Re: [PATCH 2/9] vfs: export do_splice_direct() to modules

[Jan Kara]

On Sun 17-03-13 13:06:59, David Howells wrote:
> Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> 
> > Export do_splice_direct() to modules.  Needed by overlay filesystem.
> 
> Apparently you cannot call this from any function that is holding an i_mutex
> if the target of the splice uses generic_file_splice_write().
> 
> The problem is a potential deadlock situation:
> 
> We have places already that do:
> 
> 	mnt_want_write()
> 	mutex_lock()
> 
> This can be found in do_last() for example.
> 
> However, mnt_want_write() calls sb_start_write() as does
> generic_file_splice_write().  So now in ovl_copy_up_locked() you're adding:
> 
> 	mutex_lock()
> 	sb_start_write()
> 
> which lockdep reports as a potential ABBA deadlock.
> 
> Now, looking at __sb_start_write(), I'm not entirely sure how the deadlock
> might operate, so it's possible that this is a false alarm.  Maybe Jan Kara can
> illuminate further, so I've added him to the cc list.
  IMO the deadlock is real. In freeze_super() we wait for all writers to
the filesystem to finish while blocking beginning of any further writes. So
we have a deadlock scenario like:

  THREAD1		THREAD2				THREAD3
mnt_want_write()	mutex_lock(&inode->i_mutex);
...							freeze_super()
block on mutex_lock(&inode->i_mutex)
							  sb_wait_write(sb, SB_FREEZE_WRITE);
			block in sb_start_write()

									Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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