dm crypt: fix a possible hang due to race condition on exit

A kernel thread executes __set_current_state(TASK_INTERRUPTIBLE),
__add_wait_queue, spin_unlock_irq and then tests kthread_should_stop().
It is possible that the processor reorders memory accesses so that
kthread_should_stop() is executed before __set_current_state().  If such
reordering happens, there is a possible race on thread termination:

CPU 0:
calls kthread_should_stop()
	it tests KTHREAD_SHOULD_STOP bit, returns false
CPU 1:
calls kthread_stop(cc->write_thread)
	sets the KTHREAD_SHOULD_STOP bit
	calls wake_up_process on the kernel thread, that sets the thread
	state to TASK_RUNNING
CPU 0:
sets __set_current_state(TASK_INTERRUPTIBLE)
spin_unlock_irq(&cc->write_thread_wait.lock)
schedule() - and the process is stuck and never terminates, because the
	state is TASK_INTERRUPTIBLE and wake_up_process on CPU 1 already
	terminated

Fix this race condition by using a new flag DM_CRYPT_EXIT_THREAD to
signal that the kernel thread should exit.  The flag is set and tested
while holding cc->write_thread_wait.lock, so there is no possibility of
racy access to the flag.

Also, remove the unnecessary set_task_state(current, TASK_RUNNING)
following the schedule() call.  When the process was woken up, its state
was already set to TASK_RUNNING.  Other kernel code also doesn't set the
state to TASK_RUNNING following schedule() (for example,
do_wait_for_common in completion.c doesn't do it).

Fixes: dc2676210c42 ("dm crypt: offload writes to thread")
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org # v4.0+
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
index 917d47e290ae08be08f4c964a3326f1f67acd077..3147c8d09ea84a0a76d0fd7ead35931a89e29aed 100644 (file)
--- a/drivers/md/dm-crypt.c
+++ b/drivers/md/dm-crypt.c
@@ -112,7 +112,8 @@ struct iv_tcw_private {
  * and encrypts / decrypts at the same time.
  */
 enum flags { DM_CRYPT_SUSPENDED, DM_CRYPT_KEY_VALID,
-            DM_CRYPT_SAME_CPU, DM_CRYPT_NO_OFFLOAD };
+            DM_CRYPT_SAME_CPU, DM_CRYPT_NO_OFFLOAD,
+            DM_CRYPT_EXIT_THREAD};
 
 /*
  * The fields in here must be read only after initialization.
@@ -1203,20 +1204,18 @@ continue_locked:
                if (!RB_EMPTY_ROOT(&cc->write_tree))
                        goto pop_from_list;
 
+               if (unlikely(test_bit(DM_CRYPT_EXIT_THREAD, &cc->flags))) {
+                       spin_unlock_irq(&cc->write_thread_wait.lock);
+                       break;
+               }
+
                __set_current_state(TASK_INTERRUPTIBLE);
                __add_wait_queue(&cc->write_thread_wait, &wait);
 
                spin_unlock_irq(&cc->write_thread_wait.lock);
 
-               if (unlikely(kthread_should_stop())) {
-                       set_task_state(current, TASK_RUNNING);
-                       remove_wait_queue(&cc->write_thread_wait, &wait);
-                       break;
-               }
-
                schedule();
 
-               set_task_state(current, TASK_RUNNING);
                spin_lock_irq(&cc->write_thread_wait.lock);
                __remove_wait_queue(&cc->write_thread_wait, &wait);
                goto continue_locked;
@@ -1531,8 +1530,13 @@ static void crypt_dtr(struct dm_target *ti)
        if (!cc)
                return;
 
-       if (cc->write_thread)
+       if (cc->write_thread) {
+               spin_lock_irq(&cc->write_thread_wait.lock);
+               set_bit(DM_CRYPT_EXIT_THREAD, &cc->flags);
+               wake_up_locked(&cc->write_thread_wait);
+               spin_unlock_irq(&cc->write_thread_wait.lock);
                kthread_stop(cc->write_thread);
+       }
 
        if (cc->io_queue)
                destroy_workqueue(cc->io_queue);