Patch "ACPI/processor: Replace racy task affinity logic" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

ACPI/processor: Replace racy task affinity logic

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
acpi-processor-replace-racy-task-affinity-logic.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Thomas Gleixner <[email protected]>
Date: Wed, 12 Apr 2017 22:07:34 +0200
Subject: ACPI/processor: Replace racy task affinity logic

From: Thomas Gleixner <[email protected]>

[ Upstream commit 8153f9ac43897f9f4786b30badc134fcc1a4fb11 ]

acpi_processor_get_throttling() requires to invoke the getter function on the target CPU. This is achieved by temporarily setting the affinity of the calling user space thread to the requested CPU and reset it to the original affinity afterwards.

That's racy vs. CPU hotplug and concurrent affinity settings for that thread resulting in code executing on the wrong CPU and overwriting the new affinity setting.

acpi_processor_get_throttling() is invoked in two ways:

1) The CPU online callback, which is already running on the target CPU and obviously protected against hotplug and not affected by affinity settings.

2) The ACPI driver probe function, which is not protected against hotplug during modprobe.

Switch it over to work_on_cpu() and protect the probe function against CPU hotplug.

---
 drivers/acpi/processor_driver.c     |    7 +++-
 drivers/acpi/processor_throttling.c |   62 ++++++++++++++++++++----------------
 2 files changed, 42 insertions(+), 27 deletions(-)

@@ -270,11 +270,16 @@ err_power_exit:
 static int acpi_processor_start(struct device *dev)
 {
 	struct acpi_device *device = ACPI_COMPANION(dev);
+	int ret;
 
 	if (!device)
 		return -ENODEV;
 
-	return __acpi_processor_start(device);
+	/* Protect against concurrent CPU hotplug operations */
+	get_online_cpus();
+	ret = __acpi_processor_start(device);
+	put_online_cpus();
+	return ret;
 }
 
 static int acpi_processor_stop(struct device *dev)

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/acpi-processor-fix-error-handling-in-__acpi_processor_start.patch queue-4.4/acpi-processor-replace-racy-task-affinity-logic.patch queue-4.4/time-change-posix-clocks-ops-interfaces-to-use-timespec64.patch queue-4.4/genirq-use-irqd_get_trigger_type-to-compare-the-trigger-type-for-shared-irqs.patch queue-4.4/cpufreq-sh-replace-racy-task-affinity-logic.patch