[PATCH 2/3] writeback: Introduce wb_wait_for_completion_no_hung().

[Julian Sun <sunjunchao@bytedance.com>]

This patch introduces wait_event_no_hung() and
wb_wait_for_completion_no_hung() to make hung task detector
ignore the current task if it waits for a long time.

Signed-off-by: Julian Sun <sunjunchao@bytedance.com>
---
 fs/fs-writeback.c           | 15 +++++++++++++++
 include/linux/backing-dev.h |  1 +
 include/linux/wait.h        | 15 +++++++++++++++
 3 files changed, 31 insertions(+)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index a07b8cf73ae2..eebb7f145734 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -216,6 +216,21 @@ void wb_wait_for_completion(struct wb_completion *done)
 	wait_event(*done->waitq, !atomic_read(&done->cnt));
 }
 
+/*
+ * Same as wb_wait_for_completion() but hung task warning will not be
+ * triggered if it wait for a long time. Use this function with caution
+ * unless you are sure that the hung task is undesirable.
+ * When is this undesirable? From the kernel code perspective, there is
+ * no misbehavior and it has no impact on user behavior. For example, a
+ * *background* kthread/kworker used to clean resources while waiting a
+ * long time for writeback to finish.
+ */
+ void wb_wait_for_completion_no_hung(struct wb_completion *done)
+ {
+	atomic_dec(&done->cnt);		/* put down the initial count */
+	wait_event_no_hung(*done->waitq, !atomic_read(&done->cnt));
+ }
+
 #ifdef CONFIG_CGROUP_WRITEBACK
 
 /*
diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index e721148c95d0..9d3335866f6f 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -40,6 +40,7 @@ void wb_start_background_writeback(struct bdi_writeback *wb);
 void wb_workfn(struct work_struct *work);
 
 void wb_wait_for_completion(struct wb_completion *done);
+void wb_wait_for_completion_no_hung(struct wb_completion *done);
 
 extern spinlock_t bdi_lock;
 extern struct list_head bdi_list;
diff --git a/include/linux/wait.h b/include/linux/wait.h
index 09855d819418..8f05d0bb8db7 100644
--- a/include/linux/wait.h
+++ b/include/linux/wait.h
@@ -330,6 +330,21 @@ __out:	__ret;									\
 	(void)___wait_event(wq_head, condition, TASK_UNINTERRUPTIBLE, 0, 0,	\
 			    schedule())
 
+#define __wait_event_no_hung(wq_head, condition)					\
+	(void)___wait_event(wq_head, condition, TASK_UNINTERRUPTIBLE, 0, 0,	\
+			    current_set_flags(PF_DONT_HUNG);	\
+			    schedule();						\
+			    current_clear_flags(PF_DONT_HUNG))
+
+#define wait_event_no_hung(wq_head, condition)						\
+do {										\
+	might_sleep();								\
+	if (condition)								\
+		break;								\
+	__wait_event_no_hung(wq_head, condition);					\
+} while (0)
+
+
 /**
  * wait_event - sleep until a condition gets true
  * @wq_head: the waitqueue to wait on
-- 
2.39.5