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

["Yuan Fu" <yuan.fu@gmx.cn>]

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.

  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@openvz.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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