Re: [PATCH] btrfs: qgroup: don't try to wait flushing if we're already holding a transaction

[David Sterba <dsterba@suse.cz>]

On Fri, Dec 04, 2020 at 09:24:47AM +0800, Qu Wenruo wrote:
> There is a chance of racing for qgroup flushing which may lead to
> deadlock:
> 
> 	Thread A		|	Thread B
>    (no trans handler hold)	|  (already hold a trans handler)
> --------------------------------+--------------------------------
> __btrfs_qgroup_reserve_meta()   | __btrfs_qgroup_reserve_meta()
> |- try_flush_qgroup()		| |- try_flushing_qgroup()
>    |- QGROUP_FLUSHING bit set   |    |
>    |				|    |- test_and_set_bit()
>    |				|    |- wait_event()
>    |- btrfs_join_transaction()	|
>    |- btrfs_commit_transaction()|
> 
> 			!!! DEAD LOCK !!!
> 
> Since thread A want to commit transaction, but thread B is hold a
> transaction handler, blocking the commit.
> At the same time, thread B is waiting thread A to finish it commit.
> 
> This is just a hot fix, and would lead to more EDQUOT when we're near
> the qgroup limit.
> 
> The root fix would to make all metadata/data reservation to happen
> without a transaction handler hold.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>

Added to misc-next, thanks.