From: Rafael J. Wysocki <[email protected]>
A PM-runtime device usage count underflow is potentially critical,
because it may cause a device to be suspended when it is expected to
be operational.
For this reason, (1) make rpm_check_suspend_allowed() return an error
when the device usage count is negative to prevent devices from being
suspended in that case, (2) introduce rpm_drop_usage_count() that will
detect device usage count underflows, warn about them and fix them up,
and (3) use it to drop the usage count in a few places instead of
atomic_dec_and_test().
Signed-off-by: Rafael J. Wysocki <[email protected]>
---
drivers/base/power/runtime.c | 44 ++++++++++++++++++++++++++++++++++++-------
1 file changed, 37 insertions(+), 7 deletions(-)
Index: linux-pm/drivers/base/power/runtime.c
===================================================================
--- linux-pm.orig/drivers/base/power/runtime.c
+++ linux-pm/drivers/base/power/runtime.c
@@ -263,7 +263,7 @@ static int rpm_check_suspend_allowed(str
retval = -EINVAL;
else if (dev->power.disable_depth > 0)
retval = -EACCES;
- else if (atomic_read(&dev->power.usage_count) > 0)
+ else if (atomic_read(&dev->power.usage_count))
retval = -EAGAIN;
else if (!dev->power.ignore_children &&
atomic_read(&dev->power.child_count))
@@ -1039,13 +1039,33 @@ int pm_schedule_suspend(struct device *d
}
EXPORT_SYMBOL_GPL(pm_schedule_suspend);
+static int rpm_drop_usage_count(struct device *dev)
+{
+ int ret;
+
+ ret = atomic_sub_return(1, &dev->power.usage_count);
+ if (ret >= 0)
+ return ret;
+
+ /*
+ * Because rpm_resume() does not check the usage counter, it will resume
+ * the device even if the usage counter is 0 or negative, so it is
+ * sufficient to increment the usage counter here to reverse the change
+ * made above.
+ */
+ atomic_inc(&dev->power.usage_count);
+ dev_warn(dev, "Runtime PM usage count underflow!\n");
+ return -EINVAL;
+}
+
/**
* __pm_runtime_idle - Entry point for runtime idle operations.
* @dev: Device to send idle notification for.
* @rpmflags: Flag bits.
*
* If the RPM_GET_PUT flag is set, decrement the device's usage count and
- * return immediately if it is larger than zero. Then carry out an idle
+ * return immediately if it is larger than zero (if it becomes negative, log a
+ * warning, increment it, and return an error). Then carry out an idle
* notification, either synchronous or asynchronous.
*
* This routine may be called in atomic context if the RPM_ASYNC flag is set,
@@ -1057,9 +1077,12 @@ int __pm_runtime_idle(struct device *dev
int retval;
if (rpmflags & RPM_GET_PUT) {
- if (!atomic_dec_and_test(&dev->power.usage_count)) {
+ retval = rpm_drop_usage_count(dev);
+ if (retval > 0) {
trace_rpm_usage_rcuidle(dev, rpmflags);
return 0;
+ } else if (retval < 0) {
+ return retval;
}
}
@@ -1079,7 +1102,8 @@ EXPORT_SYMBOL_GPL(__pm_runtime_idle);
* @rpmflags: Flag bits.
*
* If the RPM_GET_PUT flag is set, decrement the device's usage count and
- * return immediately if it is larger than zero. Then carry out a suspend,
+ * return immediately if it is larger than zero (if it becomes negative, log a
+ * warning, increment it, and return an error). Then carry out a suspend,
* either synchronous or asynchronous.
