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 <[email protected]> know about it.

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

From: Lorenzo Pieralisi <[email protected]>

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.

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

@@ -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 [email protected] 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