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

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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On 2020/12/5 上午1:28, David Sterba wrote:
> 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.
> 
Sorry, forgot the fixes tag:

Fixes: c53e9653605d ("btrfs: qgroup: try to flush qgroup space when we
get -EDQUOT")

Mind to add that in the branch?

Thanks,
Qu


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