linux-ext4 - Re: [PATCH v2] jbd2: fix deadlock while checkpoint thread waits commit thread to finish

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Date:   Wed, 23 Jan 2019 00:30:35 -0500
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Xiaoguang Wang <xiaoguang.wang@...ux.alibaba.com>
CC:     <linux-ext4@...r.kernel.org>, <jack@...e.cz>
Subject: Re: [PATCH v2] jbd2: fix deadlock while checkpoint thread waits
 commit thread to finish

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@...e.cz>
> Signed-off-by: Xiaoguang Wang <xiaoguang.wang@...ux.alibaba.com>

Thanks, applied.

						- Ted