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

[Logan Gunthorpe]

On 2022-06-08 11:59, Song Liu wrote:
> On Wed, Jun 8, 2022 at 9:28 AM Logan Gunthorpe <logang@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
>> Reviewed-by: Christoph Hellwig <hch@xxxxxx>
>> ---
>>  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.

Thanks,

Logan