Patch "ACPI / drivers: replace acpi_probe_lock spinlock with mutex" 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 / drivers: replace acpi_probe_lock spinlock with mutex

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-drivers-replace-acpi_probe_lock-spinlock-with-mutex.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 [off-list ref] know about it.

From 5331d9cab32ef640b4cd38a43b0858874fbb7168 Mon Sep 17 00:00:00 2001

From: Lorenzo Pieralisi <redacted>
Date: Tue, 16 Aug 2016 16:59:53 +0100
Subject: ACPI / drivers: replace acpi_probe_lock spinlock with mutex

From: Lorenzo Pieralisi <redacted>

commit 5331d9cab32ef640b4cd38a43b0858874fbb7168 upstream.

Commit e647b532275b ("ACPI: Add early device probing infrastructure")
introduced code that allows inserting driver specific
struct acpi_probe_entry probe entries into ACPI linker sections
(one per-subsystem, eg irqchip, clocksource) that are then walked
to retrieve the data and function hooks required to probe the
respective kernel components.

Probing for all entries in a section is triggered through
the __acpi_probe_device_table() function, that in turn, according
to the table ID a given probe entry reports parses the table
with the function retrieved from the respective section structures
(ie struct acpi_probe_entry). Owing to the current ACPI table
parsing implementation, the __acpi_probe_device_table() function
has to share global variables with the acpi_match_madt() function, so
in order to guarantee mutual exclusion locking is required
between the two functions.

Current kernel code implements the locking through the acpi_probe_lock
spinlock; this has the side effect of requiring all code called
within the lock (ie struct acpi_probe_entry.probe_{table/subtbl} hooks)
not to sleep.

However, kernel subsystems that make use of the early probing
infrastructure are relying on kernel APIs that may sleep (eg
irq_domain_alloc_fwnode(), among others) in the function calls
pointed at by struct acpi_probe_entry.{probe_table/subtbl} entries
(eg gic_v2_acpi_init()), which is a bug.

Since __acpi_probe_device_table() is called from context
that is allowed to sleep the acpi_probe_lock spinlock can be replaced
with a mutex; this fixes the issue whilst still guaranteeing
mutual exclusion.

Signed-off-by: Lorenzo Pieralisi <redacted>
Fixes: e647b532275b (ACPI: Add early device probing infrastructure)
Signed-off-by: Rafael J. Wysocki <redacted>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/acpi/scan.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/drivers/acpi/scan.c
+++ b/drivers/acpi/scan.c

@@ -1958,7 +1958,7 @@ int __init acpi_scan_init(void)
 
 static struct acpi_probe_entry *ape;
 static int acpi_probe_count;
-static DEFINE_SPINLOCK(acpi_probe_lock);
+static DEFINE_MUTEX(acpi_probe_mutex);
 
 static int __init acpi_match_madt(struct acpi_subtable_header *header,
 				  const unsigned long end)

@@ -1977,7 +1977,7 @@ int __init __acpi_probe_device_table(str
 	if (acpi_disabled)
 		return 0;
 
-	spin_lock(&acpi_probe_lock);
+	mutex_lock(&acpi_probe_mutex);
 	for (ape = ap_head; nr; ape++, nr--) {
 		if (ACPI_COMPARE_NAME(ACPI_SIG_MADT, ape->id)) {
 			acpi_probe_count = 0;

@@ -1990,7 +1990,7 @@ int __init __acpi_probe_device_table(str
 				count++;
 		}
 	}
-	spin_unlock(&acpi_probe_lock);
+	mutex_unlock(&acpi_probe_mutex);
 
 	return count;
 }

Patches currently in stable-queue which might be from lorenzo.pieralisi@arm.com are

queue-4.4/acpi-drivers-fix-typo-in-acpi_declare_probe_entry-macro.patch
queue-4.4/acpi-drivers-replace-acpi_probe_lock-spinlock-with-mutex.patch