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>

1 files changed, 1 insertions, 11 deletions

diff --git a/drivers/char/apm-emulation.c b/drivers/char/apm-emulation.c
index 179c7a3b6e75..ec116df919d9 100644
--- a/drivers/char/apm-emulation.c
+++ b/drivers/char/apm-emulation.c
@@ -20,6 +20,7 @@
 #include <linux/sched.h>
 #include <linux/pm.h>
 #include <linux/apm-emulation.h>
+#include <linux/freezer.h>
 #include <linux/device.h>
 #include <linux/kernel.h>
 #include <linux/list.h>
@@ -329,13 +330,8 @@ apm_ioctl(struct inode * inode, struct file *filp, u_int cmd, u_long arg)
                         /*
                          * Wait for the suspend/resume to complete.  If there
                          * are pending acknowledges, we wait here for them.
-                         *
-                         * Note: we need to ensure that the PM subsystem does
-                         * not kick us out of the wait when it suspends the
-                         * threads.
                          */
                         flags = current->flags;
-                        current->flags |= PF_NOFREEZE;
 
                         wait_event(apm_suspend_waitqueue,
                                    as->suspend_state == SUSPEND_DONE);
@@ -365,13 +361,8 @@ apm_ioctl(struct inode * inode, struct file *filp, u_int cmd, u_long arg)
                         /*
                          * Wait for the suspend/resume to complete.  If there
                          * are pending acknowledges, we wait here for them.
-                         *
-                         * Note: we need to ensure that the PM subsystem does
-                         * not kick us out of the wait when it suspends the
-                         * threads.
                          */
                         flags = current->flags;
-                        current->flags |= PF_NOFREEZE;
 
                         wait_event_interruptible(apm_suspend_waitqueue,
                                          as->suspend_state == SUSPEND_DONE);
@@ -598,7 +589,6 @@ static int __init apm_init(void)
                 kapmd_tsk = NULL;
                 return ret;
         }
-        kapmd_tsk->flags |= PF_NOFREEZE;
         wake_up_process(kapmd_tsk);
 
 #ifdef CONFIG_PROC_FS