Re: [PATCH 10/11] Btrfs: use bit operation for -fs_state

2013-01-16 Thread David Sterba 
On Mon, Jan 14, 2013 at 03:50:31PM +0800, Miao Xie wrote:
 Onthu, 10 Jan 2013 18:57:35 +0100, David Sterba wrote:
  On Thu, Jan 10, 2013 at 08:51:59PM +0800, Miao Xie wrote:
  There is no lock to protect fs_info-fs_state, it will introduce some 
  problems,
  such as the value may be covered by the other task when several tasks 
  modify
  it. Now we use bit operation for it to fix the above problem.
  
  Can you please describe in more detail how does that happen and to what
  problems it leads?
 
 For example:
   Task0 - CPU0Task1 - CPU1
   mov %fs_state rax
   or $0x1 rax
   mov %fs_state rax
   or $0x2 rax
   mov rax %fs_state
   mov rax %fs_state
 
 The expected value is 3, but in fact, it is 2

Yes, generally this happens like this, but lacks connection to btrfs.
What's the call stack on the cpus when this happens? This would mean
that abort transaction is called at the same time from 2 cpus and 2
different flag bits need to be set on fs_state. As there's only one such
flag that's not the case right now.

Losing an error flag here is potentailly bad, so I agree this needs to be
fixed and be future proof, although I doubt that the case of 2
simultaneous aborts is even remotely possible. (Personal note: look for
such occurences.)

The changelog is not very specific and wants to be improved.

david
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Re: [PATCH 10/11] Btrfs: use bit operation for -fs_state

2013-01-14 Thread Liu Bo 
On Mon, Jan 14, 2013 at 03:50:31PM +0800, Miao Xie wrote:
 Onthu, 10 Jan 2013 18:57:35 +0100, David Sterba wrote:
  On Thu, Jan 10, 2013 at 08:51:59PM +0800, Miao Xie wrote:
  There is no lock to protect fs_info-fs_state, it will introduce some 
  problems,
  such as the value may be covered by the other task when several tasks 
  modify
  it. Now we use bit operation for it to fix the above problem.
  
  Can you please describe in more detail how does that happen and to what
  problems it leads?
 
 For example:
   Task0 - CPU0Task1 - CPU1
   mov %fs_state rax
   or $0x1 rax
   mov %fs_state rax
   or $0x2 rax
   mov rax %fs_state
   mov rax %fs_state
 
 The expected value is 3, but in fact, it is 2

The code shows that fs_state is only set by open_ctree() and
save_error_info(), how could the above race can happen?

Although I'm ok with this as a harmless cleanup patch, I'm afraid the commit log
is not persuadable anyway.

thanks,
liubo

 
 Thanks
 Miao
 
  
  thanks,
  david
  
  Signed-off-by: Miao Xie mi...@cn.fujitsu.com
  ---
   fs/btrfs/ctree.h   | 4 +++-
   fs/btrfs/disk-io.c | 5 +++--
   fs/btrfs/file.c| 2 +-
   fs/btrfs/scrub.c   | 2 +-
   fs/btrfs/super.c   | 4 ++--
   fs/btrfs/transaction.c | 9 -
   6 files changed, 14 insertions(+), 12 deletions(-)
 
  diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
  index c95b539..c34e36e 100644
  --- a/fs/btrfs/ctree.h
  +++ b/fs/btrfs/ctree.h
  @@ -338,7 +338,9 @@ static inline unsigned long btrfs_chunk_item_size(int 
  num_stripes)
   /*
* File system states
*/
  +#define BTRFS_FS_STATE_ERROR  0
   
  +/* Super block flags */
   /* Errors detected */
   #define BTRFS_SUPER_FLAG_ERROR(1ULL  2)
   
  @@ -1540,7 +1542,7 @@ struct btrfs_fs_info {
 u64 qgroup_seq;
   
