From: Ted Ts'o <tytso@mit.edu>
To: Jan Kara <jack@suse.cz>
Cc: Toshiyuki Okajima <toshi.okajima@jp.fujitsu.com>,
adilger.kernel@dilger.ca, Yongqiang Yang <xiaoqiangnk@gmail.com>,
linux-ext4@vger.kernel.org
Subject: Re: [PATCH 1/2] jbd2: jbd2_journal_stop needs an exclusive control to synchronize around t_update operations
Date: Wed, 4 Jan 2012 22:12:40 -0500 [thread overview]
Message-ID: <20120105031240.GC24494@thunk.org> (raw)
In-Reply-To: <20120103153245.GE31457@quack.suse.cz>
On Tue, Jan 03, 2012 at 04:32:45PM +0100, Jan Kara wrote:
> Thanks for the analysis. Actually, you fix adds unnecessary overhead.
> The problem really is the wrong ordering of prepare_to_wait() and t_updates
> check. So attached patch should fix the issue as well without introducing
> the overhead.
Thanks, applied.
- Ted
> From 1cd5b8178893f3f186ce93eb1f47664a1a3e81fc Mon Sep 17 00:00:00 2001
> From: Jan Kara <jack@suse.cz>
> Date: Tue, 3 Jan 2012 16:13:29 +0100
> Subject: [PATCH] jbd2: Fix hung processes in jbd2_journal_lock_updates()
>
> Toshiyuki Okajima found out that when running
>
> for ((i=0; i < 100000; i++)); do
> if ((i%2 == 0)); then
> chattr +j /mnt/file
> else
> chattr -j /mnt/file
> fi
> echo "0" >> /mnt/file
> done
>
> process sometimes hangs indefinitely in jbd2_journal_lock_updates().
>
> Toshiyuki identified that the following race happens:
>
> jbd2_journal_lock_updates() |jbd2_journal_stop()
> ---------------------------------------+---------------------------------------
> write_lock(&journal->j_state_lock) | .
> ++journal->j_barrier_count | .
> spin_lock(&tran->t_handle_lock) | .
> atomic_read(&tran->t_updates) //not 0 |
> | atomic_dec_and_test(&tran->t_updates)
> | // t_updates = 0
> | wake_up(&journal->j_wait_updates)
> prepare_to_wait() | // no process is woken up.
> spin_unlock(&tran->t_handle_lock) |
> write_unlock(&journal->j_state_lock) |
> schedule() // never return |
>
> We fix the problem by first calling prepare_to_wait() and only after that
> checking t_updates in jbd2_journal_lock_updates().
>
> Reported-and-analyzed-by: Toshiyuki Okajima <toshi.okajima@jp.fujitsu.com>
> Signed-off-by: Jan Kara <jack@suse.cz>
next prev parent reply other threads:[~2012-01-05 3:12 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-12-16 11:19 [PATCH][RFC] jbd2: jbd2_journal_stop needs an exclusive control to synchronize around t_updates operations Toshiyuki Okajima
2011-12-16 12:22 ` Yongqiang Yang
2011-12-20 10:44 ` Toshiyuki Okajima
2011-12-22 11:36 ` [PATCH 0/2 take2][RFC] hangup in jbd2_journal_lock_updates Toshiyuki Okajima
2011-12-22 11:56 ` [PATCH 1/2] jbd2: jbd2_journal_stop needs an exclusive control to synchronize around t_update operations Toshiyuki Okajima
2012-01-03 15:32 ` Jan Kara
2012-01-05 3:12 ` Ted Ts'o [this message]
2011-12-22 12:00 ` [PATCH 2/2] jbd2: remove all t_handle_lock statements Toshiyuki Okajima
2011-12-22 14:07 ` Yongqiang Yang
2011-12-26 0:20 ` Toshiyuki Okajima
2011-12-26 1:17 ` [PATCH 2/2 take2] jbd2: delete spin_lock(t_handle_lock) inside wirte_lock(j_state_lock) Toshiyuki Okajima
2011-12-31 6:35 ` Yongqiang Yang
2012-01-03 15:35 ` Jan Kara
2012-01-05 3:27 ` Ted Ts'o
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