*
* This routine may be called in atomic context if the RPM_ASYNC flag is set,
@@ -1091,9 +1115,12 @@ int __pm_runtime_suspend(struct device *
int retval;
if (rpmflags & RPM_GET_PUT) {
- if (!atomic_dec_and_test(&dev->power.usage_count)) {
+ retval = rpm_drop_usage_count(dev);
+ if (retval > 0) {
trace_rpm_usage_rcuidle(dev, rpmflags);
return 0;
+ } else if (retval < 0) {
+ return retval;
}
}
@@ -1527,14 +1554,17 @@ EXPORT_SYMBOL_GPL(pm_runtime_forbid);
*/
void pm_runtime_allow(struct device *dev)
{
+ int ret;
+
spin_lock_irq(&dev->power.lock);
if (dev->power.runtime_auto)
goto out;
dev->power.runtime_auto = true;
- if (atomic_dec_and_test(&dev->power.usage_count))
+ ret = rpm_drop_usage_count(dev);
+ if (ret == 0)
rpm_idle(dev, RPM_AUTO | RPM_ASYNC);
- else
+ else if (ret > 0)
trace_rpm_usage_rcuidle(dev, RPM_AUTO | RPM_ASYNC);
out:
On Fri, Apr 8, 2022 at 4:05 PM Ulf Hansson <[email protected]> wrote:
>
> On Wed, 6 Apr 2022 at 21:03, Rafael J. Wysocki <[email protected]> wrote:
> >
> > From: Rafael J. Wysocki <[email protected]>
> >
> > A PM-runtime device usage count underflow is potentially critical,
> > because it may cause a device to be suspended when it is expected to
> > be operational.
>
> I get the point. Although, perhaps we should also state that it's a
> programming problem that we would like to catch and warn about?
OK, I can add that to the changelog.
> >
> > For this reason, (1) make rpm_check_suspend_allowed() return an error
> > when the device usage count is negative to prevent devices from being
> > suspended in that case, (2) introduce rpm_drop_usage_count() that will
> > detect device usage count underflows, warn about them and fix them up,
> > and (3) use it to drop the usage count in a few places instead of
> > atomic_dec_and_test().
> >
> > Signed-off-by: Rafael J. Wysocki <[email protected]>
> > ---
> > drivers/base/power/runtime.c | 44 ++++++++++++++++++++++++++++++++++++-------
> > 1 file changed, 37 insertions(+), 7 deletions(-)
> >
> > Index: linux-pm/drivers/base/power/runtime.c
> > ===================================================================
> > --- linux-pm.orig/drivers/base/power/runtime.c
> > +++ linux-pm/drivers/base/power/runtime.c
> > @@ -263,7 +263,7 @@ static int rpm_check_suspend_allowed(str
> > retval = -EINVAL;
> > else if (dev->power.disable_depth > 0)
> > retval = -EACCES;
> > - else if (atomic_read(&dev->power.usage_count) > 0)
> > + else if (atomic_read(&dev->power.usage_count))
> > retval = -EAGAIN;
> > else if (!dev->power.ignore_children &&
> > atomic_read(&dev->power.child_count))
> > @@ -1039,13 +1039,33 @@ int pm_schedule_suspend(struct device *d
> > }
> > EXPORT_SYMBOL_GPL(pm_schedule_suspend);
> >
> > +static int rpm_drop_usage_count(struct device *dev)
> > +{
> > + int ret;
> > +
> > + ret = atomic_sub_return(1, &dev->power.usage_count);
> > + if (ret >= 0)
> > + return ret;
> > +
> > + /*
> > + * Because rpm_resume() does not check the usage counter, it will resume
> > + * the device even if the usage counter is 0 or negative, so it is
> > + * sufficient to increment the usage counter here to reverse the change
> > + * made above.
> > + */
> > + atomic_inc(&dev->power.usage_count);
>
> Rather than this two-step process, couldn't we just do an
> "atomic_add_unless(&dev->power.usage_count, -1, 0)" - and check the
> return value?
No, we couldn't, because atomic_add_unless() returns a bool and we
need to know the new counter value (and in particular whether or not
it is 0).
I thought that it would be better to do the extra access in the
failing case only.
> > + dev_warn(dev, "Runtime PM usage count underflow!\n");
> > + return -EINVAL;
> > +}
> > +
>
> [...]
