Re: [PATCH] btrfs: fix lockdep warning chunk_mutex vs device_list_mutex

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

On 16/5/20 1:40 am, David Sterba wrote:
> On Thu, May 14, 2020 at 03:46:59AM +0800, Anand Jain wrote:
>> A full list of tests just started.
>>
>>   fs/btrfs/volumes.c | 8 +++++---
>>   1 file changed, 5 insertions(+), 3 deletions(-)
>>
>> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
>> index 60ab41c12e50..ebc8565d0f73 100644
>> --- a/fs/btrfs/volumes.c
>> +++ b/fs/btrfs/volumes.c
>> @@ -984,7 +984,6 @@ static struct btrfs_fs_devices *clone_fs_devices(struct btrfs_fs_devices *orig)
>>   	if (IS_ERR(fs_devices))
>>   		return fs_devices;
>>   
> 
> So now here's the device_list_mutex taken by a caller but inside
> clone_fs_devices there's
> 
> 	fs_devices = alloc_fs_devices(orig->fsid, NULL);
> 
> just before this line and it does a GFP_KERNEL allocation.

Oh right the allocations. Its not just about the other locks
as I thought before.

There are two ways to fix.
   Use GFP_NOFS
    I am not yet sure if it not possible. There were some previous
    work on the GFP flags. I need to review them. or,
   Move the allocation outside the locks.

  Looking into both of these choices.

Thanks, Anand

> This could
> deadlock through the allocator trying to flush data and then superblock
> write locking the device_list_mutex again.
> 
>> -	mutex_lock(&orig->device_list_mutex);
>>   	fs_devices->total_devices = orig->total_devices;
>>   
>>   	list_for_each_entry(orig_dev, &orig->devices, dev_list) {