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Message-ID: <20101122183207.2c8b85d4@notabene.brown>
Date: Mon, 22 Nov 2010 18:32:07 +1100
From: Neil Brown <neilb@...e.de>
To: <aniket@...ezza.com>
Cc: <linux-raid@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] md: Fix nr_pending race during raid10 recovery
On Wed, 17 Nov 2010 13:57:50 -0500
Aniket Kulkarni <aniket@...ezza.com> wrote:
> If a RAID10 rdev that is undergoing recovery is marked 'faulty', the rdev
> could get taken out of the array in spite of outstanding IOs leading to
> a kernel panic. There are two issues here -
>
> 1. The ref count (nr_pending) increment for sync or recovery leaves a lot of
> open windows for concurrent rdev removals
> 2. raid10 sync thread continues to submit recovery IOs to faulty devices. These get
> rejected at a later stage by management thread (raid10d).
>
> Note - rd denotes the rdev from which we are reading, and wr the one we are
> writing to
>
> Sync Thread Management Thread
>
> sync_request
> ++rd.nr_pending
> bi_end_io = end_sync_read
> generic_make_request -------> recovery_request_write
> | | wr.nr_pending++
> | | bi_end_io = end_sync_write
> V | generic_make_request
> end_sync_read -------------- |
> --rd.nr_pending |
> reschedule_retry for write |
> v
> end_sync_write
> --wr.nr_pending
>
> So a set-faulty and remove on recovery rdev between sync_request and
> recovery_request_write is allowed and will lead to a panic.
>
> The fix is -
>
> 1. Increment wr.nr_pending immediately after selecting a good target. Ofcourse
> the decrements will be added to error paths in sync_request and end_sync_read.
> 2. Don't submit recovery IOs to faulty targets
>
> Signed-off-by: Aniket Kulkarni <aniket@...ezza.com>
> ---
> drivers/md/raid10.c | 46 +++++++++++++++++++++++++++++++++++++++++++---
> 1 files changed, 43 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> index c67aa54..ec1ea43 100644
> --- a/drivers/md/raid10.c
> +++ b/drivers/md/raid10.c
> @@ -1408,6 +1408,16 @@ static void sync_request_write(mddev_t *mddev, r10bio_t *r10_bio)
>
> done:
> if (atomic_dec_and_test(&r10_bio->remaining)) {
> + for (i = 0; i < conf->copies ; i++) {
> + /* for any unsuccessful IOs give up the pending count on the
> + * write device
> + */
> + if (!test_bit(BIO_UPTODATE, &r10_bio->devs[i].bio->bi_flags) &&
> + r10_bio->devs[i].bio->bi_end_io == end_sync_write) {
> + int d = r10_bio->devs[i].devnum;
> + rdev_dec_pending(conf->mirrors[d].rdev, mddev);
> + }
> + }
> md_done_sync(mddev, r10_bio->sectors, 1);
> put_buf(r10_bio);
> }
> @@ -1443,7 +1453,6 @@ static void recovery_request_write(mddev_t *mddev, r10bio_t *r10_bio)
> }
> d = r10_bio->devs[1].devnum;
>
> - atomic_inc(&conf->mirrors[d].rdev->nr_pending);
> md_sync_acct(conf->mirrors[d].rdev->bdev, wbio->bi_size >> 9);
> if (test_bit(R10BIO_Uptodate, &r10_bio->state))
> generic_make_request(wbio);
> @@ -1906,14 +1915,24 @@ static sector_t sync_request(mddev_t *mddev, sector_t sector_nr, int *skipped, i
> int j, k;
> r10_bio = NULL;
>
> - for (i=0 ; i<conf->raid_disks; i++)
> + for (i = 0; i < conf->raid_disks; i++)
> if (conf->mirrors[i].rdev &&
> !test_bit(In_sync, &conf->mirrors[i].rdev->flags)) {
> int still_degraded = 0;
> /* want to reconstruct this device */
> r10bio_t *rb2 = r10_bio;
> - sector_t sect = raid10_find_virt(conf, sector_nr, i);
> + sector_t sect;
> int must_sync;
> +
> + /* Skip over the faulty drives */
> + if (test_bit(Faulty, &conf->mirrors[i].rdev->flags))
> + continue;
You shouldn't need to check for Faulty here as In_sync is always cleared when
Faulty is set, and we know In_sync is not clear.
> +
> + /* up the nr_pending count; this will be decremented on
> + * sync/recovery IO completion (success/erorr) to this dev
> + */
> + atomic_inc(&conf->mirrors[i].rdev->nr_pending);
If you delay that atomic_inc a little .....
> + sect = raid10_find_virt(conf, sector_nr, i);
> /* Unless we are doing a full sync, we only need
> * to recover the block if it is set in the bitmap
> */
> @@ -1927,6 +1946,7 @@ static sector_t sync_request(mddev_t *mddev, sector_t sector_nr, int *skipped, i
> * that there will never be anything to do here
> */
> chunks_skipped = -1;
> + rdev_dec_pending(conf->mirrors[i].rdev, mddev);
> continue;
> }
.... to here, you don't need the rdev_dec_pending.
>
> @@ -1997,6 +2017,10 @@ static sector_t sync_request(mddev_t *mddev, sector_t sector_nr, int *skipped, i
> if (j == conf->copies) {
> /* Cannot recover, so abort the recovery */
> put_buf(r10_bio);
> + /* we wanted to write to 'i' but we didn't; so dec the
> + * pending count
> + */
> + rdev_dec_pending(conf->mirrors[i].rdev, mddev);
> if (rb2)
> atomic_dec(&rb2->remaining);
> r10_bio = rb2;
> @@ -2014,6 +2038,22 @@ static sector_t sync_request(mddev_t *mddev, sector_t sector_nr, int *skipped, i
> r10_bio = (r10bio_t*) rb2->master_bio;
> rb2->master_bio = NULL;
> put_buf(rb2);
> + if (r10_bio) {
> + int d;
> + /* Before throwing away the complete r10bio chain, decrement
> + * the nr_pending ref counts incremented along the way
> + * We do this only for our master_bio and then on because
> + * if biolist == NULL no ref count was incremented in this
> + * r10_bio
> + */
> + for (d = 0; d < conf->copies; d++) {
> + if (r10_bio->devs[d].bio &&
> + r10_bio->devs[d].bio->bi_end_io) {
> + int dn = r10_bio->devs[d].devnum;
> + rdev_dec_pending(conf->mirrors[dn].rdev, mddev);
> + }
> + }
> + }
In raid1, we do this rdev_dec_pending loop inside put_buf. I think I'd like
to do that for raid10 too, but I'll make that a separate patch.
> }
> goto giveup;
> }
I also need some rcu locking in there and need to review the 'resync' (as
opposed to recovery) paths to make sure they are OK too.
Thanks,
NeilBrown
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