md: fix deadlock between mddev_suspend and flush bio

In the Linux kernel, the following vulnerability has been resolved:

md: fix deadlock between mddev_suspend and flush bio

Deadlock occurs when mddev is being suspended while some flush bio is in

progress. It is a complex issue.

T1. the first flush is at the ending stage, it clears 'mddev->flush_bio'

and tries to submit data, but is blocked because mddev is suspended

by T4.

T2. the second flush sets 'mddev->flush_bio', and attempts to queue

md_submit_flush_data(), which is already running (T1) and won't

execute again if on the same CPU as T1.

T3. the third flush inc active_io and tries to flush, but is blocked because

'mddev->flush_bio' is not NULL (set by T2).

T4. mddev_suspend() is called and waits for active_io dec to 0 which is inc

by T3.

T1 T2 T3 T4

(flush 1) (flush 2) (third 3) (suspend)

md_submit_flush_data

mddev->flush_bio = NULL;

.

. md_flush_request

. mddev->flush_bio = bio

. queue submit_flushes

. .

. . md_handle_request

. . active_io + 1

. . md_flush_request

. . wait !mddev->flush_bio

. .

. . mddev_suspend

. . wait !active_io

. .

. submit_flushes

. queue_work md_submit_flush_data

. //md_submit_flush_data is already running (T1)

.

md_handle_request

wait resume

The root issue is non-atomic inc/dec of active_io during flush process.

active_io is dec before md_submit_flush_data is queued, and inc soon

after md_submit_flush_data() run.

md_flush_request

active_io + 1

submit_flushes

active_io - 1

md_submit_flush_data

md_handle_request

active_io + 1

make_request

active_io - 1

If active_io is dec after md_handle_request() instead of within

submit_flushes(), make_request() can be called directly intead of

md_handle_request() in md_submit_flush_data(), and active_io will

only inc and dec once in the whole flush process. Deadlock will be

fixed.

Additionally, the only difference between fixing the issue and before is

that there is no return error handling of make_request(). But after

previous patch cleaned md_write_start(), make_requst() only return error

in raid5_make_request() by dm-raid, see commit 41425f96d7aa ("dm-raid456,

md/raid456: fix a deadlock for dm-raid456 while io concurrent with

reshape)". Since dm always splits data and flush operation into two

separate io, io size of flush submitted by dm always is 0, make_request()

will not be called in md_submit_flush_data(). To prevent future

modifications from introducing issues, add WARN_ON to ensure

make_request() no error is returned in this context.