On 2012/07/13 22:17, Jan Kara wrote:
On Thu 12-07-12 18:04:09, Fernando Luis Vázquez Cao wrote:
diff -urNp vfs-orig/fs/super.c vfs/fs/super.c
--- vfs-orig/fs/super.c 2012-07-04 18:57:54.000000000 +0900
+++ vfs/fs/super.c 2012-07-12 14:21:09.736566768 +0900
@@ -1221,23 +1221,25 @@ int freeze_super(struct super_block *sb)
EXPORT_SYMBOL(freeze_super);
/**
- * thaw_super -- unlock filesystem
+ * __thaw_super -- unlock filesystem
* @sb: the super to thaw
+ * @emergency: emergency thaw
*
* Unlocks the filesystem and marks it writeable again after freeze_super().
+ * This is the unlocked version of thaw_super and has to be called with the
+ * sb->s_umount lock held in the non-emergency thaw case.
*/
-int thaw_super(struct super_block *sb)
+static int __thaw_super(struct super_block *sb, int emergency)
{
- int error;
+ int error = 0;
- down_write(&sb->s_umount);
if (sb->s_frozen == SB_UNFROZEN) {
- up_write(&sb->s_umount);
- return -EINVAL;
+ error = -EINVAL;
+ goto out;
}
if (sb->s_flags & MS_RDONLY)
- goto out;
+ goto out_thaw;
if (sb->s_op->unfreeze_fs) {
error = sb->s_op->unfreeze_fs(sb);
@@ -1245,17 +1247,52 @@ int thaw_super(struct super_block *sb)
printk(KERN_ERR
"VFS:Filesystem thaw failed\n");
sb->s_frozen = SB_FREEZE_TRANS;
- up_write(&sb->s_umount);
- return error;
+ goto out;
}
}
-out:
+out_thaw:
sb->s_frozen = SB_UNFROZEN;
smp_wmb();
wake_up(&sb->s_wait_unfrozen);
- deactivate_locked_super(sb);
- return 0;
+ /*
+ * When called from emergency scope, we cannot grab the s_umount lock
+ * so we cannot deactivate the superblock. This may leave unbalanced
+ * superblock references which could prevent unmount, but given this is
+ * an emergency operation....
+ */
+ if (!emergency)
+ deactivate_locked_super(sb);
+out:
+ return error;
+}
This is just wrong. deactivate_locked_super() will unlock the superblock
for you (and may even free it). So just do this in thaw_super() instead of
up_write(&sb->s_umount) which is bogus there. That will also allow you to
get rid of that ugly 'emergency' argument...
I just wanted to let you know that I rewrote this patch from scratch
taking a different approach which allows me to get locking right
in the emergency thaw case too. I will be sending the updated
patch set later today.
Thanks,
Fernando
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