Re: [PATCH] btrfs: zoned: use alloc_list instead of rcu locked device_list

[Anand Jain <anand.jain@oracle.com>]

On 15/03/2022 02:55, David Sterba wrote:
> On Mon, Mar 14, 2022 at 07:16:47AM -0700, Johannes Thumshirn wrote:
>> Anand pointed out, that instead of using the rcu locked version of
>> fs_devices->device_list we cab use fs_devices->alloc_list, prrotected by
>> the chunk_mutex to traverse the list of active deviices in
>> btrfs_can_activate_zone().
> 
> Why?

We are in the chunk allocation thread. The newer chunk allocation
happens from the devices in the fs_device->alloc_list protected by
the chunk_mutex.

  btrfs_create_chunk()
    lockdep_assert_held(&info->chunk_mutex);
    gather_device_info
     list_for_each_entry(device, &fs_devices->alloc_list, dev_alloc_list) {

Also, a device reappeared after the mount won't join the alloc_list yet
and, it will be in the dev_list, which we don't want to consider in the
context of the chunk alloc.

Thanks, Anand