Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932795AbbFWMuy (ORCPT ); Tue, 23 Jun 2015 08:50:54 -0400 Received: from mail-qg0-f47.google.com ([209.85.192.47]:36534 "EHLO mail-qg0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932554AbbFWMuq (ORCPT ); Tue, 23 Jun 2015 08:50:46 -0400 MIME-Version: 1.0 In-Reply-To: <1434958282-27376-1-git-send-email-geert+renesas@glider.be> References: <1434622954-26747-3-git-send-email-geert+renesas@glider.be> <1434958282-27376-1-git-send-email-geert+renesas@glider.be> Date: Tue, 23 Jun 2015 14:50:45 +0200 Message-ID: Subject: Re: [PATCH 2/2] PM / Domains: Avoid infinite loops in attach/detach code From: Ulf Hansson To: Geert Uytterhoeven Cc: Daniel Lezcano , Thomas Gleixner , "Rafael J. Wysocki" , Kevin Hilman , Magnus Damm , Laurent Pinchart , "linux-pm@vger.kernel.org" , Linux-sh list , "linux-kernel@vger.kernel.org" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4128 Lines: 114 On 22 June 2015 at 09:31, Geert Uytterhoeven wrote: > If pm_genpd_{add,remove}_device() keeps on failing with -EAGAIN, we end > up with an infinite loop in genpd_dev_pm_{at,de}tach(). > > This may happen due to a genpd.prepared_count imbalance. This is a bug > elsewhere, but it will result in a system lock up, possibly during > reboot of an otherwise functioning system. > > To avoid this, put a limit on the maximum number of loop iterations, > including a simple back-off mechanism. If the limit is reached, the > operation will just fail. An error message is already printed. > > Signed-off-by: Geert Uytterhoeven > --- > drivers/base/power/domain.c | 16 ++++++++++++++-- > 1 file changed, 14 insertions(+), 2 deletions(-) > > diff --git a/drivers/base/power/domain.c b/drivers/base/power/domain.c > index cdd547bd67df8218..60e0309dd8dd0264 100644 > --- a/drivers/base/power/domain.c > +++ b/drivers/base/power/domain.c > @@ -6,6 +6,7 @@ > * This file is released under the GPLv2. > */ > > +#include > #include > #include > #include > @@ -19,6 +20,9 @@ > #include > #include > > +#define GENPD_RETRIES 20 > +#define GENPD_DELAY_US 10 > + > #define GENPD_DEV_CALLBACK(genpd, type, callback, dev) \ > ({ \ > type (*__routine)(struct device *__d); \ > @@ -2131,6 +2135,7 @@ EXPORT_SYMBOL_GPL(of_genpd_get_from_provider); > static void genpd_dev_pm_detach(struct device *dev, bool power_off) > { > struct generic_pm_domain *pd; > + unsigned int i; > int ret = 0; > > pd = pm_genpd_lookup_dev(dev); > @@ -2139,10 +2144,13 @@ static void genpd_dev_pm_detach(struct device *dev, bool power_off) > > dev_dbg(dev, "removing from PM domain %s\n", pd->name); > > - while (1) { > + for (i = 0; i < GENPD_RETRIES; i++) { > ret = pm_genpd_remove_device(pd, dev); > if (ret != -EAGAIN) > break; > + > + if (i > GENPD_RETRIES / 2) > + udelay(GENPD_DELAY_US); > cond_resched(); > } > > @@ -2183,6 +2191,7 @@ int genpd_dev_pm_attach(struct device *dev) > { > struct of_phandle_args pd_args; > struct generic_pm_domain *pd; > + unsigned int i; > int ret; > > if (!dev->of_node) > @@ -2218,10 +2227,13 @@ int genpd_dev_pm_attach(struct device *dev) > > dev_dbg(dev, "adding to PM domain %s\n", pd->name); > > - while (1) { > + for (i = 0; i < GENPD_RETRIES; i++) { > ret = pm_genpd_add_device(pd, dev); > if (ret != -EAGAIN) > break; > + > + if (i > GENPD_RETRIES / 2) > + udelay(GENPD_DELAY_US); In this execution path, we retry when getting -EAGAIN while believing the reason to the error are only *temporary* as we are soon waiting for all devices in the genpd to be system PM resumed. At least that's my understanding to why we want to deal with -EAGAIN here, but I might be wrong. In this regards, I wonder whether it could be better to re-try only a few times but with a far longer interval time than a couple us. What do you think? However, what if the reason to why we get -EAGAIN isn't *temporary*, because we are about to enter system PM suspend state. Then the caller of this function which comes via some bus' ->probe(), will hang until the a system PM resume is completed. Is that really going to work? So, for this case your limited re-try approach will affect this scenario as well, have you considered that? > cond_resched(); > } > > -- > 1.9.1 > Kind regards Uffe -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/