Re: [BUG] MD/RAID1 hung forever on freeze_array

[NeilBrown <neilb@suse.com>]

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> Hi Neil,
>
> I found a old mail thread below
> http://www.spinics.net/lists/raid/msg52792.html
>
> Likely Alex is trying to fix same bug, right?
> in one reply you suggested to modify the call in make_request
>
> @@ -1207,7 +1207,8 @@ read_again:
>                                 sectors_handled;
>                         goto read_again;
>                 } else
> -                       generic_make_request(read_bio);
> +                       reschedule_retry(r1_bio);
>                 return;
>         }
>
>
> I append above change, it looks fix the bug, I've run same tests over
> one hour,  no hung task anymore.
>
> Do you think this is right fix? Do we still need the change you
> suggested with punt_bios_to_rescuer?

I don't really like that fix.  I suspect it would probably hurt
performance.

I'd prefer to fix generic_make_request() to process queued requests in a
more sensible order.
Can you please try the following (with all other patches removed)?
Thanks,
NeilBrown

diff --git a/block/blk-core.c b/block/blk-core.c
index 14d7c0740dc0..3436b6fc3ef8 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -2036,10 +2036,31 @@ blk_qc_t generic_make_request(struct bio *bio)
 		struct request_queue *q = bdev_get_queue(bio->bi_bdev);
 
 		if (likely(blk_queue_enter(q, false) == 0)) {
+			struct bio_list hold;
+			struct bio_list lower, same;
+
+			/* Create a fresh bio_list for all subordinate requests */
+			bio_list_merge(&hold, &bio_list_on_stack);
+			bio_list_init(&bio_list_on_stack);
 			ret = q->make_request_fn(q, bio);
 
 			blk_queue_exit(q);
 
+			/* sort new bios into those for a lower level
+			 * and those for the same level
+			 */
+			bio_list_init(&lower);
+			bio_list_init(&same);
+			while ((bio = bio_list_pop(&bio_list_on_stack)) != NULL)
+				if (q == bdev_get_queue(bio->bi_bdev))
+					bio_list_add(&same, bio);
+				else
+					bio_list_add(&lower, bio);
+			/* now assemble so we handle the lowest level first */
+			bio_list_merge(&bio_list_on_stack, &lower);
+			bio_list_merge(&bio_list_on_stack, &same);
+			bio_list_merge(&bio_list_on_stack, &hold);
+
 			bio = bio_list_pop(current->bio_list);
 		} else {
 			struct bio *bio_next = bio_list_pop(current->bio_list);

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