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

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  [PATCH RESEND 2/2] f2fs: disable preemption when waiting on all pages  writeback Chao Yu <chao@kernel.org> - 2016-04-27 15:50 +0200
    Re: [PATCH RESEND 2/2] f2fs: disable preemption when waiting on all  pages writeback Jaegeuk Kim <jaegeuk@kernel.org> - 2016-04-27 20:10 +0200
      Re: [f2fs-dev] [PATCH RESEND 2/2] f2fs: disable preemption when  waiting on all pages writeback Chao Yu <yuchao0@huawei.com> - 2016-04-28 14:00 +0200
        Re: [f2fs-dev] [PATCH RESEND 2/2] f2fs: disable preemption when  waiting on all pages writeback Peter Zijlstra <peterz@infradead.org> - 2016-04-28 16:10 +0200
          Re: [f2fs-dev] [PATCH RESEND 2/2] f2fs: disable preemption when  waiting on all pages writeback Chao Yu <chao@kernel.org> - 2016-04-28 16:40 +0200
            Re: [f2fs-dev] [PATCH RESEND 2/2] f2fs: disable preemption when  waiting on all pages writeback Peter Zijlstra <peterz@infradead.org> - 2016-04-28 16:40 +0200
              Re: [f2fs-dev] [PATCH RESEND 2/2] f2fs: disable preemption when  waiting on all pages writeback Chao Yu <chao@kernel.org> - 2016-04-28 17:10 +0200

#1389062 — [PATCH RESEND 2/2] f2fs: disable preemption when waiting on all pages writeback

From: Chao Yu <yuchao0@huawei.com>

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@huawei.com>
---
 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

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#1389418 — Re: [PATCH RESEND 2/2] f2fs: disable preemption when waiting on all pages writeback

Hi Chao,

On Wed, Apr 27, 2016 at 09:41:48PM +0800, Chao Yu wrote:
> From: Chao Yu <yuchao0@huawei.com>
> 
> 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

Well, io_schedule_timeout should work for this?

Thanks,

> 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@huawei.com>
> ---
>  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

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#1390074 — Re: [f2fs-dev] [PATCH RESEND 2/2] f2fs: disable preemption when waiting on all pages writeback

+Cc Peter,

Hi Peter,

This patch should be RFC, I haven't saw such issue in real world though, I still
worry that it may happen. Could you help to have a look at it.

This the question I'd like to ask here is that: in a preemptible kernel, if
thread A add itself into wait queue with TASK_UNINTERRUPTIBLE status through
prepare_to_wait, after that, thread B do the preemption, if there are no waker
for thread A, will thread A sleep forever?

Thanks,

On 2016/4/28 2:09, Jaegeuk Kim wrote:
> Hi Chao,
> 
> On Wed, Apr 27, 2016 at 09:41:48PM +0800, Chao Yu wrote:
>> From: Chao Yu <yuchao0@huawei.com>
>>
>> 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
> 
> Well, io_schedule_timeout should work for this?
> 
> Thanks,
> 
>> 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@huawei.com>
>> ---
>>  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
> 
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> .
> 

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#1390237 — Re: [f2fs-dev] [PATCH RESEND 2/2] f2fs: disable preemption when waiting on all pages writeback

On Thu, Apr 28, 2016 at 07:51:04PM +0800, Chao Yu wrote:
> > On Wed, Apr 27, 2016 at 09:41:48PM +0800, Chao Yu wrote:
> >> From: Chao Yu <yuchao0@huawei.com>
> >>
> >> 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.

But here, you should be verifying if you really should go sleep; as the
code did; it tests for !get_pages(, F2FS_WRITEBACK), and if you've just
completed that very last one, this will break out.

> >>     - io_schedule_timeout
> > 

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#1390260 — Re: [f2fs-dev] [PATCH RESEND 2/2] f2fs: disable preemption when waiting on all pages writeback

On 2016/4/28 22:03, Peter Zijlstra wrote:
> On Thu, Apr 28, 2016 at 07:51:04PM +0800, Chao Yu wrote:
>>> On Wed, Apr 27, 2016 at 09:41:48PM +0800, Chao Yu wrote:
>>>> From: Chao Yu <yuchao0@huawei.com>
>>>>
>>>> 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.
> 
> But here, you should be verifying if you really should go sleep; as the
> code did; it tests for !get_pages(, F2FS_WRITEBACK), and if you've just
> completed that very last one, this will break out.

You mean after being preempted with TASK_UNINTERRUPTIBLE status, that task still
has chance to be scheduled to check '!get_pages(, F2FS_WRITEBACK)', is that right?

> 
>>>>     - io_schedule_timeout
>>>

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#1390265 — Re: [f2fs-dev] [PATCH RESEND 2/2] f2fs: disable preemption when waiting on all pages writeback

On Thu, Apr 28, 2016 at 10:30:55PM +0800, Chao Yu wrote:
> On 2016/4/28 22:03, Peter Zijlstra wrote:
> > On Thu, Apr 28, 2016 at 07:51:04PM +0800, Chao Yu wrote:
> >>> On Wed, Apr 27, 2016 at 09:41:48PM +0800, Chao Yu wrote:
> >>>> From: Chao Yu <yuchao0@huawei.com>
> >>>>
> >>>> 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.
> > 
> > But here, you should be verifying if you really should go sleep; as the
> > code did; it tests for !get_pages(, F2FS_WRITEBACK), and if you've just
> > completed that very last one, this will break out.
> 
> You mean after being preempted with TASK_UNINTERRUPTIBLE status, that task still
> has chance to be scheduled to check '!get_pages(, F2FS_WRITEBACK)', is that right?

Yes, preemption ignores task_struct::state.

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#1390292 — Re: [f2fs-dev] [PATCH RESEND 2/2] f2fs: disable preemption when waiting on all pages writeback

On 2016/4/28 22:37, Peter Zijlstra wrote:
> On Thu, Apr 28, 2016 at 10:30:55PM +0800, Chao Yu wrote:
>> On 2016/4/28 22:03, Peter Zijlstra wrote:
>>> On Thu, Apr 28, 2016 at 07:51:04PM +0800, Chao Yu wrote:
>>>>> On Wed, Apr 27, 2016 at 09:41:48PM +0800, Chao Yu wrote:
>>>>>> From: Chao Yu <yuchao0@huawei.com>
>>>>>>
>>>>>> 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.
>>>
>>> But here, you should be verifying if you really should go sleep; as the
>>> code did; it tests for !get_pages(, F2FS_WRITEBACK), and if you've just
>>> completed that very last one, this will break out.
>>
>> You mean after being preempted with TASK_UNINTERRUPTIBLE status, that task still
>> has chance to be scheduled to check '!get_pages(, F2FS_WRITEBACK)', is that right?
> 
> Yes, preemption ignores task_struct::state.

Got it. Thanks very much for your help! :)

> 

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