5:01 AM, Rafael J. Wysocki wrote: > > On Sunday, March 18, 2012, Stephen Boyd wrote: > > > >> Ok. I like where nowait() is going in the other part of the thread but > >> I'm still confused about when request_firmware() is correct to use. It > >> seems that the function is inherently racy with freezing. Does every > >> user of request_firmware() need to synchronize with freezing? > >> > >> For example, if one CPU is in the middle of a driver probe that makes a > >> request_firmware() call and another CPU is starting to suspend we will > >> have a race between usermodehelpers being disabled and the > >> request_firmware() call acquiring the usermodehelper rwsem. If the > >> suspending CPU wins the race it will disable usermodehelpers and the > >> request_firmware() call will return -EBUSY and warn. > > Yes, it will. > > That sounds wrong then, no? Why don't we have request_firmware() do a > read_lock on the usermodehelpers sem and then have suspend do a write > lock, disable usermodehelpers, wait for any users to finish, freeze > processes and then unlock the write lock? That way we don't hit a case > where request_firmware() races with suspend, and we don't have to change > the warning or conditional. So, you're postulating that the freezing of tasks be done under umhelper_sem write-locked, right? That would lead to freezing failures if a user space task waited in request_firmware() for umhelper_sem to become available for read-locking and unfortunately we don't have an interruptible variant of down_read(). However, we may catch request_firmware() and try to freeze the task calling it instead. I'll try to prototype something along these lines later today (on top of the three "firmware_class" patches I posted yesterday). Thanks, Rafael