Re: [PATCH] Btrfs: move the extent buffer radix tree into the fs_info

[Josef Bacik <jbacik@fb.com>]

On 12/16/2013 08:06 PM, David Sterba wrote:
> On Mon, Dec 16, 2013 at 01:26:26PM -0500, Josef Bacik wrote:
>> I need to create a fake tree to test qgroups and I don't want to have to setup a
>> fake btree_inode.  The fact is we only use the radix tree for the fs_info, so
>> everybody else who allocates an extent_io_tree is just wasting the space anyway.
>> This patch moves the radix tree and its lock into btrfs_fs_info so there is less
>> stuff I have to fake to do qgroup sanity tests.  Thanks,
> This would make the fs_info::buffer_lock a global hotspot if
> alloc_extent_buffer and release_extent_buffer are called frequently.
>
> But, you can get rid of the buffer_lock completely, because the radix
> tree can be safely protected by rcu_read_lock/_unlock:
>
> * alloc_extent_buffer uses radix_preload that turns off preepmtion by
> itself, so the lock here would be pointless

Except you still need a lock for other inserts.
>
> * release_extent_buffer locks around radix_tree_delete, here a rcu
> locking will be ok as well

No it won't.  RCU just makes sure readers don't get screwed, you still 
need to have real locking around the insertions/deletions, look at 
pagecache, we have mapping->tree_lock for this even though it uses rcu 
for the lookups.  Thanks,

Josef