On Wed, Jun 22 2016, Lars Ellenberg wrote:

For a long time, generic_make_request() converts recursion into
iteration by queuing recursive arguments on current->bio_list.

This is convenient for stacking drivers,
the top-most driver would take the originally submitted bio,
and re-submit a re-mapped version of it, or one or more clones,
or one or more new allocated bios to its backend(s). Which
are then simply processed in turn, and each can again queue
more "backend-bios" until we reach the bottom of the driver stack,
and actually dispatch to the real backend device.

Any stacking driver ->make_request_fn() could expect that,
once it returns, any backend-bios it submitted via recursive calls
to generic_make_request() would now be processed and dispatched, before
the current task would call into this driver again.

This is changed by commit
  54efd50 block: make generic_make_request handle arbitrarily sized bios

Drivers may call blk_queue_split() inside their ->make_request_fn(),
which may split the current bio into a front-part to be dealt with
immediately, and a remainder-part, which may need to be split even
further. That remainder-part will simply also be pushed to
current->bio_list, and would end up being head-of-queue, in front
of any backend-bios the current make_request_fn() might submit during
processing of the fron-part.

Which means the current task would immediately end up back in the same
make_request_fn() of the same driver again, before any of its backend
bios have even been processed.

This can lead to resource starvation deadlock.
Drivers could avoid this by learning to not need blk_queue_split(),
or by submitting their backend bios in a different context (dedicated
kernel thread, work_queue context, ...). Or by playing funny re-ordering
games with entries on current->bio_list.

Instead, I suggest to distinguish between recursive calls to
generic_make_request(), and pushing back the remainder part in
blk_queue_split(), by pointing current->bio_lists to a
	struct recursion_to_iteration_bio_lists {
		struct bio_list recursion;
		struct bio_list remainder;
	}

To have all bios targeted to drivers lower in the stack processed before
processing the next piece of a bio targeted at the higher levels,
as long as queued bios resulting from recursion are available,
they will continue to be processed in FIFO order.
Pushed back bio-parts resulting from blk_queue_split() will be processed
in LIFO order, one-by-one, whenever the recursion list becomes empty.

I really like this change.  It seems to precisely address the problem.
The "problem" being that requests for "this" device are potentially
mixed up with requests from underlying devices.
However I'm not sure it is quite general enough.

The "remainder" list is a stack of requests aimed at "this" level or
higher, and I think it will always exactly fit that description.
The "recursion" list needs to be a queue of requests aimed at the next
level down, and that doesn't quiet work, because once you start acting
on the first entry in that list, all the rest become "this" level.

I think you can address this by always calling ->make_request_fn with an
empty "recursion", then after the call completes, splice the "recursion"
list that resulted (if any) on top of the "remainder" stack.

This way, the "remainder" stack is always "requests for lower-level
devices before request for upper level devices" and the "recursion"
queue is always "requests for devices below the current level".

I also really *don't* like the idea of punting to a separate thread - it
seems to be just delaying the problem.

Can you try move the bio_list_init(->recursion) call to just before
the ->make_request_fn() call, and adding
    bio_list_merge_head(->remainder, ->recursion)
just after?
(or something like that) and confirm it makes sense, and works?

Thanks!

NeilBrown