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Date: Thu, 10 Jun 2010 17:19:51 +1000
From: Dave Chinner <david@...morbit.com>
To: linux-fsdevel@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, viro@...IV.linux.org.uk,
josef@...hat.com, jeffmerkey@...il.com
Subject: [PATCH 2/5] fsfreeze: emergency thaw will deadlock on s_umount
From: Dave Chinner <dchinner@...hat.com>
The emergency thaw process uses iterate_super() which holds the
sb->s_umount lock in read mode. The current thaw_super() code takes
the sb->s_umount lock in write mode, hence leading to an instant
deadlock.
Pass the emergency state into the thaw_bdev/thaw_super code to avoid
taking the s_umount lock in this case. We are running under the bdev
freeze mutex, so this is still serialised against freeze despite
only having a read lock on the sb->s_umount. Hence it should be safe
to execute in this manner, especially given that emergency thaw is a
rarely executed "get-out-of-jail" feature.
Signed-off-by: Dave Chinner <dchinner@...hat.com>
---
fs/block_dev.c | 26 ++++++++++++++++++++--
fs/buffer.c | 2 +-
fs/super.c | 58 +++++++++++++++++++++++++++++++++++++++++++---------
include/linux/fs.h | 9 ++++++++
4 files changed, 81 insertions(+), 14 deletions(-)
diff --git a/fs/block_dev.c b/fs/block_dev.c
index 366ac38..a8c8224 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -262,13 +262,14 @@ struct super_block *freeze_bdev(struct block_device *bdev)
EXPORT_SYMBOL(freeze_bdev);
/**
- * thaw_bdev -- unlock filesystem
+ * __thaw_bdev -- unlock filesystem
* @bdev: blockdevice to unlock
* @sb: associated superblock
+ * @emergency: emergency thaw
*
* Unlocks the filesystem and marks it writeable again after freeze_bdev().
*/
-int thaw_bdev(struct block_device *bdev, struct super_block *sb)
+static int __thaw_bdev(struct block_device *bdev, struct super_block *sb, int emergency)
{
int error = -EINVAL;
@@ -283,15 +284,34 @@ int thaw_bdev(struct block_device *bdev, struct super_block *sb)
if (--bdev->bd_fsfreeze_count > 0)
goto out;
- error = thaw_super(sb);
+ if (emergency)
+ error = thaw_super_emergency(sb);
+ else
+ error = thaw_super(sb);
if (error)
bdev->bd_fsfreeze_count++;
out:
mutex_unlock(&bdev->bd_fsfreeze_mutex);
return error;
}
+/**
+ * thaw_bdev -- unlock filesystem
+ * @bdev: blockdevice to unlock
+ * @sb: associated superblock
+ *
+ * Unlocks the filesystem and marks it writeable again after freeze_bdev().
+ */
+int thaw_bdev(struct block_device *bdev, struct super_block *sb)
+{
+ return __thaw_bdev(bdev, sb, 0);
+}
EXPORT_SYMBOL(thaw_bdev);
+int thaw_bdev_emergency(struct block_device *bdev, struct super_block *sb)
+{
+ return __thaw_bdev(bdev, sb, 1);
+}
+
static int blkdev_writepage(struct page *page, struct writeback_control *wbc)
{
return block_write_full_page(page, blkdev_get_block, wbc);
diff --git a/fs/buffer.c b/fs/buffer.c
index d54812b..f0c55d9 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -564,7 +564,7 @@ repeat:
static void do_thaw_one(struct super_block *sb, void *unused)
{
char b[BDEVNAME_SIZE];
- while (sb->s_bdev && !thaw_bdev(sb->s_bdev, sb))
+ while (sb->s_bdev && !thaw_bdev_emergency(sb->s_bdev, sb))
printk(KERN_WARNING "Emergency Thaw on %s\n",
bdevname(sb->s_bdev, b));
}
diff --git a/fs/super.c b/fs/super.c
index 5c35bc7..76ed922 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -987,19 +987,24 @@ 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().
+ * If we are doing an emergency thaw, we don't need to grab the sb->s_umount
+ * lock as it is already held.
*/
-int thaw_super(struct super_block *sb)
+static int __thaw_super(struct super_block *sb, int emergency)
{
- int error;
+ int error = 0;
+
+ if (!emergency)
+ down_write(&sb->s_umount);
- down_write(&sb->s_umount);
if (sb->s_frozen == SB_UNFROZEN) {
- up_write(&sb->s_umount);
- return -EINVAL;
+ error = -EINVAL;
+ goto out_unlock;
}
if (sb->s_flags & MS_RDONLY)
@@ -1011,8 +1016,7 @@ 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_unlock;
}
}
@@ -1020,12 +1024,46 @@ out:
sb->s_frozen = SB_UNFROZEN;
smp_wmb();
wake_up(&sb->s_wait_unfrozen);
- deactivate_locked_super(sb);
-
+ /*
+ * 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);
return 0;
+
+out_unlock:
+ if (!emergency)
+ up_write(&sb->s_umount);
+ return error;
+}
+
+/**
+ * thaw_super -- unlock filesystem
+ * @sb: the super to thaw
+ *
+ * Unlocks the filesystem and marks it writeable again after freeze_super().
+ */
+int thaw_super(struct super_block *sb)
+{
+ return __thaw_super(sb, 0);
}
EXPORT_SYMBOL(thaw_super);
+/**
+ * thaw_super_emergency -- unlock filesystem
+ * @sb: the super to thaw
+ *
+ * Unlocks the filesystem and marks it writeable again after freeze_super().
+ * This avoids taking the s_umount lock if it is already held.
+ */
+int thaw_super_emergency(struct super_block *sb)
+{
+ return __thaw_super(sb, 1);
+}
+
static struct vfsmount *fs_set_subtype(struct vfsmount *mnt, const char *fstype)
{
int err;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 471e1ff..e246389 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1803,6 +1803,7 @@ extern int iterate_mounts(int (*)(struct vfsmount *, void *), void *,
extern int vfs_statfs(struct dentry *, struct kstatfs *);
extern int freeze_super(struct super_block *super);
extern int thaw_super(struct super_block *super);
+extern int thaw_super_emergency(struct super_block *super);
extern int current_umask(void);
@@ -1953,6 +1954,8 @@ extern int sync_blockdev(struct block_device *bdev);
extern struct super_block *freeze_bdev(struct block_device *);
extern void emergency_thaw_all(void);
extern int thaw_bdev(struct block_device *bdev, struct super_block *sb);
+extern int thaw_bdev_emergency(struct block_device *bdev,
+ struct super_block *sb);
extern int fsync_bdev(struct block_device *);
#else
static inline void bd_forget(struct inode *inode) {}
@@ -1968,6 +1971,12 @@ static inline int thaw_bdev(struct block_device *bdev, struct super_block *sb)
{
return 0;
}
+
+static inline int thaw_bdev_emergency(struct block_device *bdev,
+ struct super_block *sb)
+{
+ return 0;
+}
#endif
extern int sync_filesystem(struct super_block *);
extern const struct file_operations def_blk_fops;
--
1.7.1
--
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