 /* filesystem state */
  -  u64 fs_state;
  +  unsigned long fs_state;
   
 struct btrfs_delayed_root *delayed_root;
   
  diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
  index cf03a45..d06e50c 100644
  --- a/fs/btrfs/disk-io.c
  +++ b/fs/btrfs/disk-io.c
  @@ -2196,7 +2196,8 @@ int open_ctree(struct super_block *sb,
 goto fail_alloc;
   
 /* check FS state, whether FS is broken. */
  -  fs_info-fs_state |= btrfs_super_flags(disk_super);
  +  if (btrfs_super_flags(disk_super)  BTRFS_SUPER_FLAG_ERROR)
  +  set_bit(BTRFS_FS_STATE_ERROR, fs_info-fs_state);
   
 ret = btrfs_check_super_valid(fs_info, sb-s_flags  MS_RDONLY);
 if (ret) {
  @@ -3354,7 +3355,7 @@ int close_ctree(struct btrfs_root *root)
 printk(KERN_ERR btrfs: commit super ret %d\n, ret);
 }
   
  -  if (fs_info-fs_state  BTRFS_SUPER_FLAG_ERROR)
  +  if (test_bit(BTRFS_FS_STATE_ERROR, fs_info-fs_state))
 btrfs_error_commit_super(root);
   
 btrfs_put_block_group_cache(fs_info);
  diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
  index 3e9fa0e..ec87b69 100644
  --- a/fs/btrfs/file.c
  +++ b/fs/btrfs/file.c
  @@ -1531,7 +1531,7 @@ static ssize_t btrfs_file_aio_write(struct kiocb 
  *iocb,
  * although we have opened a file as writable, we have
  * to stop this write operation to ensure FS consistency.
  */
  -  if (root-fs_info-fs_state  BTRFS_SUPER_FLAG_ERROR) {
  +  if (test_bit(BTRFS_FS_STATE_ERROR, root-fs_info-fs_state)) {
 mutex_unlock(inode-i_mutex);
 err = -EROFS;
 goto out;
  diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
  index af0b566..2e91b56 100644
  --- a/fs/btrfs/scrub.c
  +++ b/fs/btrfs/scrub.c
  @@ -2700,7 +2700,7 @@ static noinline_for_stack int scrub_supers(struct 
  scrub_ctx *sctx,
 int ret;
 struct btrfs_root *root = sctx-dev_root;
   
  -  if (root-fs_info-fs_state  BTRFS_SUPER_FLAG_ERROR)
  +  if (test_bit(BTRFS_FS_STATE_ERROR, root-fs_info-fs_state))
 return -EIO;
   
 gen = atomic64_read(root-fs_info-last_trans_committed);
  diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
  index 6f0524d..f714379 100644
  --- a/fs/btrfs/super.c
  +++ b/fs/btrfs/super.c
  @@ -98,7 +98,7 @@ static void __save_error_info(struct btrfs_fs_info 
  *fs_info)
  * today we only save the error info into ram.  Long term we'll
  * also send it down to the disk
  */
  -  fs_info-fs_state = BTRFS_SUPER_FLAG_ERROR;
  +  set_bit(BTRFS_FS_STATE_ERROR, fs_info-fs_state);
   }
   
   static void save_error_info(struct btrfs_fs_info *fs_info)
  @@ -114,7 +114,7 @@ static void btrfs_handle_error(struct btrfs_fs_info 
  *fs_info)
 if (sb-s_flags  MS_RDONLY)
 return;
   
  -  if (fs_info-fs_state  BTRFS_SUPER_FLAG_ERROR) {
  +  if (test_bit(BTRFS_FS_STATE_ERROR, fs_info-fs_state)) {
 sb-s_flags |= MS_RDONLY;
 printk(KERN_INFO btrfs is forced readonly\n);
 /*
  diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
  index 7999bf8..a950d48

Re: [PATCH 10/11] Btrfs: use bit operation for -fs_state

2013-01-14 Thread Miao Xie 
On tue, 15 Jan 2013 12:03:03 +0800, Liu Bo wrote:
 On Mon, Jan 14, 2013 at 03:50:31PM +0800, Miao Xie wrote:
 On   thu, 10 Jan 2013 18:57:35 +0100, David Sterba wrote:
 On Thu, Jan 10, 2013 at 08:51:59PM +0800, Miao Xie wrote:
 There is no lock to protect fs_info-fs_state, it will introduce some 
 problems,
 such as the value may be covered by the other task when several tasks 
 modify
 it. Now we use bit operation for it to fix the above problem.