On Wed, 6 Apr 2022 at 21:03, Rafael J. Wysocki <[email protected]> wrote:
>
> From: Rafael J. Wysocki <[email protected]>
>
> A PM-runtime device usage count underflow is potentially critical,
> because it may cause a device to be suspended when it is expected to
> be operational.
I get the point. Although, perhaps we should also state that it's a
programming problem that we would like to catch and warn about?
>
> For this reason, (1) make rpm_check_suspend_allowed() return an error
> when the device usage count is negative to prevent devices from being
> suspended in that case, (2) introduce rpm_drop_usage_count() that will
> detect device usage count underflows, warn about them and fix them up,
> and (3) use it to drop the usage count in a few places instead of
> atomic_dec_and_test().
>
> Signed-off-by: Rafael J. Wysocki <[email protected]>
> ---
> drivers/base/power/runtime.c | 44 ++++++++++++++++++++++++++++++++++++-------
> 1 file changed, 37 insertions(+), 7 deletions(-)
>
> Index: linux-pm/drivers/base/power/runtime.c
> ===================================================================
> --- linux-pm.orig/drivers/base/power/runtime.c
> +++ linux-pm/drivers/base/power/runtime.c
> @@ -263,7 +263,7 @@ static int rpm_check_suspend_allowed(str
> retval = -EINVAL;
> else if (dev->power.disable_depth > 0)
> retval = -EACCES;
> - else if (atomic_read(&dev->power.usage_count) > 0)
> + else if (atomic_read(&dev->power.usage_count))
> retval = -EAGAIN;
> else if (!dev->power.ignore_children &&
> atomic_read(&dev->power.child_count))
> @@ -1039,13 +1039,33 @@ int pm_schedule_suspend(struct device *d
> }
> EXPORT_SYMBOL_GPL(pm_schedule_suspend);
>
> +static int rpm_drop_usage_count(struct device *dev)
> +{
> + int ret;
> +
> + ret = atomic_sub_return(1, &dev->power.usage_count);
> + if (ret >= 0)
> + return ret;
> +
> + /*
> + * Because rpm_resume() does not check the usage counter, it will resume
> + * the device even if the usage counter is 0 or negative, so it is
> + * sufficient to increment the usage counter here to reverse the change
> + * made above.
> + */
> + atomic_inc(&dev->power.usage_count);
Rather than this two-step process, couldn't we just do an
"atomic_add_unless(&dev->power.usage_count, -1, 0)" - and check the
return value?
> + dev_warn(dev, "Runtime PM usage count underflow!\n");
> + return -EINVAL;
> +}
> +
[...]
Kind regards
Uffe
On Mon, Apr 11, 2022 at 6:17 PM Rafael J. Wysocki <[email protected]> wrote:
> On Mon, Apr 11, 2022 at 5:09 PM Andy Shevchenko
> <[email protected]> wrote:
> > On Wed, Apr 6, 2022 at 11:49 PM Rafael J. Wysocki <[email protected]> wrote:
...
> > > + retval = rpm_drop_usage_count(dev);
> > > + if (retval > 0) {
> > > trace_rpm_usage_rcuidle(dev, rpmflags);
> > > return 0;
> > > + } else if (retval < 0) {
> > > + return retval;
> > > }
> >
> > Can be written in a form
> >
> > if (retval < 0)
> > return retval;
> > if (retval > 0) {
> > trace_rpm_usage_rcuidle(dev, rpmflags);
> > return 0;
> > }
> >
>
> I know.
>
> And why would it be better?
Depends on the perception:
a) less characters to parse (no 'else');
b) checking for errors first, which seems more or less standard pattern.
--
With Best Regards,
Andy Shevchenko
On Fri, 8 Apr 2022 at 19:05, Rafael J. Wysocki <[email protected]> wrote:
>
> On Fri, Apr 8, 2022 at 4:05 PM Ulf Hansson <[email protected]> wrote:
> >
> > On Wed, 6 Apr 2022 at 21:03, Rafael J. Wysocki <[email protected]> wrote:
> > >
> > > From: Rafael J. Wysocki <[email protected]>
> > >
> > > A PM-runtime device usage count underflow is potentially critical,
> > > because it may cause a device to be suspended when it is expected to
> > > be operational.
