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Subject[PATCH 4/6 v2] PM / Domains: Allow callbacks to execute all runtime PM helpers
Date
From: Rafael J. Wysocki <rjw@sisk.pl>

A deadlock may occur if one of the PM domains' .start_device() or
.stop_device() callbacks or a device driver's .runtime_suspend() or
.runtime_resume() callback executed by the core generic PM domain
code uses a "wrong" runtime PM helper function. This happens, for
example, if .runtime_resume() from one device's driver calls
pm_runtime_resume() for another device in the same PM domain.
A similar situation may take place if a device's parent is in the
same PM domain, in which case the runtime PM framework may execute
pm_genpd_runtime_resume() automatically for the parent (if it is
suspended at the moment). This, of course, is undesirable, so
the generic PM domains code should be modified to prevent it from
happening.

The runtime PM framework guarantees that pm_genpd_runtime_suspend()
and pm_genpd_runtime_resume() won't be executed in parallel for
the same device, so the generic PM domains code need not worry
about those cases. Still, it needs to prevent the other possible
race conditions between pm_genpd_runtime_suspend(),
pm_genpd_runtime_resume(), pm_genpd_poweron() and pm_genpd_poweroff()
from happening and it needs to avoid deadlocks at the same time.
To this end, modify the generic PM domains code to relax
synchronization rules so that:

* pm_genpd_poweron() doesn't wait for the PM domain status to
change from GPD_STATE_BUSY. If it finds that the status is
not GPD_STATE_POWER_OFF, it returns without powering the domain on
(it may modify the status depending on the circumstances).

* pm_genpd_poweroff() returns as soon as it finds that the PM
domain's status changed from GPD_STATE_BUSY after it's released
the PM domain's lock.

* pm_genpd_runtime_suspend() doesn't wait for the PM domain status
to change from GPD_STATE_BUSY after executing the domain's
.stop_device() callback and executes pm_genpd_poweroff() only
if pm_genpd_runtime_resume() is not executed in parallel.

* pm_genpd_runtime_resume() doesn't wait for the PM domain status
to change from GPD_STATE_BUSY after executing pm_genpd_poweron()
and sets the domain's status to GPD_STATE_BUSY and increments its
counter of resuming devices (introduced by this change) immediately
after acquiring the lock. The counter of resuming devices is then
decremented after executing __pm_genpd_runtime_resume() for the
device and the domain's status is reset to GPD_STATE_ACTIVE (unless
there are more resuming devices in the domain, in which case the
status remains GPD_STATE_BUSY).

This way, for example, if a device driver's .runtime_resume()
callback executes pm_runtime_resume() for another device in the same
PM domain, pm_genpd_poweron() called by pm_genpd_runtime_resume()
invoked by the runtime PM framework will not block and it will see
that there's nothing to do for it. Next, the PM domain's lock will
be acquired without waiting for its status to change from
GPD_STATE_BUSY and the device driver's .runtime_resume() callback
will be executed. In turn, if pm_runtime_suspend() is executed by
one device driver's .runtime_resume() callback for another device in
the same PM domain, pm_genpd_poweroff() executed by
pm_genpd_runtime_suspend() invoked by the runtime PM framework as a
result will notice that one of the devices in the domain is being
resumed, so it will return immediately.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
---
drivers/base/power/domain.c | 144 ++++++++++++++++++++++++++++++--------------
include/linux/pm_domain.h | 3
2 files changed, 102 insertions(+), 45 deletions(-)

Index: linux-2.6/drivers/base/power/domain.c
===================================================================
--- linux-2.6.orig/drivers/base/power/domain.c
+++ linux-2.6/drivers/base/power/domain.c
@@ -44,7 +44,8 @@ static void genpd_acquire_lock(struct ge
for (;;) {
prepare_to_wait(&genpd->status_wait_queue, &wait,
TASK_UNINTERRUPTIBLE);
- if (genpd->status != GPD_STATE_BUSY)
+ if (genpd->status == GPD_STATE_ACTIVE
+ || genpd->status == GPD_STATE_POWER_OFF)
break;
mutex_unlock(&genpd->lock);

@@ -60,6 +61,12 @@ static void genpd_release_lock(struct ge
mutex_unlock(&genpd->lock);
}

