linux-kernel - [RFC PATCH RT] PM: runtime: avoid retry loops on RT

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Message-Id: <20211005155427.1591196-1-john@metanate.com>
Date:   Tue,  5 Oct 2021 16:54:27 +0100
From:   John Keeping <john@...anate.com>
To:     linux-rt-users@...r.kernel.org
Cc:     John Keeping <john@...anate.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Pavel Machek <pavel@....cz>, Len Brown <len.brown@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [RFC PATCH RT] PM: runtime: avoid retry loops on RT

With PREEMPT_RT spin_unlock() is identical to spin_unlock_irq() so there
is no reason to have a special case using the former.  Furthermore,
spin_unlock() enables preemption meaning that a task in RESUMING or
SUSPENDING state may be preempted by a higher priority task running
pm_runtime_get_sync() leading to a livelock.

Use the non-irq_safe path for all waiting so that the waiting task will
block.

Note that this changes only the waiting behaviour of irq_safe, other
uses are left unchanged so that the parent device always remains active
in the same way as !RT.

Signed-off-by: John Keeping <john@...anate.com>
---
 drivers/base/power/runtime.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/base/power/runtime.c b/drivers/base/power/runtime.c
index 96972d5f6ef3..5e0d349fab4e 100644
--- a/drivers/base/power/runtime.c
+++ b/drivers/base/power/runtime.c
@@ -347,8 +347,9 @@ static int __rpm_callback(int (*cb)(struct device *), struct device *dev)
 {
 	int retval = 0, idx;
 	bool use_links = dev->power.links_count > 0;
+	bool irq_safe = dev->power.irq_safe && !IS_ENABLED(CONFIG_PREEMPT_RT);
 
-	if (dev->power.irq_safe) {
+	if (irq_safe) {
 		spin_unlock(&dev->power.lock);
 	} else {
 		spin_unlock_irq(&dev->power.lock);
@@ -376,7 +377,7 @@ static int __rpm_callback(int (*cb)(struct device *), struct device *dev)
 	if (cb)
 		retval = cb(dev);
 
-	if (dev->power.irq_safe) {
+	if (irq_safe) {
 		spin_lock(&dev->power.lock);
 	} else {
 		/*
@@ -596,7 +597,7 @@ 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();
@@ -777,7 +778,7 @@ 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();
-- 
2.33.0