 Can you please describe in more detail how does that happen and to what
 problems it leads?

 For example:
  Task0 - CPU0Task1 - CPU1
  mov %fs_state rax
  or $0x1 rax
  mov %fs_state rax
  or $0x2 rax
  mov rax %fs_state
  mov rax %fs_state

 The expected value is 3, but in fact, it is 2
 
 The code shows that fs_state is only set by open_ctree() and
 save_error_info(), how could the above race can happen?

The reason that the above race can not happen is because there is only one flag 
currently.
But as we know, -fs_state can be accessed and updated by multi-task, so the 
current code
is error prone, if we add other flags, the above problem will happen to a 
certainty. (Adding
new flags is very likely) So why not write right and robust code at the 
beginning?

 Although I'm ok with this as a harmless cleanup patch, I'm afraid the commit 
 log
 is not persuadable anyway.

I think the changelog is right since -fs_state can be accessed and updated by 
multi-task
actually.

Thanks
Miao

 
 thanks,
 liubo
 

 Thanks
 Miao


 thanks,
 david

 Signed-off-by: Miao Xie mi...@cn.fujitsu.com
 ---
  fs/btrfs/ctree.h   | 4 +++-
  fs/btrfs/disk-io.c | 5 +++--
  fs/btrfs/file.c| 2 +-
  fs/btrfs/scrub.c   | 2 +-
  fs/btrfs/super.c   | 4 ++--
  fs/btrfs/transaction.c | 9 -
  6 files changed, 14 insertions(+), 12 deletions(-)

 diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
 index c95b539..c34e36e 100644
 --- a/fs/btrfs/ctree.h
 +++ b/fs/btrfs/ctree.h
 @@ -338,7 +338,9 @@ static inline unsigned long btrfs_chunk_item_size(int 
 num_stripes)
  /*
   * File system states
   */
 +#define BTRFS_FS_STATE_ERROR  0
  
 +/* Super block flags */
  /* Errors detected */
  #define BTRFS_SUPER_FLAG_ERROR(1ULL  2)
  
 @@ -1540,7 +1542,7 @@ struct btrfs_fs_info {
u64 qgroup_seq;
  
/* filesystem state */
 -  u64 fs_state;
 +  unsigned long fs_state;
  
struct btrfs_delayed_root *delayed_root;
  
 diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
 index cf03a45..d06e50c 100644
 --- a/fs/btrfs/disk-io.c
 +++ b/fs/btrfs/disk-io.c
 @@ -2196,7 +2196,8 @@ int open_ctree(struct super_block *sb,
goto fail_alloc;
  
/* check FS state, whether FS is broken. */
 -  fs_info-fs_state |= btrfs_super_flags(disk_super);
 +  if (btrfs_super_flags(disk_super)  BTRFS_SUPER_FLAG_ERROR)
 +  set_bit(BTRFS_FS_STATE_ERROR, fs_info-fs_state);
  
ret = btrfs_check_super_valid(fs_info, sb-s_flags  MS_RDONLY);
if (ret) {
 @@ -3354,7 +3355,7 @@ int close_ctree(struct btrfs_root *root)
printk(KERN_ERR btrfs: commit super ret %d\n, ret);
}
  