+static void genpd_set_active(struct generic_pm_domain *genpd)
+{
+ if (genpd->resume_count == 0)
+ genpd->status = GPD_STATE_ACTIVE;
+}
+
/**
* pm_genpd_poweron - Restore power to a given PM domain and its parents.
* @genpd: PM domain to power up.
@@ -75,42 +82,24 @@ static int pm_genpd_poweron(struct gener

start:
if (parent) {
- mutex_lock(&parent->lock);
+ genpd_acquire_lock(parent);
mutex_lock_nested(&genpd->lock, SINGLE_DEPTH_NESTING);
} else {
mutex_lock(&genpd->lock);
}
- /*
- * Wait for the domain to transition into either the active,
- * or the power off state.
- */
- for (;;) {
- prepare_to_wait(&genpd->status_wait_queue, &wait,
- TASK_UNINTERRUPTIBLE);
- if (genpd->status != GPD_STATE_BUSY)
- break;
- mutex_unlock(&genpd->lock);
- if (parent)
- mutex_unlock(&parent->lock);
-
- schedule();
-
- if (parent) {
- mutex_lock(&parent->lock);
- mutex_lock_nested(&genpd->lock, SINGLE_DEPTH_NESTING);
- } else {
- mutex_lock(&genpd->lock);
- }
- }
- finish_wait(&genpd->status_wait_queue, &wait);

if (genpd->status == GPD_STATE_ACTIVE
|| (genpd->prepared_count > 0 && genpd->suspend_power_off))
goto out;

+ if (genpd->status != GPD_STATE_POWER_OFF) {
+ genpd_set_active(genpd);
+ goto out;
+ }
+
if (parent && parent->status != GPD_STATE_ACTIVE) {
mutex_unlock(&genpd->lock);
- mutex_unlock(&parent->lock);
+ genpd_release_lock(parent);

ret = pm_genpd_poweron(parent);
if (ret)
@@ -125,14 +114,14 @@ static int pm_genpd_poweron(struct gener
goto out;
}

- genpd->status = GPD_STATE_ACTIVE;
+ genpd_set_active(genpd);
if (parent)
parent->sd_count++;

out:
mutex_unlock(&genpd->lock);
if (parent)
- mutex_unlock(&parent->lock);
+ genpd_release_lock(parent);

return ret;
}
@@ -210,6 +199,20 @@ static void __pm_genpd_restore_device(st
}

/**
+ * genpd_abort_poweroff - Check if a PM domain power off should be aborted.
+ * @genpd: PM domain to check.
+ *
+ * Return true if a PM domain's status changed to GPD_STATE_ACTIVE during
+ * a "power off" operation, which means that a "power on" has occured in the
+ * meantime, or if its resume_count field is different from zero, which means
+ * that one of its devices has been resumed in the meantime.
+ */
+static bool genpd_abort_poweroff(struct generic_pm_domain *genpd)
+{
+ return genpd->status == GPD_STATE_ACTIVE || genpd->resume_count > 0;
+}
+
+/**
* pm_genpd_poweroff - Remove power from a given PM domain.
* @genpd: PM domain to power down.
*
@@ -223,9 +226,17 @@ static int pm_genpd_poweroff(struct gene
struct generic_pm_domain *parent;
struct dev_list_entry *dle;
unsigned int not_suspended;
- int ret;
+ int ret = 0;

- if (genpd->status == GPD_STATE_POWER_OFF || genpd->prepared_count > 0)
+ start:
+ /*
+ * Do not try to power off the domain in the following situations:
+ * (1) The domain is already in the "power off" state.
+ * (2) System suspend is in progress.
+ * (3) One of the domain's devices is being resumed right now.
+ */
+ if (genpd->status == GPD_STATE_POWER_OFF || genpd->prepared_count > 0
+ || genpd->resume_count > 0)
return 0;

if (genpd->sd_count > 0)
@@ -239,34 +250,54 @@ static int pm_genpd_poweroff(struct gene
if (not_suspended > genpd->in_progress)
return -EBUSY;

+ if (genpd->poweroff_task) {
+ /*
+ * Another instance of pm_genpd_poweroff() is executing
+ * callbacks, so tell it to start over and return.
+ */
+ genpd->status = GPD_STATE_REPEAT;
+ return 0;
+ }
+
if (genpd->gov && genpd->gov->power_down_ok) {
if (!genpd->gov->power_down_ok(&genpd->domain))
return -EAGAIN;
}

genpd->status = GPD_STATE_BUSY;
+ genpd->poweroff_task = current;

list_for_each_entry_reverse(dle, &genpd->dev_list, node) {
ret = __pm_genpd_save_device(dle, genpd);
if (ret)
goto err_dev;
- }

