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Date:   Tue, 9 Jun 2020 15:03:12 +0800
From:   "zhangyi (F)" <yi.zhang@...wei.com>
To:     Jan Kara <jack@...e.cz>
CC:     <linux-ext4@...r.kernel.org>, <tytso@....edu>,
        <adilger.kernel@...ger.ca>, <zhangxiaoxu5@...wei.com>
Subject: Re: [PATCH] ext4, jbd2: switch to use completion variable instead of
 JBD2_REC_ERR

On 2020/6/8 23:36, Jan Kara wrote:
> On Mon 08-06-20 23:08:42, zhangyi (F) wrote:
>> Hi, Jan.
>>
>> On 2020/6/8 15:57, Jan Kara wrote:
>>> On Tue 26-05-20 22:20:39, zhangyi (F) wrote:
>>>> In the ext4 filesystem with errors=panic, if one process is recording
>>>> errno in the superblock when invoking jbd2_journal_abort() due to some
>>>> error cases, it could be raced by another __ext4_abort() which is
>>>> setting the SB_RDONLY flag but missing panic because errno has not been
>>>> recorded.
>>>>
>>>> jbd2_journal_abort()
>>>>  journal->j_flags |= JBD2_ABORT;
>>>>  jbd2_journal_update_sb_errno()
>>>>                                    | __ext4_abort()
>>>>                                    |  sb->s_flags |= SB_RDONLY;
>>>>                                    |  if (!JBD2_REC_ERR)
>>>>                                    |       return;
>>>>  journal->j_flags |= JBD2_REC_ERR;
>>>>
>>>> Finally, it will no longer trigger panic because the filesystem has
>>>> already been set read-only. Fix this by remove JBD2_REC_ERR and switch
>>>> to use completion variable instead.
>>>
>>> Thanks for the patch! I don't quite understand how this last part can
>>> happen: "Finally, it will no longer trigger panic because the filesystem has
>>> already been set read-only."
>>>
>>> AFAIU jbd2_journal_abort() gets called somewhere from jbd2 so ext4 doesn't
>>> know about it. At the same time ext4_abort() gets called somewhere from
>>> ext4 and races as you describe above. OK. But then the next ext4_abort()
>>> call should panic() just fine. What am I missing? I understand that we
>>> might want that the first ext4_abort() already triggers the panic but I'd
>>> like to understand whether that's the bug you're trying to fix or something
>>> else...
>>>
>> Since the fs is marked to read-only in the first ext4_abort(), the
>> ext4_journal_check_start() will return -EROFS immediately, so we
>> have no chance to invoke ext4_abort() again and trigger panic.
>>
>> static int ext4_journal_check_start(struct super_block *sb)
>> {
>> ...
>> 	if (sb_rdonly(sb))
>> 		return -EROFS;
>> ...
>> }
> 
> Ah, I see. I didn't look into ext4_journal_check_start() in particular.
> Thanks for explanation.
> 
>>> WRT the solution I think that the completion you add unnecessarily
>>> complicates matters. I'd rather introduce j_abort_mutex to the journal and
>>> all jbd2_journal_abort() calls will take it and release it once everything
>>> is done. That way we can remove JBD2_REC_ERR, races are avoided, and the
>>> filesystem (ext4 or ocfs2) knows that after its call to
>>> jbd2_journal_abort() completes, journal abort is completed (either by us or
>>> someone else) and so we are free to panic. No need for strange
>>> wait_for_completion() calls in ext4_handle_error() or __ext4_abort() and
>>> the error handling is again fully self-contained within the jbd2 layer.
>>>
>>
>> Now, the race condition is between jbd2_journal_abort() and
>> ext4_handle_error()/__ext4_abort(), so if we only use j_abort_mutex, it
>> will re-introduce the problem which 4327ba52afd03 want to fix, think
>> about below case:
>>
>> jbd2_journal_commit_transaction()   ext4_journal_check_start()   ext4_journal_check_start()
>>  jbd2_journal_abort()
>>    lock j_abort_mutex
>>    journal->j_flags |= JBD2_ABORT;
>>                                      __ext4_abort()
>>                                                                    __ext4_abort()
>>                                       sb->s_flags |= SB_RDONLY;
>>                                                                      panic()  <-- system panic here due to "sb_rdonly()==true"
>>                                       jbd2_journal_abort() <-- block
>>    jbd2_journal_update_sb_errno  <-- not write to disk
>>    unlock j_abort_mutex
>>
>> The system will panic before the error info is written to the journal's
>> super block. Use j_abort_mutex to avoid the race between jbd2_journal_abort()
>> and ext4_handle_error()/__ext4_abort() is depends on the both of those two
>> ext4 error handlers invoke jbd2_journal_abort(), if not, the race will
>> re-open.
> 
> Yes, you're right. Or we could move sb->s_flags |= SB_RDONLY in
> __ext4_abort() after jbd2_journal_abort() call, can't we?
> 

