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SubjectRe: Possible deadlock with ->writepaged version of flush_dirty_buffers() and 2.4.0


On Wednesday, January 10, 2001 05:56:09 PM -0200 Marcelo Tosatti
<marcelo@conectiva.com.br> wrote:

>
> 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:
>
Yes, good catch.

>
> 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.
>
It won't happen until someone other than fs/buffer.c starts marking ext2
pages dirty. The normal file write path will make sure that any dirty
buffers are mapped, so the ext2_get_block code is never run.

> 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)
>

This is probably the best solution, since it is a good idea regardless of
my patch.

-chris



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