- mutex_unlock(&genpd->lock);
+ if (genpd_abort_poweroff(genpd))
+ goto out;
+
+ if (genpd->status == GPD_STATE_REPEAT) {
+ genpd->poweroff_task = NULL;
+ goto start;
+ }
+ }

parent = genpd->parent;
if (parent) {
+ mutex_unlock(&genpd->lock);
+
genpd_acquire_lock(parent);
mutex_lock_nested(&genpd->lock, SINGLE_DEPTH_NESTING);
- } else {
- mutex_lock(&genpd->lock);
+
+ if (genpd_abort_poweroff(genpd)) {
+ genpd_release_lock(parent);
+ goto out;
+ }
}

if (genpd->power_off)
genpd->power_off(genpd);

genpd->status = GPD_STATE_POWER_OFF;
- wake_up_all(&genpd->status_wait_queue);

if (parent) {
genpd_sd_counter_dec(parent);
@@ -276,16 +307,17 @@ static int pm_genpd_poweroff(struct gene
genpd_release_lock(parent);
}

- return 0;
+ out:
+ genpd->poweroff_task = NULL;
+ wake_up_all(&genpd->status_wait_queue);
+ return ret;

err_dev:
list_for_each_entry_continue(dle, &genpd->dev_list, node)
__pm_genpd_restore_device(dle, genpd);

- genpd->status = GPD_STATE_ACTIVE;
- wake_up_all(&genpd->status_wait_queue);
-
- return ret;
+ genpd_set_active(genpd);
+ goto out;
}

/**
@@ -327,11 +359,11 @@ static int pm_genpd_runtime_suspend(stru
return ret;
}

- genpd_acquire_lock(genpd);
+ mutex_lock(&genpd->lock);
genpd->in_progress++;
pm_genpd_poweroff(genpd);
genpd->in_progress--;
- genpd_release_lock(genpd);
+ mutex_unlock(&genpd->lock);

return 0;
}
@@ -365,6 +397,7 @@ static void __pm_genpd_runtime_resume(st
static int pm_genpd_runtime_resume(struct device *dev)
{
struct generic_pm_domain *genpd;
+ DEFINE_WAIT(wait);
int ret;

dev_dbg(dev, "%s()\n", __func__);
@@ -377,12 +410,31 @@ static int pm_genpd_runtime_resume(struc
if (ret)
return ret;

- genpd_acquire_lock(genpd);
+ mutex_lock(&genpd->lock);
genpd->status = GPD_STATE_BUSY;
+ genpd->resume_count++;
+ for (;;) {
+ prepare_to_wait(&genpd->status_wait_queue, &wait,
+ TASK_UNINTERRUPTIBLE);
+ /*
+ * If current is the powering off task, we have been called
+ * reentrantly from one of the device callbacks, so we should
+ * not wait.
+ */
+ if (!genpd->poweroff_task || genpd->poweroff_task == current)
+ break;
+ mutex_unlock(&genpd->lock);
+
+ schedule();
+
+ mutex_lock(&genpd->lock);
+ }
+ finish_wait(&genpd->status_wait_queue, &wait);
__pm_genpd_runtime_resume(dev, genpd);
- genpd->status = GPD_STATE_ACTIVE;
+ genpd->resume_count--;
+ genpd_set_active(genpd);
wake_up_all(&genpd->status_wait_queue);
- genpd_release_lock(genpd);
+ mutex_unlock(&genpd->lock);

if (genpd->start_device)
genpd->start_device(dev);
@@ -1130,6 +1182,8 @@ void pm_genpd_init(struct generic_pm_dom
genpd->sd_count = 0;
genpd->status = is_off ? GPD_STATE_POWER_OFF : GPD_STATE_ACTIVE;
init_waitqueue_head(&genpd->status_wait_queue);
+ genpd->poweroff_task = NULL;
+ genpd->resume_count = 0;
genpd->device_count = 0;
genpd->suspended_count = 0;
genpd->domain.ops.runtime_suspend = pm_genpd_runtime_suspend;
Index: linux-2.6/include/linux/pm_domain.h
===================================================================
--- linux-2.6.orig/include/linux/pm_domain.h
+++ linux-2.6/include/linux/pm_domain.h
@@ -14,6 +14,7 @@
enum gpd_status {
GPD_STATE_ACTIVE = 0, /* PM domain is active */
GPD_STATE_BUSY, /* Something is happening to the PM domain */
+ GPD_STATE_REPEAT, /* Power off in progress, to be repeated */
GPD_STATE_POWER_OFF, /* PM domain is off */
};

@@ -34,6 +35,8 @@ struct generic_pm_domain {
unsigned int sd_count; /* Number of subdomains with power "on" */
enum gpd_status status; /* Current state of the domain */
wait_queue_head_t status_wait_queue;
+ struct task_struct *poweroff_task; /* Powering off task */
+ unsigned int resume_count; /* Number of devices being resumed */
unsigned int device_count; /* Number of devices */
unsigned int suspended_count; /* System suspend device counter */
unsigned int prepared_count; /* Suspend counter of prepared devices */


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