Re: locking rules for ->dirty_inode()

Nikita Danilov
2002-09-20T16:27:52Z

Andrew Morton writes:
> 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.

Actually, I came over this while trying to describe lock ordering in
reiser4 after I just started integrating other kernel locks there. I
wonder, has somebody already done this, writing up kernel lock
hierarchy, that is?

Nikita.