 -  if (fs_info-fs_state  BTRFS_SUPER_FLAG_ERROR)
 +  if (test_bit(BTRFS_FS_STATE_ERROR, fs_info-fs_state))
btrfs_error_commit_super(root);
  
btrfs_put_block_group_cache(fs_info);
 diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
 index 3e9fa0e..ec87b69 100644
 --- a/fs/btrfs/file.c
 +++ b/fs/btrfs/file.c
 @@ -1531,7 +1531,7 @@ static ssize_t btrfs_file_aio_write(struct kiocb 
 *iocb,
 * although we have opened a file as writable, we have
 * to stop this write operation to ensure FS consistency.
 */
 -  if (root-fs_info-fs_state  BTRFS_SUPER_FLAG_ERROR) {
 +  if (test_bit(BTRFS_FS_STATE_ERROR, root-fs_info-fs_state)) {
mutex_unlock(inode-i_mutex);
err = -EROFS;
goto out;
 diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
 index af0b566..2e91b56 100644
 --- a/fs/btrfs/scrub.c
 +++ b/fs/btrfs/scrub.c
 @@ -2700,7 +2700,7 @@ static noinline_for_stack int scrub_supers(struct 
 scrub_ctx *sctx,
int ret;
struct btrfs_root *root = sctx-dev_root;
  
 -  if (root-fs_info-fs_state  BTRFS_SUPER_FLAG_ERROR)
 +  if (test_bit(BTRFS_FS_STATE_ERROR, root-fs_info-fs_state))
return -EIO;
  
gen = atomic64_read(root-fs_info-last_trans_committed);
 diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
 index 6f0524d..f714379 100644
 --- a/fs/btrfs/super.c
 +++ b/fs/btrfs/super.c
 @@ -98,7 +98,7 @@ static void __save_error_info(struct btrfs_fs_info 
 *fs_info)
 * today we only save the error info into ram.  Long term we'll
 * also send it down to the disk
 */
 -  fs_info-fs_state = BTRFS_SUPER_FLAG_ERROR;
 +  set_bit(BTRFS_FS_STATE_ERROR, fs_info-fs_state);
  }
  
  static void save_error_info(struct btrfs_fs_info *fs_info)
 @@ -114,7 +114,7 @@ static void

Re: [PATCH 10/11] Btrfs: use bit operation for -fs_state

2013-01-10 Thread David Sterba 
On Thu, Jan 10, 2013 at 08:51:59PM +0800, Miao Xie wrote:
 There is no lock to protect fs_info-fs_state, it will introduce some 
 problems,
 such as the value may be covered by the other task when several tasks modify
 it. Now we use bit operation for it to fix the above problem.

Can you please describe in more detail how does that happen and to what
problems it leads?

thanks,
david

 Signed-off-by: Miao Xie mi...@cn.fujitsu.com
 ---
  fs/btrfs/ctree.h   | 4 +++-
  fs/btrfs/disk-io.c | 5 +++--
  fs/btrfs/file.c| 2 +-
  fs/btrfs/scrub.c   | 2 +-
  fs/btrfs/super.c   | 4 ++--
  fs/btrfs/transaction.c | 9 -
  6 files changed, 14 insertions(+), 12 deletions(-)
 
 diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
 index c95b539..c34e36e 100644
 --- a/fs/btrfs/ctree.h
 +++ b/fs/btrfs/ctree.h
 @@ -338,7 +338,9 @@ static inline unsigned long btrfs_chunk_item_size(int 
 num_stripes)
  /*
   * File system states
   */
 +#define BTRFS_FS_STATE_ERROR 0
  
 +/* Super block flags */
  /* Errors detected */
  #define BTRFS_SUPER_FLAG_ERROR   (1ULL  2)
  
 @@ -1540,7 +1542,7 @@ struct btrfs_fs_info {
   u64 qgroup_seq;
  
   /* filesystem state */
 - u64 fs_state;
 + unsigned long fs_state;
  
   struct btrfs_delayed_root *delayed_root;
  
 diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
 index cf03a45..d06e50c 100644
 --- a/fs/btrfs/disk-io.c
 +++ b/fs/btrfs/disk-io.c
 @@ -2196,7 +2196,8 @@ int open_ctree(struct super_block *sb,
   goto fail_alloc;
  
