Re: [PATCH v4 04/11] md/raid5: Ensure array is suspended for calls to log_exit()

[Logan Gunthorpe <logang@deltatee.com>]

On 2022-06-08 16:02, Song Liu wrote:
> On Wed, Jun 8, 2022 at 11:21 AM Logan Gunthorpe <logang@deltatee.com> wrote:
>>
>>
>>
>> On 2022-06-08 11:59, Song Liu wrote:
>>> 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:
>>
>> Ah, yes. What a tangle. I think we can just drop this patch. Now that we
>> are removing RCU it isn't actually necessary to fix the bug I was
>> seeing. It's still probably broken as the comment notes though.
> 
> How about we keep the suspend/resume in raid5_change_consistency_policy()?
> Like the one I just pushed to md-next:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/song/md.git/commit/?h=md-next&id=ac1506992459fe45a085c1f93df74d51c381887b

I'm good with that. Thanks.

I'll do some testing on md-next shortly.

Thanks,

Logan