Re: locking rules for ->dirty_inode()

Andrew Morton
2002-09-20T15:49:53Z

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.