PM / Hibernate: Freeze kernel threads after preallocating memory

There is a problem with the current ordering of hibernate code which
leads to deadlocks in some filesystems' memory shrinkers.  Namely,
some filesystems use freezable kernel threads that are inactive when
the hibernate memory preallocation is carried out.  Those same
filesystems use memory shrinkers that may be triggered by the
hibernate memory preallocation.  If those memory shrinkers wait for
the frozen kernel threads, the hibernate process deadlocks (this
happens with XFS, for one example).

Apparently, it is not technically viable to redesign the filesystems
in question to avoid the situation described above, so the only
possible solution of this issue is to defer the freezing of kernel
threads until the hibernate memory preallocation is done, which is
implemented by this change.

Unfortunately, this requires the memory preallocation to be done
before the "prepare" stage of device freeze, so after this change the
only way drivers can allocate additional memory for their freeze
routines in a clean way is to use PM notifiers.

Reported-by: Christoph <cr2005@u-club.de>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

1 files changed, 20 insertions, 10 deletions

diff --git a/kernel/power/process.c b/kernel/power/process.c
index 0cf3a27a6c9d5..addbbe5531bc4 100644
--- a/kernel/power/process.c
+++ b/kernel/power/process.c
@@ -135,7 +135,7 @@ static int try_to_freeze_tasks(bool sig_only)
 }
 
 /**
- *	freeze_processes - tell processes to enter the refrigerator
+ * freeze_processes - Signal user space processes to enter the refrigerator.
  */
 int freeze_processes(void)
 {
@@ -143,20 +143,30 @@ int freeze_processes(void)
 
 	printk("Freezing user space processes ... ");
 	error = try_to_freeze_tasks(true);
-	if (error)
-		goto Exit;
-	printk("done.\n");
+	if (!error) {
+		printk("done.");
+		oom_killer_disable();
+	}
+	printk("\n");
+	BUG_ON(in_atomic());
+
+	return error;
+}
+
+/**
+ * freeze_kernel_threads - Make freezable kernel threads go to the refrigerator.
+ */
+int freeze_kernel_threads(void)
+{
+	int error;
 
 	printk("Freezing remaining freezable tasks ... ");
 	error = try_to_freeze_tasks(false);
-	if (error)
-		goto Exit;
-	printk("done.");
+	if (!error)
+		printk("done.");
 
-	oom_killer_disable();
- Exit:
-	BUG_ON(in_atomic());
 	printk("\n");
+	BUG_ON(in_atomic());
 
 	return error;
 }