Re: locking rules for ->dirty_inode()

[Andrew Morton <akpm@digeo.com>]

Nikita Danilov wrote:
> 
> Hello,
> 
> Documentation/filesystems/Locking states that all super operations may
> block, but __set_page_dirty_buffers() calls
> 
>    __mark_inode_dirty()->s_op->dirty_inode()
> 
> under mapping->private_lock spin lock. This seems strange, because file
> systems' ->dirty_inode() assume that they are allowed to block. For
> example, ext3_dirty_inode() allocates memory in
> 
>    ext3_journal_start()->journal_start()->new_handle()->...
> 

OK, thanks.

mapping->private_lock is taken there to pin page->buffers()
(Can't lock the page because set_page_dirty is called under
page_table_lock, and other locks).

I'm sure we can just move the spin_unlock up to above the
TestSetPageDirty(), but I need to zenuflect for a while over
why I did it that way.

It's necessary to expose buffer-dirtiness and page-dirtiness
to the rest of the world in the correct order.  If we set the
page dirty and then the buffers, there is a window in which writeback
could find the dirty page, try to write it, discover clean buffers
and mark the page clean.  We would end up with a !PageDirty page,
on mapping->clean_pages, with dirty buffers.  It would never be
written.

Yup.  We can move that spin_unlock up ten lines.