acpi: bus: check once more for an empty list after locking it

[Chris Wright]

-stable review patch.  If anyone has any objections, please let us know.
---------------------

From: Chuck Ebbert <cebbert@xxxxxxxxxx>

upstream commit: f0a37e008750ead1751b7d5e89d220a260a46147

List could have become empty after the unlocked check that was made earlier,
so check again inside the lock.

Should fix https://bugzilla.redhat.com/show_bug.cgi?id=427765

Signed-off-by: Chuck Ebbert <cebbert@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxx>
Cc: Len Brown <lenb@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Chris Wright <chrisw@xxxxxxxxxxxx>
---
 drivers/acpi/bus.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

--- a/drivers/acpi/bus.c
+++ b/drivers/acpi/bus.c
@@ -350,10 +350,11 @@ int acpi_bus_receive_event(struct acpi_b
 	}
 
 	spin_lock_irqsave(&acpi_bus_event_lock, flags);
-	entry =
-	    list_entry(acpi_bus_event_list.next, struct acpi_bus_event, node);
-	if (entry)
+	if (!list_empty(&acpi_bus_event_list)) {
+		entry = list_entry(acpi_bus_event_list.next,
+				   struct acpi_bus_event, node);
 		list_del(&entry->node);
+	}
 	spin_unlock_irqrestore(&acpi_bus_event_lock, flags);
 
 	if (!entry)

-- 
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