Re: [PATCH 1/5] btrfs: check rw_devices, not num_devices for restriping

[Josef Bacik <josef@toxicpanda.com>]

On 1/16/20 10:59 AM, David Sterba wrote:
> On Tue, Jan 14, 2020 at 01:07:22PM -0800, Josef Bacik wrote:
>>>> -	num_devices = btrfs_num_devices(fs_info);
>>>> +	/*
>>>> +	 * rw_devices can be messed with by rm_device and device replace, so
>>>> +	 * take the chunk_mutex to make sure we have a relatively consistent
>>>> +	 * view of the fs at this point.
>>>
>>> Well, what does 'relatively consistent' mean here? There are enough
>>> locks and exclusion that device remove or replace should not change the
>>> value until btrfs_balance ends, no?
>>>
>>
>> Again I don't have the code in front of me, but there's nothing at this point to
>> stop us from running in at the tail end of device replace or device rm.
> 
> This should be prevented by the EXCL_OP mechanism, so even the end of
> device remove or replace will not be running at this time because it
> cannot even start.
> 
>> The
>> mutex keeps us from getting weirdly inflated values when we increment and
>> decrement at the end of device replace, but there's nothing (that I can
>> remember) that will stop rw devices from changing right after we check it, thus
>> relatively.
> 
> rw_devices is changed in a handful of places on a mounted filesystem,
> not counting device open/close. Device remove and replace are excluded
> from running at that time, rw_devices can't change at this point of
> balance.
> 
> btrfs_dev_replace_finishing
>   - when removing srcdev, rw_devices--
>   - when adding the target device as new, rw_devices++
> 
> btrfs_rm_device
>   - rw_devices--
> 
> btrfs_init_new_device (called by device add)
>   - rw_devices++
> 
> So the chunk mutex is either redundant or there's something I'm missing.
> 

Nope you're right, I missed the EXCL_OP thing, so we can just read rw_devices 
normally.  Thanks,

Josef