> >
> > I get the point. Although, perhaps we should also state that it's a
> > programming problem that we would like to catch and warn about?
>
> OK, I can add that to the changelog.
>
> > >
> > > For this reason, (1) make rpm_check_suspend_allowed() return an error
> > > when the device usage count is negative to prevent devices from being
> > > suspended in that case, (2) introduce rpm_drop_usage_count() that will
> > > detect device usage count underflows, warn about them and fix them up,
> > > and (3) use it to drop the usage count in a few places instead of
> > > atomic_dec_and_test().
> > >
> > > Signed-off-by: Rafael J. Wysocki <[email protected]>
> > > ---
> > > drivers/base/power/runtime.c | 44 ++++++++++++++++++++++++++++++++++++-------
> > > 1 file changed, 37 insertions(+), 7 deletions(-)
> > >
> > > Index: linux-pm/drivers/base/power/runtime.c
> > > ===================================================================
> > > --- linux-pm.orig/drivers/base/power/runtime.c
> > > +++ linux-pm/drivers/base/power/runtime.c
> > > @@ -263,7 +263,7 @@ static int rpm_check_suspend_allowed(str
> > > retval = -EINVAL;
> > > else if (dev->power.disable_depth > 0)
> > > retval = -EACCES;
> > > - else if (atomic_read(&dev->power.usage_count) > 0)
> > > + else if (atomic_read(&dev->power.usage_count))
> > > retval = -EAGAIN;
> > > else if (!dev->power.ignore_children &&
> > > atomic_read(&dev->power.child_count))
> > > @@ -1039,13 +1039,33 @@ int pm_schedule_suspend(struct device *d
> > > }
> > > EXPORT_SYMBOL_GPL(pm_schedule_suspend);
> > >
> > > +static int rpm_drop_usage_count(struct device *dev)
> > > +{
> > > + int ret;
> > > +
> > > + ret = atomic_sub_return(1, &dev->power.usage_count);
> > > + if (ret >= 0)
> > > + return ret;
> > > +
> > > + /*
> > > + * Because rpm_resume() does not check the usage counter, it will resume
> > > + * the device even if the usage counter is 0 or negative, so it is
> > > + * sufficient to increment the usage counter here to reverse the change
> > > + * made above.
> > > + */
> > > + atomic_inc(&dev->power.usage_count);
> >
> > Rather than this two-step process, couldn't we just do an
> > "atomic_add_unless(&dev->power.usage_count, -1, 0)" - and check the
> > return value?
>
> No, we couldn't, because atomic_add_unless() returns a bool and we
> need to know the new counter value (and in particular whether or not
> it is 0).
atomic_add_unless(&dev->power.usage_count, -1, 0) would return true as
long as the counter value is greater than 0.
If the counter has become 0, atomic_add_unless() would return false
and not continue to decrease the value below zero. Isn't this exactly
what we want?
>
> I thought that it would be better to do the extra access in the
> failing case only.
>
> > > + dev_warn(dev, "Runtime PM usage count underflow!\n");
> > > + return -EINVAL;
> > > +}
> > > +
> >
> > [...]
Kind regards
Uffe
On Mon, Apr 11, 2022 at 12:36 PM Ulf Hansson <[email protected]> wrote:
>
> On Fri, 8 Apr 2022 at 19:05, Rafael J. Wysocki <[email protected]> wrote:
> >
> > On Fri, Apr 8, 2022 at 4:05 PM Ulf Hansson <[email protected]> wrote:
> > >
> > > On Wed, 6 Apr 2022 at 21:03, Rafael J. Wysocki <[email protected]> wrote:
> > > >
> > > > From: Rafael J. Wysocki <[email protected]>
> > > >
> > > > A PM-runtime device usage count underflow is potentially critical,
> > > > because it may cause a device to be suspended when it is expected to
> > > > be operational.
