Re: [PATCH] ext4, jbd2: switch to use completion variable instead of JBD2_REC_ERR

zhangyi (F)
2020-06-08T15:08:56Z

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;
...
}

> 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.

Thanks,
Yi.

>
>> Fixes: 4327ba52afd03 ("ext4, jbd2: ensure entering into panic after recording an error in superblock")
>> Signed-off-by: zhangyi (F) <yi.zhang@huawei.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
>>