[PATCH RESEND 2/2] f2fs: disable preemption when waiting on all pages writeback

[Chao Yu]

From: Chao Yu <yuchao0@xxxxxxxxxx>

The following condition can happen in a preemptible kernel, it may cause
checkpointer hunging.

CPU0:					CPU1:
 - write_checkpoint
  - do_checkpoint
   - wait_on_all_pages_writeback
					 - f2fs_write_end_io
					  - wake_up
					this is last writebacked page, but
					no sleeper in sbi->cp_wait wait
					queue, wake_up is not been called.
    - prepare_to_wait(TASK_UNINTERRUPTIBLE)
    Here, current task can been preempted,
    but there will be no waker since last
    write_end_io has bypassed wake_up. So
    current task will sleep forever.
    - io_schedule_timeout

Now we use spinlock to create a critical section to guarantee preemption
was disabled during racing in between wait_on_all_pages_writeback and
f2fs_write_end_io, so that above condition can be avoided.

Signed-off-by: Chao Yu <yuchao0@xxxxxxxxxx>
---
 fs/f2fs/checkpoint.c | 14 +++++++++-----
 fs/f2fs/data.c       |  9 +++++++--
 fs/f2fs/f2fs.h       |  3 ++-
 fs/f2fs/super.c      |  1 +
 4 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
index bf040b5..817cda7 100644
--- a/fs/f2fs/checkpoint.c
+++ b/fs/f2fs/checkpoint.c
@@ -914,15 +914,19 @@ static void wait_on_all_pages_writeback(struct
f2fs_sb_info *sbi)
 {
 	DEFINE_WAIT(wait);

-	for (;;) {
+	spin_lock(&sbi->cp_wb_lock);
+
+	while (get_pages(sbi, F2FS_WRITEBACK)) {
 		prepare_to_wait(&sbi->cp_wait, &wait, TASK_UNINTERRUPTIBLE);

-		if (!get_pages(sbi, F2FS_WRITEBACK))
-			break;
+		spin_unlock(&sbi->cp_wb_lock);
+		io_schedule();
+		spin_lock(&sbi->cp_wb_lock);

-		io_schedule_timeout(5*HZ);
+		finish_wait(&sbi->cp_wait, &wait);
 	}
-	finish_wait(&sbi->cp_wait, &wait);
+
+	spin_unlock(&sbi->cp_wb_lock);
 }

 static int do_checkpoint(struct f2fs_sb_info *sbi, struct cp_control *cpc)
diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index 38ce5d6..8faeada 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -59,6 +59,7 @@ static void f2fs_write_end_io(struct bio *bio)
 {
 	struct f2fs_sb_info *sbi = bio->bi_private;
 	struct bio_vec *bvec;
+	unsigned long flags;
 	int i;

 	bio_for_each_segment_all(bvec, bio, i) {
@@ -74,8 +75,12 @@ static void f2fs_write_end_io(struct bio *bio)
 		dec_page_count(sbi, F2FS_WRITEBACK);
 	}

-	if (!get_pages(sbi, F2FS_WRITEBACK) && wq_has_sleeper(&sbi->cp_wait))
-		wake_up(&sbi->cp_wait);
+	if (!get_pages(sbi, F2FS_WRITEBACK)) {
+		spin_lock_irqsave(&sbi->cp_wb_lock, flags);
+		if (wq_has_sleeper(&sbi->cp_wait))
+			wake_up(&sbi->cp_wait);
+		spin_unlock_irqrestore(&sbi->cp_wb_lock, flags);
+	}

 	bio_put(bio);
 }
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index 0786a45..cf646b3 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -725,7 +725,8 @@ struct f2fs_sb_info {
 	struct rw_semaphore cp_rwsem;		/* blocking FS operations */
 	struct rw_semaphore node_write;		/* locking node writes */
 	struct mutex writepages;		/* mutex for writepages() */
-	wait_queue_head_t cp_wait;
+	wait_queue_head_t cp_wait;		/* for wait pages writeback */
+	spinlock_t cp_wb_lock;			/* for protect cp_wait */
 	unsigned long last_time[MAX_TIME];	/* to store time in jiffies */
 	long interval_time[MAX_TIME];		/* to store thresholds */

diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 19a85cf..8b25ac1 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -1436,6 +1436,7 @@ try_onemore:

 	init_rwsem(&sbi->cp_rwsem);
 	init_waitqueue_head(&sbi->cp_wait);
+	spin_lock_init(&sbi->cp_wb_lock);
 	init_sb_info(sbi);

 	/* get an inode for meta space */
-- 
2.7.2