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

[Chao Yu]

+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@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
> 
> 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@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
> 
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