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Date: Thu, 12 May 2011 12:40:40 +0300
From: Surbhi Palande <surbhi.palande@...onical.com>
To: Andreas Dilger <adilger@...ger.ca>
CC: Eric Sandeen <sandeen@...hat.com>, "jack@...e.cz" <jack@...e.cz>,
"marco.stornelli@...il.com" <marco.stornelli@...il.com>,
"adilger.kernel@...ger.ca" <adilger.kernel@...ger.ca>,
"toshi.okajima@...fujitsu.com" <toshi.okajima@...fujitsu.com>,
"tytso@....edu" <tytso@....edu>,
"m.mizuma@...fujitsu.com" <m.mizuma@...fujitsu.com>,
"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v3] Adding support to freeze and unfreeze a journal
On 05/11/2011 12:05 PM, Andreas Dilger wrote:
> On 2011-05-11, at 1:06 AM, Surbhi Palande<surbhi.palande@...onical.com> wrote:
>
>> On 05/09/2011 06:23 PM, Eric Sandeen wrote:
>>> On 5/9/11 9:51 AM, Surbhi Palande wrote:
>>>> The journal should be frozen when a F.S freezes. What this means is that till
>>>> the F.S is thawed again, no new transactions should be accepted by the
>>>> journal. When the F.S thaws, inturn it should thaw the journal and this should
>>>> allow the journal to resume accepting new transactions.
>>>> While the F.S has frozen the journal, the clients of journal on calling
>>>> jbd2_journal_start() will sleep on a wait queue. Thawing the journal will wake
>>>> up the sleeping clients and journalling can progress normally.
>>>
>>> Can I ask how this was tested?
>>
>> Yes! I did the following on an ext4 fs mount:
>> 1. fsfreeze -f $MNT
>> 2. dd if=/dev/zero of=$MNT/file count=10 bs=1024&
>> 3. sync
>> 4. fsfreeze -u $MNT
>>
>> If the dd blocks on the start_handle, then the page cache is clean and sync should have nothing to write and everything will work fine. But otherwise this should sequence should create a deadlock.
>
> Sorry to ask the obvious question, but presumably this test fails without your patch? It isn't clear from your comment that this is the case.
Actually since this is a race its very difficult to see this
deterministically. The deadlock is apparently regulary seen by running
iozone on multipath - when a path comes back to service.
I imagined that this could be simulated by running a lot of I/O in the
background and trying fsfreeze, unfreeze parallely (multiple times).
Unfortunately its not easy to hit the bug - its never deterministic. I
really smoke tested my patch using this method
{
dd i/o in a loop (10000 times) & (background process)
touch some file & in the same loop (background process)
}
{
freeze &, sync &, unfreeze &, sleep in a loop & (100 times)
(background processes)
}
and saw that there was not any deadlock in this case. But I have not
tested/seen the deadlock with this script without the patch.
Sorry for not being clear before :(
Warm Regards,
Surbhi.
>
>> I have attempted to create a patch for xfs-test. Shall send it out as a reply to this email soon!
>>
>> Warm Regards,
>> Surbhi.
>>
>>
>>
>>>
>>> Ideally anything you found useful for testing should probably be integrated
>>> into the xfstests test suite so that we don't regresss in the future.
