[dm-devel] [PATCH v2] block: flush queued bios when the process blocks

[Mike Snitzer]

On Fri, Oct 09 2015 at  3:52pm -0400,
Mike Snitzer <snitzer at redhat.com> wrote:
 
> Turns out that this change:
> http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/commit/?h=wip&id=2639638c77768a86216be456c2764e32a2bcd841
> 
> needed to be reverted with:
> http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/commit/?h=wip&id=ad3ccd760da7c05b90775372f9b39dc2964086fe
> 
> Because nested plugs caused generic_make_request()'s onstack bio_list to
> go out of scope (blk_finish_plug() wouldn't actually flush the list
> within generic_make_request because XFS already added an outermost
> plug).
> 
> But even after fixing that I then hit issues with these changes now
> resulting in imperfect 'in_generic_make_request' accounting that happens
> lazily once the outermost plug completes blk_finish_plug.  manifested as
> dm-bufio.c:dm_bufio_prefetch's BUG_ON(dm_bufio_in_request()); hitting.
> 
> Basically using the blk-core's onstack plugging isn't workable for
> fixing this deadlock and we're back to having to seriously consider
> this (with its additional hook in the scheduler): 

http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/commit/?h=wip&id=a91709cd32b5ca7ca047b68c9299e747f2ae6ca2