> > >
> > > I get the point. Although, perhaps we should also state that it's a
> > > programming problem that we would like to catch and warn about?
> >
> > OK, I can add that to the changelog.
> >
> > > >
> > > > For this reason, (1) make rpm_check_suspend_allowed() return an error
> > > > when the device usage count is negative to prevent devices from being
> > > > suspended in that case, (2) introduce rpm_drop_usage_count() that will
> > > > detect device usage count underflows, warn about them and fix them up,
> > > > and (3) use it to drop the usage count in a few places instead of
> > > > atomic_dec_and_test().
> > > >
> > > > Signed-off-by: Rafael J. Wysocki <[email protected]>
> > > > ---
> > > > drivers/base/power/runtime.c | 44 ++++++++++++++++++++++++++++++++++++-------
> > > > 1 file changed, 37 insertions(+), 7 deletions(-)
> > > >
> > > > Index: linux-pm/drivers/base/power/runtime.c
> > > > ===================================================================
> > > > --- linux-pm.orig/drivers/base/power/runtime.c
> > > > +++ linux-pm/drivers/base/power/runtime.c
> > > > @@ -263,7 +263,7 @@ static int rpm_check_suspend_allowed(str
> > > > retval = -EINVAL;
> > > > else if (dev->power.disable_depth > 0)
> > > > retval = -EACCES;
> > > > - else if (atomic_read(&dev->power.usage_count) > 0)
> > > > + else if (atomic_read(&dev->power.usage_count))
> > > > retval = -EAGAIN;
> > > > else if (!dev->power.ignore_children &&
> > > > atomic_read(&dev->power.child_count))
> > > > @@ -1039,13 +1039,33 @@ int pm_schedule_suspend(struct device *d
> > > > }
> > > > EXPORT_SYMBOL_GPL(pm_schedule_suspend);
> > > >
> > > > +static int rpm_drop_usage_count(struct device *dev)
> > > > +{
> > > > + int ret;
> > > > +
> > > > + ret = atomic_sub_return(1, &dev->power.usage_count);
> > > > + if (ret >= 0)
> > > > + return ret;
> > > > +
> > > > + /*
> > > > + * Because rpm_resume() does not check the usage counter, it will resume
> > > > + * the device even if the usage counter is 0 or negative, so it is
> > > > + * sufficient to increment the usage counter here to reverse the change
> > > > + * made above.
> > > > + */
> > > > + atomic_inc(&dev->power.usage_count);
> > >
> > > Rather than this two-step process, couldn't we just do an
> > > "atomic_add_unless(&dev->power.usage_count, -1, 0)" - and check the
> > > return value?
> >
> > No, we couldn't, because atomic_add_unless() returns a bool and we
> > need to know the new counter value (and in particular whether or not
> > it is 0).
>
> atomic_add_unless(&dev->power.usage_count, -1, 0) would return true as
> long as the counter value is greater than 0.
Yes, and it in particular, when the current value of the counter is 1
before the operation IIUC.
So after the operation it is 0 and true will be returned, won't it?
But that's exactly the case we want to catch.
> If the counter has become 0, atomic_add_unless() would return false
> and not continue to decrease the value below zero. Isn't this exactly
> what we want?
Not really.
We want to detect transitions from 0 to 1 which is the case when the
device can be suspended.
On Wed, Apr 6, 2022 at 11:49 PM Rafael J. Wysocki <[email protected]> wrote:
>
> From: Rafael J. Wysocki <[email protected]>
>
> A PM-runtime device usage count underflow is potentially critical,
> because it may cause a device to be suspended when it is expected to
> be operational.
>
> For this reason, (1) make rpm_check_suspend_allowed() return an error
> when the device usage count is negative to prevent devices from being
> suspended in that case, (2) introduce rpm_drop_usage_count() that will
> detect device usage count underflows, warn about them and fix them up,
> and (3) use it to drop the usage count in a few places instead of
> atomic_dec_and_test().
