Re: [PATCH] Btrfs: fix task hang under heavy compressed write

[Liu Bo <bo.li.liu@oracle.com>]

On Wed, Aug 13, 2014 at 01:54:40PM +0200, Martin Steigerwald wrote:
> Am Dienstag, 12. August 2014, 15:44:59 schrieb Liu Bo:
> > This has been reported and discussed for a long time, and this hang occurs
> > in both 3.15 and 3.16.
> 
> Liu, is this safe for testing yet?

Yes, I've confirmed that this hang doesn't occur by running my tests for 2
days(usually it hangs in 2 hours).

But...
As Chris said in the thread, this is more a workaround, there're other potential
issues that would lead to similar deadlock.

I'm trying to write a real fix instead of a workaround.

thanks,
-liubo

> 
> Thanks,
> Martin
> 
> > Btrfs now migrates to use kernel workqueue, but it introduces this hang
> > problem.
> > 
> > Btrfs has a kind of work queued as an ordered way, which means that its
> > ordered_func() must be processed in the way of FIFO, so it usually looks
> > like --
> > 
> > normal_work_helper(arg)
> >     work = container_of(arg, struct btrfs_work, normal_work);
> > 
> >     work->func() <---- (we name it work X)
> >     for ordered_work in wq->ordered_list
> >             ordered_work->ordered_func()
> >             ordered_work->ordered_free()
> > 
> > The hang is a rare case, first when we find free space, we get an uncached
> > block group, then we go to read its free space cache inode for free space
> > information, so it will
> > 
> > file a readahead request
> >     btrfs_readpages()
> >          for page that is not in page cache
> >                 __do_readpage()
> >                      submit_extent_page()
> >                            btrfs_submit_bio_hook()
> >                                  btrfs_bio_wq_end_io()
> >                                  submit_bio()
> >                                  end_workqueue_bio() <--(ret by the 1st
> > endio) queue a work(named work Y) for the 2nd also the real endio()
> > 
> > So the hang occurs when work Y's work_struct and work X's work_struct
> > happens to share the same address.
> > 
> > A bit more explanation,
> > 
> > A,B,C -- struct btrfs_work
> > arg   -- struct work_struct
> > 
> > kthread:
> > worker_thread()
> >     pick up a work_struct from @worklist
> >     process_one_work(arg)
> > 	worker->current_work = arg;  <-- arg is A->normal_work
> > 	worker->current_func(arg)
> > 		normal_work_helper(arg)
> > 		     A = container_of(arg, struct btrfs_work, normal_work);
> > 
> > 		     A->func()
> > 		     A->ordered_func()
> > 		     A->ordered_free()  <-- A gets freed
> > 
> > 		     B->ordered_func()
> > 			  submit_compressed_extents()
> > 			      find_free_extent()
> > 				  load_free_space_inode()
> > 				      ...   <-- (the above readhead stack)
> > 				      end_workqueue_bio()
> > 					   btrfs_queue_work(work C)
> > 		     B->ordered_free()
> > 
> > As if work A has a high priority in wq->ordered_list and there are more
> > ordered works queued after it, such as B->ordered_func(), its memory could
> > have been freed before normal_work_helper() returns, which means that
> > kernel workqueue code worker_thread() still has worker->current_work
> > pointer to be work A->normal_work's, ie. arg's address.
> > 
> > Meanwhile, work C is allocated after work A is freed, work C->normal_work
> > and work A->normal_work are likely to share the same address(I confirmed
> > this with ftrace output, so I'm not just guessing, it's rare though).
> > 
> > When another kthread picks up work C->normal_work to process, and finds our
> > kthread is processing it(see find_worker_executing_work()), it'll think
> > work C as a collision and skip then, which ends up nobody processing work C.
> > 
> > So the situation is that our kthread is waiting forever on work C.
> > 
> > The key point is that they shouldn't have the same address, so this defers
> > ->ordered_free() and does a batched free to avoid that.
> > 
> > Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
> > ---
> >  fs/btrfs/async-thread.c | 12 ++++++++++--
> >  1 file changed, 10 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/btrfs/async-thread.c b/fs/btrfs/async-thread.c
> > index 5a201d8..2ac01b3 100644
> > --- a/fs/btrfs/async-thread.c
> > +++ b/fs/btrfs/async-thread.c
> > @@ -195,6 +195,7 @@ static void run_ordered_work(struct __btrfs_workqueue
> > *wq) struct btrfs_work *work;
> >  	spinlock_t *lock = &wq->list_lock;
> >  	unsigned long flags;
> > +	LIST_HEAD(free_list);
> > 
> >  	while (1) {
> >  		spin_lock_irqsave(lock, flags);
> > @@ -219,17 +220,24 @@ static void run_ordered_work(struct __btrfs_workqueue
> > *wq)
> > 
> >  		/* now take the lock again and drop our item from the list */
> >  		spin_lock_irqsave(lock, flags);
> > -		list_del(&work->ordered_list);
> > +		list_move_tail(&work->ordered_list, &free_list);
> >  		spin_unlock_irqrestore(lock, flags);
> > 
> >  		/*
> >  		 * we don't want to call the ordered free functions
> >  		 * with the lock held though
> >  		 */
> > +	}
> > +	spin_unlock_irqrestore(lock, flags);
> > +
> > +	while (!list_empty(&free_list)) {
> > +		work = list_entry(free_list.next, struct btrfs_work,
> > +				  ordered_list);
> > +
> > +		list_del(&work->ordered_list);
> >  		work->ordered_free(work);
> >  		trace_btrfs_all_work_done(work);
> >  	}
> > -	spin_unlock_irqrestore(lock, flags);
> >  }
> > 
> >  static void normal_work_helper(struct work_struct *arg)
> 
> -- 
> Martin 'Helios' Steigerwald - http://www.Lichtvoll.de
> GPG: 03B0 0D6C 0040 0710 4AFA  B82F 991B EAAC A599 84C7