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>

4 files changed, 6 insertions, 2 deletions

diff --git a/drivers/usb/atm/ueagle-atm.c b/drivers/usb/atm/ueagle-atm.c
index 4973e147bc79..8f046659b4e9 100644
--- a/drivers/usb/atm/ueagle-atm.c
+++ b/drivers/usb/atm/ueagle-atm.c
@@ -1168,6 +1168,7 @@ static int uea_kthread(void *data)
         struct uea_softc *sc = data;
         int ret = -EAGAIN;
 
+        set_freezable();
         uea_enters(INS_TO_USBDEV(sc));
         while (!kthread_should_stop()) {
                 if (ret < 0 || sc->reset)
diff --git a/drivers/usb/core/hub.c b/drivers/usb/core/hub.c
index 50e79010401c..fd74c50b1804 100644
--- a/drivers/usb/core/hub.c
+++ b/drivers/usb/core/hub.c
@@ -2728,6 +2728,7 @@ loop:
 
 static int hub_thread(void *__unused)
 {
+        set_freezable();
         do {
                 hub_events();
                 wait_event_interruptible(khubd_wait,
diff --git a/drivers/usb/gadget/file_storage.c b/drivers/usb/gadget/file_storage.c
index 8712ef987179..be7a1bd2823b 100644
--- a/drivers/usb/gadget/file_storage.c
+++ b/drivers/usb/gadget/file_storage.c
@@ -3434,6 +3434,9 @@ static int fsg_main_thread(void *fsg_)
         allow_signal(SIGKILL);
         allow_signal(SIGUSR1);
 
+        /* Allow the thread to be frozen */
+        set_freezable();
+
         /* Arrange for userspace references to be interpreted as kernel
          * pointers.  That way we can pass a kernel pointer to a routine
          * that expects a __user pointer and it will work okay. */
diff --git a/drivers/usb/storage/usb.c b/drivers/usb/storage/usb.c
index bef8bcd9bd98..28842d208bb0 100644
--- a/drivers/usb/storage/usb.c
+++ b/drivers/usb/storage/usb.c
@@ -311,8 +311,6 @@ static int usb_stor_control_thread(void * __us)
         struct Scsi_Host *host = us_to_host(us);
         int autopm_rc;
 
-        current->flags |= PF_NOFREEZE;
-
         for(;;) {
                 US_DEBUGP("*** thread sleeping.\n");
                 if(down_interruptible(&us->sema))
@@ -920,6 +918,7 @@ static int usb_stor_scan_thread(void * __us)
         printk(KERN_DEBUG
                 "usb-storage: device found at %d\n", us->pusb_dev->devnum);
 
+        set_freezable();
         /* Wait for the timeout to expire or for a disconnect */
         if (delay_use > 0) {
                 printk(KERN_DEBUG "usb-storage: waiting for device "