Freezer: make kernel threads nonfreezable by default

Currently, the freezer treats all tasks as freezable, except for the kernel
threads that explicitly set the PF_NOFREEZE flag for themselves.  This
approach is problematic, since it requires every kernel thread to either
set PF_NOFREEZE explicitly, or call try_to_freeze(), even if it doesn't
care for the freezing of tasks at all.

It seems better to only require the kernel threads that want to or need to
be frozen to use some freezer-related code and to remove any
freezer-related code from the other (nonfreezable) kernel threads, which is
done in this patch.

The patch causes all kernel threads to be nonfreezable by default (ie.  to
have PF_NOFREEZE set by default) and introduces the set_freezable()
function that should be called by the freezable kernel threads in order to
unset PF_NOFREEZE.  It also makes all of the currently freezable kernel
threads call set_freezable(), so it shouldn't cause any (intentional)
change of behaviour to appear.  Additionally, it updates documentation to
describe the freezing of tasks more accurately.

[akpm@linux-foundation.org: build fixes]
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Nigel Cunningham <nigel@nigel.suspend2.net>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Gautham R Shenoy <ego@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 3 insertions, 1 deletions

diff --git a/drivers/net/wireless/airo.c b/drivers/net/wireless/airo.c
index 1c54908fdc4c..ee1cc14db389 100644
--- a/drivers/net/wireless/airo.c
+++ b/drivers/net/wireless/airo.c
@@ -3086,7 +3086,8 @@ static int airo_thread(void *data) {
         struct net_device *dev = data;
         struct airo_info *ai = dev->priv;
         int locked;
-        
+
+        set_freezable();
         while(1) {
                 /* make swsusp happy with our thread */
                 try_to_freeze();
diff --git a/drivers/net/wireless/libertas/main.c b/drivers/net/wireless/libertas/main.c
index 4a59306a3f05..9f366242c392 100644
--- a/drivers/net/wireless/libertas/main.c
+++ b/drivers/net/wireless/libertas/main.c
@@ -613,6 +613,7 @@ static int wlan_service_main_thread(void *data)
 
         init_waitqueue_entry(&wait, current);
 
+        set_freezable();
         for (;;) {
                 lbs_deb_thread( "main-thread 111: intcounter=%d "
                        "currenttxskb=%p dnld_sent=%d\n",