OK, it looks good, thanks for your suggestion, will do.

Thanks,
Yi.

> 
>>>> Fixes: 4327ba52afd03 ("ext4, jbd2: ensure entering into panic after recording an error in superblock")
>>>> Signed-off-by: zhangyi (F) <yi.zhang@...wei.com>
>>>> ---
>>>>  fs/ext4/super.c      | 25 +++++++++++++------------
>>>>  fs/jbd2/journal.c    |  6 ++----
>>>>  include/linux/jbd2.h |  6 +++++-
>>>>  3 files changed, 20 insertions(+), 17 deletions(-)
>>>>
>>>> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
>>>> index bf5fcb477f66..987a0bd5b78a 100644
>>>> --- a/fs/ext4/super.c
>>>> +++ b/fs/ext4/super.c
>>>> @@ -495,6 +495,8 @@ static bool system_going_down(void)
>>>>  
>>>>  static void ext4_handle_error(struct super_block *sb)
>>>>  {
>>>> +	struct ext4_sb_info *sbi = EXT4_SB(sb);
>>>> +
>>>>  	if (test_opt(sb, WARN_ON_ERROR))
>>>>  		WARN_ON_ONCE(1);
>>>>  
>>>> @@ -502,9 +504,9 @@ static void ext4_handle_error(struct super_block *sb)
>>>>  		return;
>>>>  
>>>>  	if (!test_opt(sb, ERRORS_CONT)) {
>>>> -		journal_t *journal = EXT4_SB(sb)->s_journal;
>>>> +		journal_t *journal = sbi->s_journal;
>>>>  
>>>> -		EXT4_SB(sb)->s_mount_flags |= EXT4_MF_FS_ABORTED;
>>>> +		sbi->s_mount_flags |= EXT4_MF_FS_ABORTED;
>>>>  		if (journal)
>>>>  			jbd2_journal_abort(journal, -EIO);
>>>>  	}
>>>> @@ -522,9 +524,8 @@ static void ext4_handle_error(struct super_block *sb)
>>>>  		smp_wmb();
>>>>  		sb->s_flags |= SB_RDONLY;
>>>>  	} else if (test_opt(sb, ERRORS_PANIC)) {
>>>> -		if (EXT4_SB(sb)->s_journal &&
>>>> -		  !(EXT4_SB(sb)->s_journal->j_flags & JBD2_REC_ERR))
>>>> -			return;
>>>> +		if (sbi->s_journal && is_journal_aborted(sbi->s_journal))
>>>> +			wait_for_completion(&sbi->s_journal->j_record_errno);
>>>>  		panic("EXT4-fs (device %s): panic forced after error\n",
>>>>  			sb->s_id);
>>>>  	}
>>>> @@ -710,10 +711,11 @@ void __ext4_std_error(struct super_block *sb, const char *function,
>>>>  void __ext4_abort(struct super_block *sb, const char *function,
>>>>  		  unsigned int line, int error, const char *fmt, ...)
>>>>  {
>>>> +	struct ext4_sb_info *sbi = EXT4_SB(sb);
>>>>  	struct va_format vaf;
>>>>  	va_list args;
>>>>  
>>>> -	if (unlikely(ext4_forced_shutdown(EXT4_SB(sb))))
>>>> +	if (unlikely(ext4_forced_shutdown(sbi)))
>>>>  		return;
>>>>  
>>>>  	save_error_info(sb, error, 0, 0, function, line);
>>>> @@ -726,20 +728,19 @@ void __ext4_abort(struct super_block *sb, const char *function,
>>>>  
>>>>  	if (sb_rdonly(sb) == 0) {
>>>>  		ext4_msg(sb, KERN_CRIT, "Remounting filesystem read-only");
>>>> -		EXT4_SB(sb)->s_mount_flags |= EXT4_MF_FS_ABORTED;
>>>> +		sbi->s_mount_flags |= EXT4_MF_FS_ABORTED;
>>>>  		/*
