From bcbd94ff481ec1d7b5c824d90df82d0faafabd35 Mon Sep 17 00:00:00 2001 From: Mikulas Patocka Date: Thu, 19 Nov 2015 07:36:50 -0500 Subject: [PATCH] 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 Cc: stable@vger.kernel.org # v4.0+ Signed-off-by: Mike Snitzer --- drivers/md/dm-crypt.c | 22 +++++++++++++--------- 1 file changed, 13 insertions(+), 9 deletions(-) diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c index 917d47e290ae..3147c8d09ea8 100644 --- 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);