Received-SPF: Pass (protection.outlook.com: domain of mit.edu designates
 18.9.28.11 as permitted sender) receiver=protection.outlook.com;
 client-ip=18.9.28.11; helo=outgoing.mit.edu;
Date:   Wed, 23 Jan 2019 00:30:35 -0500
From:   "Theodore Y. Ts'o" <tytso@mit.edu>
To:     Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
CC:     <linux-ext4@vger.kernel.org>, <jack@suse.cz>
Subject: Re: [PATCH v2] jbd2: fix deadlock while checkpoint thread waits
 commit thread to finish
Message-ID: <20190123053035.GB8785@mit.edu>
References: <20181125083606.5273-1-xiaoguang.wang@linux.alibaba.com>
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Disposition: inline
In-Reply-To: <20181125083606.5273-1-xiaoguang.wang@linux.alibaba.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
SpamDiagnosticOutput: 1:99
SpamDiagnosticMetadata: NSPM
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 23 Jan 2019 05:30:38.9558
 (UTC)
X-MS-Exchange-CrossTenant-Network-Message-Id: 7e7befbd-feda-4211-dccf-08d680f3e8eb
X-MS-Exchange-CrossTenant-Id: 64afd9ba-0ecf-4acf-bc36-935f6235ba8b
X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=64afd9ba-0ecf-4acf-bc36-935f6235ba8b;Ip=[18.9.28.11];Helo=[outgoing.mit.edu]
X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem
X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM6PR01MB3753
Sender: linux-ext4-owner@vger.kernel.org
Precedence: bulk
List-ID: <linux-ext4.vger.kernel.org>
X-Mailing-List: linux-ext4@vger.kernel.org

On Sun, Nov 25, 2018 at 04:36:06PM +0800, Xiaoguang Wang wrote:
> This issue was found when I tried to put checkpoint work in a separate thread,
> the deadlock below happened:
>          Thread1                                |   Thread2
> __jbd2_log_wait_for_space                       |
> jbd2_log_do_checkpoint (hold j_checkpoint_mutex)|
>   if (jh->b_transaction != NULL)                |
>     ...                                         |
>     jbd2_log_start_commit(journal, tid);        |jbd2_update_log_tail
>                                                 |  will lock j_checkpoint_mutex,
>                                                 |  but will be blocked here.
>                                                 |
>     jbd2_log_wait_commit(journal, tid);         |
>     wait_event(journal->j_wait_done_commit,     |
>      !tid_gt(tid, journal->j_commit_sequence)); |
>      ...                                        |wake_up(j_wait_done_commit)
>   }                                             |
> 
> then deadlock occurs, Thread1 will never be waken up.
> 
> To fix this issue, drop j_checkpoint_mutex in jbd2_log_do_checkpoint()
> when we are going to wait for transaction commit.
> 
> Reviewed-by: Jan Kara <jack@suse.cz>
> Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>

Thanks, applied.

						- Ted