...
> + retval = rpm_drop_usage_count(dev);
> + if (retval > 0) {
> trace_rpm_usage_rcuidle(dev, rpmflags);
> return 0;
> + } else if (retval < 0) {
> + return retval;
> }
Can be written in a form
if (retval < 0)
return retval;
if (retval > 0) {
trace_rpm_usage_rcuidle(dev, rpmflags);
return 0;
}
...
> + if (retval > 0) {
> trace_rpm_usage_rcuidle(dev, rpmflags);
> return 0;
> + } else if (retval < 0) {
> + return retval;
> }
Ditto.
--
With Best Regards,
Andy Shevchenko
On Mon, Apr 11, 2022 at 6:53 PM Andy Shevchenko
<[email protected]> wrote:
>
> On Mon, Apr 11, 2022 at 6:17 PM Rafael J. Wysocki <[email protected]> wrote:
> > On Mon, Apr 11, 2022 at 5:09 PM Andy Shevchenko
> > <[email protected]> wrote:
> > > On Wed, Apr 6, 2022 at 11:49 PM Rafael J. Wysocki <[email protected]> wrote:
>
> ...
>
> > > > + retval = rpm_drop_usage_count(dev);
> > > > + if (retval > 0) {
> > > > trace_rpm_usage_rcuidle(dev, rpmflags);
> > > > return 0;
> > > > + } else if (retval < 0) {
> > > > + return retval;
> > > > }
> > >
> > > Can be written in a form
> > >
> > > if (retval < 0)
> > > return retval;
> > > if (retval > 0) {
> > > trace_rpm_usage_rcuidle(dev, rpmflags);
> > > return 0;
> > > }
> > >
> >
> > I know.
> >
> > And why would it be better?
>
> Depends on the perception:
Well, exactly.
> a) less characters to parse (no 'else');
But to me, with the "else" it is clear that the conditionals are
related to each other which is not so clear otherwise at first sight.
YMMV
> b) checking for errors first, which seems more or less standard pattern.
So the checks can be reversed no problem, but this is such a minor point ,,,
On Mon, Apr 11, 2022 at 5:09 PM Andy Shevchenko
<[email protected]> wrote:
>
> On Wed, Apr 6, 2022 at 11:49 PM Rafael J. Wysocki <[email protected]> wrote:
> >
> > From: Rafael J. Wysocki <[email protected]>
> >
> > A PM-runtime device usage count underflow is potentially critical,
> > because it may cause a device to be suspended when it is expected to
> > be operational.
> >
> > For this reason, (1) make rpm_check_suspend_allowed() return an error
> > when the device usage count is negative to prevent devices from being
> > suspended in that case, (2) introduce rpm_drop_usage_count() that will
> > detect device usage count underflows, warn about them and fix them up,
> > and (3) use it to drop the usage count in a few places instead of
> > atomic_dec_and_test().
>
> ...
>
> > + retval = rpm_drop_usage_count(dev);
> > + if (retval > 0) {
> > trace_rpm_usage_rcuidle(dev, rpmflags);
> > return 0;
> > + } else if (retval < 0) {
> > + return retval;
> > }
>
> Can be written in a form
>
> if (retval < 0)
> return retval;
> if (retval > 0) {
> trace_rpm_usage_rcuidle(dev, rpmflags);
> return 0;
> }
>
I know.
And why would it be better?
On Mon, 11 Apr 2022 at 13:29, Rafael J. Wysocki <[email protected]> wrote:
>
> On Mon, Apr 11, 2022 at 12:36 PM Ulf Hansson <[email protected]> wrote:
> >
> > On Fri, 8 Apr 2022 at 19:05, Rafael J. Wysocki <[email protected]> wrote:
> > >
> > > On Fri, Apr 8, 2022 at 4:05 PM Ulf Hansson <[email protected]> wrote:
> > > >
> > > > On Wed, 6 Apr 2022 at 21:03, Rafael J. Wysocki <[email protected]> wrote:
> > > > >
> > > > > From: Rafael J. Wysocki <[email protected]>
> > > > >
> > > > > A PM-runtime device usage count underflow is potentially critical,
> > > > > because it may cause a device to be suspended when it is expected to
> > > > > be operational.
