[PATCH v2 1/3] sched: Avoid that __wait_on_bit_lock() hangs

[Bart Van Assche <bart.vanassche@sandisk.com>]

If delivery of a signal and __wake_up_common() happen concurrently
it is possible that the signal is delivered after __wake_up_common()
woke up the affected task and before bit_wait_io() checks whether a
signal is pending. Avoid that the next waiter is not woken up if this
happens. 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")
References: https://lkml.org/lkml/2012/8/24/185
References: http://www.spinics.net/lists/raid/msg53056.html
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Neil Brown <neilb@suse.de>
Cc: Michael Shaver <jmshaver@gmail.com>
Cc: <stable@vger.kernel.org> # v2.6.29+
---
 kernel/sched/wait.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
index f15d6b6..fa12939 100644
--- a/kernel/sched/wait.c
+++ b/kernel/sched/wait.c
@@ -266,12 +266,16 @@ EXPORT_SYMBOL(finish_wait);
  * the wait descriptor from the given waitqueue if still
  * queued.
  *
- * Wakes up the next waiter if the caller is concurrently
- * woken up through the queue.
- *
- * This prevents waiter starvation where an exclusive waiter
- * aborts and is woken up concurrently and no one wakes up
- * the next waiter.
+ * Wakes up the next waiter to prevent waiter starvation
+ * when an exclusive waiter aborts and is woken up
+ * concurrently and no one wakes up the next waiter. Note:
+ * even when a signal is pending it is possible that
+ * __wake_up_common() wakes up the current thread and hence
+ * that @wait has been removed from the wait queue @q. Hence
+ * test whether there are more waiters on the wait queue
+ * even if @wait is not on the wait queue @q. This approach
+ * will cause a spurious wakeup if a signal is delivered and
+ * no other thread calls __wake_up_common() concurrently.
  */
 void abort_exclusive_wait(wait_queue_head_t *q, wait_queue_t *wait,
 			unsigned int mode, void *key)
@@ -282,7 +286,7 @@ void abort_exclusive_wait(wait_queue_head_t *q, wait_queue_t *wait,
 	spin_lock_irqsave(&q->lock, flags);
 	if (!list_empty(&wait->task_list))
 		list_del_init(&wait->task_list);
-	else if (waitqueue_active(q))
+	if (waitqueue_active(q))
 		__wake_up_locked_key(q, mode, key);
 	spin_unlock_irqrestore(&q->lock, flags);
 }
-- 
2.9.2