Messages in this thread | | | Date | Tue, 23 Jun 2015 15:38:46 +0200 | Subject | Re: [PATCH 2/2] PM / Domains: Avoid infinite loops in attach/detach code | From | "Rafael J. Wysocki" <> |
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On Tue, Jun 23, 2015 at 3:20 PM, Geert Uytterhoeven <geert@linux-m68k.org> wrote: > Hi Ulf, > > On Tue, Jun 23, 2015 at 2:50 PM, Ulf Hansson <ulf.hansson@linaro.org> wrote: >> On 22 June 2015 at 09:31, Geert Uytterhoeven <geert+renesas@glider.be> 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 <geert+renesas@glider.be> >>> --- >>> 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 <linux/delay.h> >>> #include <linux/kernel.h> >>> #include <linux/io.h> >>> #include <linux/platform_device.h> >>> @@ -19,6 +20,9 @@ >>> #include <linux/suspend.h> >>> #include <linux/export.h> >>> >>> +#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? > > That's indeed viable. I have no idea for how long this temporary state can > extend.
A usual approach to this kind of thing is to use exponential fallback where you increase the delay twice with respect to the previous one every time.
Rafael
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