> > > >
> > > > I get the point. Although, perhaps we should also state that it's a
> > > > programming problem that we would like to catch and warn about?
> > >
> > > OK, I can add that to the changelog.
> > >
> > > > >
> > > > > For this reason, (1) make rpm_check_suspend_allowed() return an error
> > > > > when the device usage count is negative to prevent devices from being
> > > > > suspended in that case, (2) introduce rpm_drop_usage_count() that will
> > > > > detect device usage count underflows, warn about them and fix them up,
> > > > > and (3) use it to drop the usage count in a few places instead of
> > > > > atomic_dec_and_test().
> > > > >
> > > > > Signed-off-by: Rafael J. Wysocki <[email protected]>
> > > > > ---
> > > > > drivers/base/power/runtime.c | 44 ++++++++++++++++++++++++++++++++++++-------
> > > > > 1 file changed, 37 insertions(+), 7 deletions(-)
> > > > >
> > > > > Index: linux-pm/drivers/base/power/runtime.c
> > > > > ===================================================================
> > > > > --- linux-pm.orig/drivers/base/power/runtime.c
> > > > > +++ linux-pm/drivers/base/power/runtime.c
> > > > > @@ -263,7 +263,7 @@ static int rpm_check_suspend_allowed(str
> > > > > retval = -EINVAL;
> > > > > else if (dev->power.disable_depth > 0)
> > > > > retval = -EACCES;
> > > > > - else if (atomic_read(&dev->power.usage_count) > 0)
> > > > > + else if (atomic_read(&dev->power.usage_count))
> > > > > retval = -EAGAIN;
> > > > > else if (!dev->power.ignore_children &&
> > > > > atomic_read(&dev->power.child_count))
> > > > > @@ -1039,13 +1039,33 @@ int pm_schedule_suspend(struct device *d
> > > > > }
> > > > > EXPORT_SYMBOL_GPL(pm_schedule_suspend);
> > > > >
> > > > > +static int rpm_drop_usage_count(struct device *dev)
> > > > > +{
> > > > > + int ret;
> > > > > +
> > > > > + ret = atomic_sub_return(1, &dev->power.usage_count);
> > > > > + if (ret >= 0)
> > > > > + return ret;
> > > > > +
> > > > > + /*
> > > > > + * Because rpm_resume() does not check the usage counter, it will resume
> > > > > + * the device even if the usage counter is 0 or negative, so it is
> > > > > + * sufficient to increment the usage counter here to reverse the change
> > > > > + * made above.
> > > > > + */
> > > > > + atomic_inc(&dev->power.usage_count);
> > > >
> > > > Rather than this two-step process, couldn't we just do an
> > > > "atomic_add_unless(&dev->power.usage_count, -1, 0)" - and check the
> > > > return value?
> > >
> > > No, we couldn't, because atomic_add_unless() returns a bool and we
> > > need to know the new counter value (and in particular whether or not
> > > it is 0).
> >
> > atomic_add_unless(&dev->power.usage_count, -1, 0) would return true as
> > long as the counter value is greater than 0.
>
> Yes, and it in particular, when the current value of the counter is 1
> before the operation IIUC.
>
> So after the operation it is 0 and true will be returned, won't it?
> But that's exactly the case we want to catch.
>
> > If the counter has become 0, atomic_add_unless() would return false
> > and not continue to decrease the value below zero. Isn't this exactly
> > what we want?
>
> Not really.
>
> We want to detect transitions from 0 to 1 which is the case when the
> device can be suspended.
I assume you mean from 1 to 0. In any case, I see what you mean by
now, sorry for the noise.
Then feel free to add:
Reviewed-by: Ulf Hansson <[email protected]>
Kind regards
Uffe