Re: [PATCH 09/10] md: only delete entries from all_mddevs when the disk is freed

[Christoph Hellwig <hch@lst.de>]

On Mon, Jul 18, 2022 at 09:17:42AM +0200, Hannes Reinecke wrote:
> Why can't we use 'atomic_inc_unless_zero' here and do away with the 
> additional 'deleted' flag?

See my previous reply.

>> @@ -3338,6 +3342,8 @@ static bool md_rdev_overlaps(struct md_rdev *rdev)
>>     	spin_lock(&all_mddevs_lock);
>>   	list_for_each_entry(mddev, &all_mddevs, all_mddevs) {
>> +		if (mddev->deleted)
>> +			continue;
>
> Any particular reason why you can't use the 'mddev_get()' / 'mddev_put()' 
> sequence here?

Mostly because it would be pretty mess.  mdev_put takes all_mddevs_lock,
so we'd have to add an unlock/relock cycle for each loop iteration.

> After all, I don't think that 'mddev' should vanish while being in this 
> loop, no?

It won't, at least without the call to mddev_put.  Once mddev_put is
in the game things aren't that easy, and I suspect the exising and
new code might have bugs in that area in the reboot notifier and
md_exit for extreme corner cases.