>>>
>>> thanks,
>>> -Eric
>>>
>>>> Signed-off-by: Surbhi Palande<surbhi.palande@...onical.com>
>>>> ---
>>>> Changes since the last patch:
>>>> * Changed to the shorter forms of expressions eg: x |= y
>>>> * removed the unnecessary barrier
>>>>
>>>> fs/ext4/super.c | 20 ++++++--------------
>>>> fs/jbd2/journal.c | 1 +
>>>> fs/jbd2/transaction.c | 42 ++++++++++++++++++++++++++++++++++++++++++
>>>> include/linux/jbd2.h | 10 ++++++++++
>>>> 4 files changed, 59 insertions(+), 14 deletions(-)
>>>>
>>>> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
>>>> index 8553dfb..796aa4c 100644
>>>> --- a/fs/ext4/super.c
>>>> +++ b/fs/ext4/super.c
>>>> @@ -4179,23 +4179,15 @@ static int ext4_freeze(struct super_block *sb)
>>>>
>>>> journal = EXT4_SB(sb)->s_journal;
>>>>
>>>> - /* Now we set up the journal barrier. */
>>>> - jbd2_journal_lock_updates(journal);
>>>> -
>>>> + error = jbd2_journal_freeze(journal);
>>>> /*
>>>> - * Don't clear the needs_recovery flag if we failed to flush
>>>> + * Don't clear the needs_recovery flag if we failed to freeze
>>>> * the journal.
>>>> */
>>>> - error = jbd2_journal_flush(journal);
>>>> - if (error< 0)
>>>> - goto out;
>>>> -
>>>> - /* Journal blocked and flushed, clear needs_recovery flag. */
>>>> - EXT4_CLEAR_INCOMPAT_FEATURE(sb, EXT4_FEATURE_INCOMPAT_RECOVER);
>>>> - error = ext4_commit_super(sb, 1);
>>>> -out:
>>>> - /* we rely on s_frozen to stop further updates */
>>>> - jbd2_journal_unlock_updates(EXT4_SB(sb)->s_journal);
>>>> + if (error>= 0) {
>>>> + EXT4_CLEAR_INCOMPAT_FEATURE(sb, EXT4_FEATURE_INCOMPAT_RECOVER);
>>>> + error = ext4_commit_super(sb, 1);
>>>> + }
>>>> return error;
>>>> }
>>>>
>>>> diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
>>>> index e0ec3db..5e46333 100644
>>>> --- a/fs/jbd2/journal.c
>>>> +++ b/fs/jbd2/journal.c
>>>> @@ -842,6 +842,7 @@ static journal_t * journal_init_common (void)
>>>> init_waitqueue_head(&journal->j_wait_checkpoint);
>>>> init_waitqueue_head(&journal->j_wait_commit);
>>>> init_waitqueue_head(&journal->j_wait_updates);
>>>> + init_waitqueue_head(&journal->j_wait_frozen);
>>>> mutex_init(&journal->j_barrier);
>>>> mutex_init(&journal->j_checkpoint_mutex);
>>>> spin_lock_init(&journal->j_revoke_lock);
>>>> diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
>>>> index 05fa77a..b040293 100644
>>>> --- a/fs/jbd2/transaction.c
>>>> +++ b/fs/jbd2/transaction.c
>>>> @@ -171,6 +171,17 @@ repeat:
>>>> journal->j_barrier_count == 0);
>>>> goto repeat;
>>>> }
>>>> + /* dont let a new handle start when a journal is frozen.
>>>> + * jbd2_journal_freeze calls jbd2_journal_unlock_updates() only after
>>>> + * the jflags indicate that the journal is frozen. So if the
>>>> + * j_barrier_count is 0, then check if this was made 0 by the freezing
>>>> + * process
>>>> + */
>>>> + if (journal->j_flags& JBD2_FROZEN) {
>>>> + read_unlock(&journal->j_state_lock);
>>>> + jbd2_check_frozen(journal);
>>>> + goto repeat;
>>>> + }
>>>>
>>>> if (!journal->j_running_transaction) {
>>>> read_unlock(&journal->j_state_lock);
>>>> @@ -489,6 +500,37 @@ int jbd2_journal_restart(handle_t *handle, int nblocks)
>>>> }
>>>> EXPORT_SYMBOL(jbd2_journal_restart);
>>>>
>>>> +int jbd2_journal_freeze(journal_t *journal)
>>>> +{
>>>> + int error = 0;
>>>> + /* Now we set up the journal barrier. */
>>>> + jbd2_journal_lock_updates(journal);
>>>> +
>>>> + /*
>>>> + * Don't clear the needs_recovery flag if we failed to flush
>>>> + * the journal.