>>>>  		 * Make sure updated value of ->s_mount_flags will be visible
>>>>  		 * before ->s_flags update
>>>>  		 */
>>>>  		smp_wmb();
>>>>  		sb->s_flags |= SB_RDONLY;
>>>> -		if (EXT4_SB(sb)->s_journal)
>>>> -			jbd2_journal_abort(EXT4_SB(sb)->s_journal, -EIO);
>>>> +		if (sbi->s_journal)
>>>> +			jbd2_journal_abort(sbi->s_journal, -EIO);
>>>>  	}
>>>>  	if (test_opt(sb, ERRORS_PANIC) && !system_going_down()) {
>>>> -		if (EXT4_SB(sb)->s_journal &&
>>>> -		  !(EXT4_SB(sb)->s_journal->j_flags & JBD2_REC_ERR))
>>>> -			return;
>>>> +		if (sbi->s_journal && is_journal_aborted(sbi->s_journal))
>>>> +			wait_for_completion(&sbi->s_journal->j_record_errno);
>>>>  		panic("EXT4-fs panic from previous error\n");
>>>>  	}
>>>>  }
>>>> diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
>>>> index a49d0e670ddf..b8acdb2f7ac7 100644
>>>> --- a/fs/jbd2/journal.c
>>>> +++ b/fs/jbd2/journal.c
>>>> @@ -1140,6 +1140,7 @@ static journal_t *journal_init_common(struct block_device *bdev,
>>>>  	init_waitqueue_head(&journal->j_wait_commit);
>>>>  	init_waitqueue_head(&journal->j_wait_updates);
>>>>  	init_waitqueue_head(&journal->j_wait_reserved);
>>>> +	init_completion(&journal->j_record_errno);
>>>>  	mutex_init(&journal->j_barrier);
>>>>  	mutex_init(&journal->j_checkpoint_mutex);
>>>>  	spin_lock_init(&journal->j_revoke_lock);
>>>> @@ -2188,10 +2189,7 @@ void jbd2_journal_abort(journal_t *journal, int errno)
>>>>  	 * layer could realise that a filesystem check is needed.
>>>>  	 */
>>>>  	jbd2_journal_update_sb_errno(journal);
>>>> -
>>>> -	write_lock(&journal->j_state_lock);
>>>> -	journal->j_flags |= JBD2_REC_ERR;
>>>> -	write_unlock(&journal->j_state_lock);
>>>> +	complete_all(&journal->j_record_errno);
>>>>  }
>>>>  
>>>>  /**
>>>> diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
>>>> index f613d8529863..0f623b0c347f 100644
>>>> --- a/include/linux/jbd2.h
>>>> +++ b/include/linux/jbd2.h
>>>> @@ -765,6 +765,11 @@ struct journal_s
>>>>  	 */
>>>>  	int			j_errno;
>>>>  
>>>> +	/**
>>>> +	 * @j_record_errno: complete to record errno in the journal superblock
>>>> +	 */
>>>> +	struct completion	j_record_errno;
>>>> +
>>>>  	/**
>>>>  	 * @j_sb_buffer: The first part of the superblock buffer.
>>>>  	 */
>>>> @@ -1247,7 +1252,6 @@ JBD2_FEATURE_INCOMPAT_FUNCS(csum3,		CSUM_V3)
>>>>  #define JBD2_ABORT_ON_SYNCDATA_ERR	0x040	/* Abort the journal on file
>>>>  						 * data write error in ordered
>>>>  						 * mode */
>>>> -#define JBD2_REC_ERR	0x080	/* The errno in the sb has been recorded */
>>>>  
>>>>  /*
>>>>   * Function declarations for the journaling transaction and buffer
>>>> -- 
>>>> 2.21.3
>>>>
>>

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