[linux-lvm] [RFC] Reverting "bd_mount_mutex" to "bd_mount_sem"

[Srinivasa Ds]

Hi all
  When I was executing "dmsetup resume <device-name>" command,I got the 
error shown below. Which basically tells that, "bd_mount_mutex"  in 
thaw_bdev() is not locked by "dmsetup resume" command and hence it is 
not allowing it to unlock also.
=========================================================
Badness in debug_mutex_unlock at kernel/mutex-debug.c:80

Call Trace:
[C0000000634DB260] [C000000000010948] .show_stack+0x68/0x1b0 (unreliable)
[C0000000634DB300] [C0000000003376C4] .program_check_exception+0x1cc/0x5b0
[C0000000634DB3D0] [C0000000000047EC] program_check_common+0xec/0x100
--- Exception: 700 at .debug_mutex_unlock+0x3c/0xc4
    LR = .debug_mutex_unlock+0x30/0xc4
[C0000000634DB6C0] [C0000000634DB750] 0xc0000000634db750 (unreliable)
[C0000000634DB740] [C000000000335950] .__mutex_unlock_slowpath+0xd8/0x144
[C0000000634DB7E0] [C0000000000E2370] .thaw_bdev+0x9c/0xb8
[C0000000634DB870] [D000000000480830] .unlock_fs+0x34/0x70 [dm_mod]
[C0000000634DB900] [D000000000481720] .dm_resume+0x110/0x1ac [dm_mod]
[C0000000634DB9A0] [D000000000485C54] .dev_suspend+0x1b0/0x204 [dm_mod]
[C0000000634DBA40] [D000000000486728] .ctl_ioctl+0x29c/0x318 [dm_mod]
[C0000000634DBC30] [C0000000000F8310] .do_ioctl+0xbc/0xf0
[C0000000634DBCD0] [C0000000000F879C] .vfs_ioctl+0x458/0x498
[C0000000634DBD80] [C0000000000F8874] .sys_ioctl+0x98/0xe0
[C0000000634DBE30] [C00000000000871C] syscall_exit+0x0/0x40
======================================================================

   On debugging I found out that,"dmsetup suspend <device name>" calls 
"freeze_bdev()",which locks "bd_mount_mutex" to make sure that no new 
mounts happen on bdev until thaw_bdev() is called.
   This "thaw_bdev()" is getting called when we resume the device through
"dmsetup resume <device-name>".
   Hence we have 2 processes,one of which locks "bd_mount_mutex"(dmsetup
suspend) and Another(dmsetup resume) unlocks it.

   Since this is not allowed in mutex,I reverted back to 
bd_mount_sem(semaphore),It worked for me.

I posted the below patch to lkml also. So Please let me know your 
comments on this.
===================================================================

diff -Naurp linux-2.6.18-rc6-orig/fs/block_dev.c 
linux-2.6.18-rc6-mod/fs/block_dev.c
--- linux-2.6.18-rc6-orig/fs/block_dev.c	2006-09-27 04:47:25.000000000 -0700
+++ linux-2.6.18-rc6-mod/fs/block_dev.c	2006-09-27 04:53:29.000000000 -0700
@@ -261,7 +261,7 @@ static void init_once(void * foo, kmem_c
  	{
  		memset(bdev, 0, sizeof(*bdev));
  		mutex_init(&bdev->bd_mutex);
-		mutex_init(&bdev->bd_mount_mutex);
+		sema_init(&bdev->bd_mount_sem, 1);
  		INIT_LIST_HEAD(&bdev->bd_inodes);
  		INIT_LIST_HEAD(&bdev->bd_list);
  #ifdef CONFIG_SYSFS
diff -Naurp linux-2.6.18-rc6-orig/fs/buffer.c 
linux-2.6.18-rc6-mod/fs/buffer.c
--- linux-2.6.18-rc6-orig/fs/buffer.c	2006-09-27 04:46:14.000000000 -0700
+++ linux-2.6.18-rc6-mod/fs/buffer.c	2006-09-27 04:49:55.000000000 -0700
@@ -213,7 +213,7 @@ struct super_block *freeze_bdev(struct b
  {
  	struct super_block *sb;

-	mutex_lock(&bdev->bd_mount_mutex);
+	down(&bdev->bd_mount_sem);
  	sb = get_super(bdev);
  	if (sb && !(sb->s_flags & MS_RDONLY)) {
  		sb->s_frozen = SB_FREEZE_WRITE;
@@ -255,7 +255,7 @@ void thaw_bdev(struct block_device *bdev
  		drop_super(sb);
  	}

-	mutex_unlock(&bdev->bd_mount_mutex);
+	up(&bdev->bd_mount_sem);
  }
  EXPORT_SYMBOL(thaw_bdev);

diff -Naurp linux-2.6.18-rc6-orig/fs/super.c linux-2.6.18-rc6-mod/fs/super.c
--- linux-2.6.18-rc6-orig/fs/super.c	2006-09-27 04:46:51.000000000 -0700
+++ linux-2.6.18-rc6-mod/fs/super.c	2006-09-27 04:50:56.000000000 -0700
@@ -700,9 +700,9 @@ int get_sb_bdev(struct file_system_type
  	 * will protect the lockfs code from trying to start a snapshot
  	 * while we are mounting
  	 */
-	mutex_lock(&bdev->bd_mount_mutex);
+	down(&bdev->bd_mount_sem);
  	s = sget(fs_type, test_bdev_super, set_bdev_super, bdev);
-	mutex_unlock(&bdev->bd_mount_mutex);
+	up(&bdev->bd_mount_sem);
  	if (IS_ERR(s))
  		goto error_s;

diff -Naurp linux-2.6.18-rc6-orig/include/linux/fs.h 
linux-2.6.18-rc6-mod/include/linux/fs.h
--- linux-2.6.18-rc6-orig/include/linux/fs.h	2006-09-27 
04:45:31.000000000 -0700
+++ linux-2.6.18-rc6-mod/include/linux/fs.h	2006-09-27 
04:52:30.000000000 -0700
@@ -414,7 +414,7 @@ struct block_device {
  	struct inode *		bd_inode;	/* will die */
  	int			bd_openers;
  	struct mutex		bd_mutex;	/* open/close mutex */
-	struct mutex		bd_mount_mutex;	/* mount mutex */
+	struct semaphore        bd_mount_sem;
  	struct list_head	bd_inodes;
  	void *			bd_holder;
  	int			bd_holders;
==============================================================================

Thanks
  Srinivasa DS
  Linux Technology Centre