>>>> + */
>>>> + error = jbd2_journal_flush(journal);
>>>> + if (error>= 0) {
>>>> + write_lock(&journal->j_state_lock);
>>>> + journal->j_flags |= JBD2_FROZEN;
>>>> + write_unlock(&journal->j_state_lock);
>>>> + }
>>>> + jbd2_journal_unlock_updates(journal);
>>>> + return error;
>>>> +}
>>>> +EXPORT_SYMBOL(jbd2_journal_freeze);
>>>> +
>>>> +void jbd2_journal_thaw(journal_t * journal)
>>>> +{
>>>> + write_lock(&journal->j_state_lock);
>>>> + journal->j_flags&= ~JBD2_FROZEN;
>>>> + write_unlock(&journal->j_state_lock);
>>>> + wake_up(&journal->j_wait_frozen);
>>>> +}
>>>> +EXPORT_SYMBOL(jbd2_journal_thaw);
>>>> +
>>>> +
>>>> /**
>>>> * void jbd2_journal_lock_updates () - establish a transaction barrier.
>>>> * @journal: Journal to establish a barrier on.
>>>> diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
>>>> index a32dcae..c7885b2 100644
>>>> --- a/include/linux/jbd2.h
>>>> +++ b/include/linux/jbd2.h
>>>> @@ -718,6 +718,7 @@ jbd2_time_diff(unsigned long start, unsigned long end)
>>>> * @j_wait_checkpoint: Wait queue to trigger checkpointing
>>>> * @j_wait_commit: Wait queue to trigger commit
>>>> * @j_wait_updates: Wait queue to wait for updates to complete
>>>> + * @j_wait_frozen: Wait queue to wait for journal to thaw
>>>> * @j_checkpoint_mutex: Mutex for locking against concurrent checkpoints
>>>> * @j_head: Journal head - identifies the first unused block in the journal
>>>> * @j_tail: Journal tail - identifies the oldest still-used block in the
>>>> @@ -835,6 +836,9 @@ struct journal_s
>>>> /* Wait queue to wait for updates to complete */
>>>> wait_queue_head_t j_wait_updates;
>>>>
>>>> + /* Wait queue to wait for journal to thaw*/
>>>> + wait_queue_head_t j_wait_frozen;
>>>> +
>>>> /* Semaphore for locking against concurrent checkpoints */
>>>> struct mutex j_checkpoint_mutex;
>>>>
>>>> @@ -1013,7 +1017,11 @@ struct journal_s
>>>> #define JBD2_ABORT_ON_SYNCDATA_ERR 0x040 /* Abort the journal on file
>>>> * data write error in ordered
>>>> * mode */
>>>> +#define JBD2_FROZEN 0x080 /* Journal thread is frozen as the filesystem is frozen */
>>>> +
>>>>
>>>> +#define jbd2_check_frozen(journal) \
>>>> + wait_event(journal->j_wait_frozen, (journal->j_flags& JBD2_FROZEN))
>>>> /*
>>>> * Function declarations for the journaling transaction and buffer
>>>> * management
>>>> @@ -1121,6 +1129,8 @@ extern void jbd2_journal_invalidatepage(journal_t *,
>>>> struct page *, unsigned long);
>>>> extern int jbd2_journal_try_to_free_buffers(journal_t *, struct page *, gfp_t);
>>>> extern int jbd2_journal_stop(handle_t *);
>>>> +extern int jbd2_journal_freeze(journal_t *);
>>>> +extern void jbd2_journal_thaw(journal_t *);
>>>> extern int jbd2_journal_flush (journal_t *);
>>>> extern void jbd2_journal_lock_updates (journal_t *);
>>>> extern void jbd2_journal_unlock_updates (journal_t *);
>>>
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