[PATCH v2] PM: runtime: avoid priority inversion on PREEMPT_RT

[John Keeping]

With PREEMPT_RT the cpu_relax() loops in rpm_suspend and rpm_resume can
cause unbounded latency if they preempt an asynchronous suspend.  The
main scenario where this can happen is when a realtime thread resumes a
device while it is asynchronously suspending on a worker thread.

I'm not convinced this can actually happen in the rpm_suspend case, or
at least it's a lot less likely for a synchronous suspend to run at the
same time as an asynchronous suspend, but both functions are updated
here for symmetry.

For devices setting power.irq_safe, it is possible that RPM functions
will be called with a spinlock held (for example in
pl330_issue_pending()).  This means a normal call to schedule() can't be
used, but to avoid the priority inversion it is necessary to wait and
schedule.  schedule_rtlock() is only available when CONFIG_PREEMPT_RT is
defined, so even though the logic is correct without any preprocessor
guards around schedule_rtlock(), they are necessary for compilation.

Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: John Keeping <john@xxxxxxxxxxxx>
---
Changes since v1:
- Use schedule_rtlock() instead of schedule() for PREEMPT_RT & irq_safe
- Rewritten commit description

 drivers/base/power/runtime.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/drivers/base/power/runtime.c b/drivers/base/power/runtime.c
index f3de7bfc7f5b..fdf461bfae8c 100644
--- a/drivers/base/power/runtime.c
+++ b/drivers/base/power/runtime.c
@@ -596,7 +596,7 @@ static int rpm_suspend(struct device *dev, int rpmflags)
 			goto out;
 		}
 
-		if (dev->power.irq_safe) {
+		if (dev->power.irq_safe && !IS_ENABLED(CONFIG_PREEMPT_RT)) {
 			spin_unlock(&dev->power.lock);
 
 			cpu_relax();
@@ -614,7 +614,12 @@ static int rpm_suspend(struct device *dev, int rpmflags)
 
 			spin_unlock_irq(&dev->power.lock);
 
-			schedule();
+#ifdef CONFIG_PREEMPT_RT
+			if (dev->power.irq_safe)
+				schedule_rtlock();
+			else
+#endif
+				schedule();
 
 			spin_lock_irq(&dev->power.lock);
 		}
@@ -779,7 +784,7 @@ static int rpm_resume(struct device *dev, int rpmflags)
 			goto out;
 		}
 
-		if (dev->power.irq_safe) {
+		if (dev->power.irq_safe && !IS_ENABLED(CONFIG_PREEMPT_RT)) {
 			spin_unlock(&dev->power.lock);
 
 			cpu_relax();
@@ -798,7 +803,12 @@ static int rpm_resume(struct device *dev, int rpmflags)
 
 			spin_unlock_irq(&dev->power.lock);
 
-			schedule();
+#ifdef CONFIG_PREEMPT_RT
+			if (dev->power.irq_safe)
+				schedule_rtlock();
+			else
+#endif
+				schedule();
 
 			spin_lock_irq(&dev->power.lock);
 		}
-- 
2.34.0