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

[Martin Steigerwald <Martin@lichtvoll.de>]

Am Mittwoch, 13. August 2014, 23:20:46 schrieb Liu Bo:
> 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, so this one goes together with the fixed compressed write corruption 
one? I would put them onto 3.16.1. With 3.17 I want to wait till rc2 I think.

> 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
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