   /* check FS state, whether FS is broken. */
 - fs_info-fs_state |= btrfs_super_flags(disk_super);
 + if (btrfs_super_flags(disk_super)  BTRFS_SUPER_FLAG_ERROR)
 + set_bit(BTRFS_FS_STATE_ERROR, fs_info-fs_state);
  
   ret = btrfs_check_super_valid(fs_info, sb-s_flags  MS_RDONLY);
   if (ret) {
 @@ -3354,7 +3355,7 @@ int close_ctree(struct btrfs_root *root)
   printk(KERN_ERR btrfs: commit super ret %d\n, ret);
   }
  
 - if (fs_info-fs_state  BTRFS_SUPER_FLAG_ERROR)
 + if (test_bit(BTRFS_FS_STATE_ERROR, fs_info-fs_state))
   btrfs_error_commit_super(root);
  
   btrfs_put_block_group_cache(fs_info);
 diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
 index 3e9fa0e..ec87b69 100644
 --- a/fs/btrfs/file.c
 +++ b/fs/btrfs/file.c
 @@ -1531,7 +1531,7 @@ static ssize_t btrfs_file_aio_write(struct kiocb *iocb,
* although we have opened a file as writable, we have
* to stop this write operation to ensure FS consistency.
*/
 - if (root-fs_info-fs_state  BTRFS_SUPER_FLAG_ERROR) {
 + if (test_bit(BTRFS_FS_STATE_ERROR, root-fs_info-fs_state)) {
   mutex_unlock(inode-i_mutex);
   err = -EROFS;
   goto out;
 diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
 index af0b566..2e91b56 100644
 --- a/fs/btrfs/scrub.c
 +++ b/fs/btrfs/scrub.c
 @@ -2700,7 +2700,7 @@ static noinline_for_stack int scrub_supers(struct 
 scrub_ctx *sctx,
   int ret;
   struct btrfs_root *root = sctx-dev_root;
  
 - if (root-fs_info-fs_state  BTRFS_SUPER_FLAG_ERROR)
 + if (test_bit(BTRFS_FS_STATE_ERROR, root-fs_info-fs_state))
   return -EIO;
  
   gen = atomic64_read(root-fs_info-last_trans_committed);
 diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
 index 6f0524d..f714379 100644
 --- a/fs/btrfs/super.c
 +++ b/fs/btrfs/super.c
 @@ -98,7 +98,7 @@ static void __save_error_info(struct btrfs_fs_info *fs_info)
* today we only save the error info into ram.  Long term we'll
* also send it down to the disk
*/
 - fs_info-fs_state = BTRFS_SUPER_FLAG_ERROR;
 + set_bit(BTRFS_FS_STATE_ERROR, fs_info-fs_state);
  }
  
  static void save_error_info(struct btrfs_fs_info *fs_info)
 @@ -114,7 +114,7 @@ static void btrfs_handle_error(struct btrfs_fs_info 
 *fs_info)
   if (sb-s_flags  MS_RDONLY)
   return;
  
 - if (fs_info-fs_state  BTRFS_SUPER_FLAG_ERROR) {
 + if (test_bit(BTRFS_FS_STATE_ERROR, fs_info-fs_state)) {
   sb-s_flags |= MS_RDONLY;
   printk(KERN_INFO btrfs is forced readonly\n);
   /*
 diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
 index 7999bf8..a950d48 100644
 --- a/fs/btrfs/transaction.c
 +++ b/fs/btrfs/transaction.c
 @@ -62,7 +62,7 @@ static noinline int join_transaction(struct btrfs_root 
 *root, int type)
   spin_lock(fs_info-trans_lock);
  loop:
   /* The file system has been taken offline. No new transactions. */
 - if (fs_info-fs_state  BTRFS_SUPER_FLAG_ERROR) {
 + if (test_bit(BTRFS_FS_STATE_ERROR, fs_info-fs_state)) {
   spin_unlock(fs_info-trans_lock);
   return -EROFS;
   }
 @@ -114,7 +114,7 @@ loop:
   kmem_cache_free(btrfs_transaction_cachep, cur_trans);
   cur_trans = fs_info-running_transaction;
   goto loop;
 - } else if (fs_info-fs_state