linux-ext4 - Re: [PATCH 1/4] ext4: Fix fsync error handling after filesysteb abort.

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Date:	Fri, 17 May 2013 11:10:28 +0400
From:	Dmitry Monakhov <dmonakhov@...nvz.org>
To:	Yuan Fu <yuan.fu@....cn>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 1/4] ext4: Fix fsync error handling after filesysteb abort.

On Fri, 17 May 2013 05:24:15 +0200, "Yuan Fu" <yuan.fu@....cn> wrote:
> Dear Dmitry Monakhov, 
> 
>   I see a race condition,  
>       __ext4_abort()     
>           ...         
>              EXT4_SB(sb)->s_mount_flags |= EXT4_MF_FS_ABORTED;
>           ...
>              smp_wmb()
>  
>              [if scheduled at this point ]
>           ...
>              sb->s_flags |= MS_RDONLY;
>  
> 
>    if schedule occur above point(in red). There comes race condition
>    the s_mount_flags set to EXT4_MF_FS_ABORTED. On the other hand   
>    sb->s_flags is not set to MS_RDONLY. Now if ext4_fsync_file() is 
>    called from   some process, the check s_flags to MS_RDONLY will fail,
>    and it will flush   unwritten io and not return -EORFS.
If flush_unwritten_io() was called after fs was aborted it will return
appropriate code (EROFS or EIO) so fsync(2) will fail as expected.
> 
>   thanks         
> 
>   ----- Original Message -----
> From: Dmitry Monakhov
> Sent: 05/16/13 05:58 PM
> Subject: [PATCH 1/4] ext4: Fix fsync error handling after filesysteb abort.
>  If filesystem was aborted after inode's write back complete 
> but before it's metadata was updated we may return success 
> due to (sb->s_flags & MS_RDONLY) which is incorrect and 
> result in data loss. 
> In order to handle fs abort correctly we have to check 
> fs state once we discover that it is in MS_RDONLY state 
> 
> Test case: http://patchwork.ozlabs.org/patch/244297/ 
> 
> Signed-off-by: Dmitry Monakhov <dmonakhov@...nvz.org> 
> --- 
>  fs/ext4/fsync.c | 8 ++++++-- 
>  fs/ext4/super.c | 13 ++++++++++++- 
>  2 files changed, 18 insertions(+), 3 deletions(-) 
> 
> diff --git a/fs/ext4/fsync.c b/fs/ext4/fsync.c 
> index e0ba8a4..d7df2f1 100644 
> --- a/fs/ext4/fsync.c 
> +++ b/fs/ext4/fsync.c 
> @@ -129,9 +129,13 @@ int ext4_sync_file(struct file *file, loff_t start, loff_t end, int datasync) 
>  return ret; 
>  mutex_lock(&inode->i_mutex); 
>  
> - if (inode->i_sb->s_flags & MS_RDONLY) 
> + if (inode->i_sb->s_flags & MS_RDONLY) { 
> + /* Make shure that we read updated s_mount_flags value */ 
> + smp_rmb(); 
> + if (EXT4_SB(inode->i_sb)->s_mount_flags & EXT4_MF_FS_ABORTED) 
> + ret = -EROFS; 
>  goto out; 
> - 
> + } 
>  ret = ext4_flush_unwritten_io(inode); 
>  if (ret < 0) 
>  goto out; 
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c 
> index dbc7c09..6c91c8e 100644 
> --- a/fs/ext4/super.c 
> +++ b/fs/ext4/super.c 
> @@ -398,6 +398,11 @@ static void ext4_handle_error(struct super_block *sb) 
>  } 
>  if (test_opt(sb, ERRORS_RO)) { 
>  ext4_msg(sb, KERN_CRIT, "Remounting filesystem read-only"); 
> + /* 
> + * Make shure updated value of ->s_mount_flags will be visiable 
> + * before ->s_flags update 
> + */ 
> + smp_wmb(); 
>  sb->s_flags |= MS_RDONLY; 
>  } 
>  if (test_opt(sb, ERRORS_PANIC)) 
> @@ -552,6 +557,7 @@ void __ext4_std_error(struct super_block *sb, const char *function, 
>  * 
>  * We unconditionally force the filesystem into an ABORT|READONLY state, 
>  * unless the error response on the fs has been set to panic in which 
> + 
>  * case we take the easy way out and panic immediately. 
>  */ 
>  
> @@ -570,8 +576,13 @@ void __ext4_abort(struct super_block *sb, const char *function, 
>  
>  if ((sb->s_flags & MS_RDONLY) == 0) { 
>  ext4_msg(sb, KERN_CRIT, "Remounting filesystem read-only"); 
> - sb->s_flags |= MS_RDONLY; 
>  EXT4_SB(sb)->s_mount_flags |= EXT4_MF_FS_ABORTED; 
> + /* 
> + * Make shure updated value of ->s_mount_flags will be visiable 
> + * before ->s_flags update 
> + */ 
> + smp_wmb(); 
> + sb->s_flags |= MS_RDONLY; 
>  if (EXT4_SB(sb)->s_journal) 
>  jbd2_journal_abort(EXT4_SB(sb)->s_journal, -EIO); 
>  save_error_info(sb, function, line); 
> -- 
> 1.7.1 
> 
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