[PATCH] sched: Avoid that __wait_on_bit_lock() hangs

[Bart Van Assche]

If try_to_wakeup() reads the task state before abort_exclusive_wait()
sets the task state and if autoremove_wake_function() is called after
abort_exclusive_wait() has removed a task from a wait list then the
cascading mechanism for exclusive wakeups in abort_exclusive_wait()
won't be triggered. Avoid this by serializing the task state change
in abort_exclusive_wait() and try_to_wakeup(). This patch fixes the
following hang:

INFO: task systemd-udevd:10111 blocked for more than 480 seconds.
      Not tainted 4.7.0-dbg+ #1
Call Trace:
 [<ffffffff8161f397>] schedule+0x37/0x90
 [<ffffffff816239ef>] schedule_timeout+0x27f/0x470
 [<ffffffff8161e76f>] io_schedule_timeout+0x9f/0x110
 [<ffffffff8161fb36>] bit_wait_io+0x16/0x60
 [<ffffffff8161f929>] __wait_on_bit_lock+0x49/0xa0
 [<ffffffff8114fe69>] __lock_page+0xb9/0xc0
 [<ffffffff81165d90>] truncate_inode_pages_range+0x3e0/0x760
 [<ffffffff81166120>] truncate_inode_pages+0x10/0x20
 [<ffffffff81212a20>] kill_bdev+0x30/0x40
 [<ffffffff81213d41>] __blkdev_put+0x71/0x360
 [<ffffffff81214079>] blkdev_put+0x49/0x170
 [<ffffffff812141c0>] blkdev_close+0x20/0x30
 [<ffffffff811d48e8>] __fput+0xe8/0x1f0
 [<ffffffff811d4a29>] ____fput+0x9/0x10
 [<ffffffff810842d3>] task_work_run+0x83/0xb0
 [<ffffffff8106606e>] do_exit+0x3ee/0xc40
 [<ffffffff8106694b>] do_group_exit+0x4b/0xc0
 [<ffffffff81073d9a>] get_signal+0x2ca/0x940
 [<ffffffff8101bf43>] do_signal+0x23/0x660
 [<ffffffff810022b3>] exit_to_usermode_loop+0x73/0xb0
 [<ffffffff81002cb0>] syscall_return_slowpath+0xb0/0xc0
 [<ffffffff81624e33>] entry_SYSCALL_64_fastpath+0xa6/0xa8

Fixes: 777c6c5f1f6e ("wait: prevent exclusive waiter starvation")
Signed-off-by: Bart Van Assche <bart.vanassche@xxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Neil Brown <neilb@xxxxxxx>
Cc: Michael Shaver <jmshaver@xxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # v2.6.33+
---
 kernel/sched/wait.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
index f15d6b6..93d1f50 100644
--- a/kernel/sched/wait.c
+++ b/kernel/sched/wait.c
@@ -277,10 +277,17 @@ void abort_exclusive_wait(wait_queue_head_t *q, wait_queue_t *wait,
 			unsigned int mode, void *key)
 {
 	unsigned long flags;
+	long wake_up;
+
+	/* Serialize against try_to_wake_up() */
+	raw_spin_lock_irqsave(&current->pi_lock, flags);
+	wake_up = current->state & (TASK_INTERRUPTIBLE | TASK_UNINTERRUPTIBLE);
+	if (wake_up)
+		__set_current_state(TASK_RUNNING);
+	raw_spin_unlock_irqrestore(&current->pi_lock, flags);
 
-	__set_current_state(TASK_RUNNING);
 	spin_lock_irqsave(&q->lock, flags);
-	if (!list_empty(&wait->task_list))
+	if (wake_up)
 		list_del_init(&wait->task_list);
 	else if (waitqueue_active(q))
 		__wake_up_locked_key(q, mode, key);
-- 
2.9.2