Possible deadlock with ->writepaged version of flush_dirty_buffers()

[Marcelo Tosatti]

Hi Chris,

It seems there is a possible deadlock condition with your patch which
changes flush_dirty_buffers() to use ->writepage (something which we
_definately_ want for 2.5). Take a look:

mark_buffer_dirty->balance_dirty->wakeup_bdflush->flush_dirty_buffers->
writepage->block_write_full_page->__block_write_full_page->get_block->
ext2_get_block->ext2_alloc_branch->

	ext2_alloc_block->ext2_new_block->lock_super
	or 
	getblk()->lock_super

I dont see any reason why this deadlock could'nt happen in practice now.

If I'm right, it will pretty nasty to fix this. One possible solution is
to _never_ call mark_buffer_dirty() with the superblock lock held (ext2
has a lot of places likes this right now)

Comments?