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SubjectRe: [PATCH v4 04/11] md/raid5: Ensure array is suspended for calls to log_exit()
On Wed, Jun 8, 2022 at 9:28 AM Logan Gunthorpe <logang@deltatee.com> wrote:
>
> The raid5-cache code relies on there being no IO in flight when
> log_exit() is called. There are two places where this is not
> guaranteed so add mddev_suspend() and mddev_resume() calls to these
> sites.
>
> The site in raid5_remove_disk() has a comment saying that it is
> called in raid5d and thus cannot wait for pending writes; however that
> does not appear to be correct anymore (if it ever was) as
> raid5_remove_disk() is called from hot_remove_disk() which only
> appears to be called in the md_ioctl(). Thus, the comment is removed,
> as well as the racy check and replaced with calls to suspend/resume.
>
> The site in raid5_change_consistency_policy() is in the error path,
> and another similar call site already has suspend/resume calls just
> below it; so it should be equally safe to make that change here.
>
> Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> ---
> drivers/md/raid5.c | 18 ++++++------------
> 1 file changed, 6 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index 5d09256d7f81..3ad37dd4c5cd 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -7938,18 +7938,9 @@ static int raid5_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
>
> print_raid5_conf(conf);
> if (test_bit(Journal, &rdev->flags) && conf->log) {
> - /*
> - * we can't wait pending write here, as this is called in
> - * raid5d, wait will deadlock.
> - * neilb: there is no locking about new writes here,
> - * so this cannot be safe.
> - */
> - if (atomic_read(&conf->active_stripes) ||
> - atomic_read(&conf->r5c_cached_full_stripes) ||
> - atomic_read(&conf->r5c_cached_partial_stripes)) {
> - return -EBUSY;
> - }
> + mddev_suspend(mddev);

Unfortunately, the comment about deadlock is still true, and we cannot call
mddev_suspend here. To trigger it:

# create raid5 with journal:
mdadm --create /dev/md0 -l 5 -n 3 /dev/nvme[0-2]n1 --write-journal
/dev/nvme3n1
# start some writes
fio ...
# fail the journal
mdadm --fail /dev/md0 /dev/nvme3n1

This will cause deadlock of the thread:
[<0>] raid5_quiesce+0x2a8/0x5f0
[<0>] mddev_suspend+0x26b/0x530
[<0>] raid5_remove_disk+0x12e/0x6f3
[<0>] remove_and_add_spares+0x5b2/0xef0
[<0>] md_check_recovery+0xe68/0x12b0
[<0>] raid5d+0xf4/0xf30
[<0>] md_thread+0x299/0x350
[<0>] kthread+0x29d/0x340
[<0>] ret_from_fork+0x22/0x30

Thanks,
Song


> log_exit(conf);
> + mddev_resume(mddev);
> return 0;
> }
> if (rdev == rcu_access_pointer(p->rdev))
> @@ -8697,8 +8688,11 @@ static int raid5_change_consistency_policy(struct mddev *mddev, const char *buf)
> err = log_init(conf, NULL, true);
> if (!err) {
> err = resize_stripes(conf, conf->pool_size);
> - if (err)
> + if (err) {
> + mddev_suspend(mddev);
> log_exit(conf);
> + mddev_resume(mddev);
> + }
> }
> } else
> err = -EINVAL;
> --
> 2.30.2
>

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