Btrfs: bring back balance pause/resume logic

Balance pause/resume logic got broken by 5ac00add (went in into 3.8-rc1
as part of dev-replace merge).  Offending commit took a stab at making
mutually exclusive volume operations (add_dev, rm_dev, resize, balance,
replace_dev) not block behind volume_mutex if another such operation is
in progress and instead return an error right away.  Balancing front-end
relied on the blocking behaviour, so the fix is ugly, but short of a
complete rework, it's the best we can do.

Reported-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

1 files changed, 64 insertions, 14 deletions

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 982c0b9ceea5..77d8273e394c 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -3440,8 +3440,8 @@ static long btrfs_ioctl_balance(struct file *file, void __user *arg)
         struct btrfs_fs_info *fs_info = root->fs_info;
         struct btrfs_ioctl_balance_args *bargs;
         struct btrfs_balance_control *bctl;
+        bool need_unlock; /* for mut. excl. ops lock */
         int ret;
-        int need_to_clear_lock = 0;
 
         if (!capable(CAP_SYS_ADMIN))
                 return -EPERM;
@@ -3450,14 +3450,61 @@ static long btrfs_ioctl_balance(struct file *file, void __user *arg)
         if (ret)
                 return ret;
 
-        mutex_lock(&fs_info->volume_mutex);
+again:
+        if (!atomic_xchg(&fs_info->mutually_exclusive_operation_running, 1)) {
+                mutex_lock(&fs_info->volume_mutex);
+                mutex_lock(&fs_info->balance_mutex);
+                need_unlock = true;
+                goto locked;
+        }
+
+        /*
+         * mut. excl. ops lock is locked.  Three possibilites:
+         *   (1) some other op is running
+         *   (2) balance is running
+         *   (3) balance is paused -- special case (think resume)
+         */
         mutex_lock(&fs_info->balance_mutex);
+        if (fs_info->balance_ctl) {
+                /* this is either (2) or (3) */
+                if (!atomic_read(&fs_info->balance_running)) {
+                        mutex_unlock(&fs_info->balance_mutex);
+                        if (!mutex_trylock(&fs_info->volume_mutex))
+                                goto again;
+                        mutex_lock(&fs_info->balance_mutex);
+
+                        if (fs_info->balance_ctl &&
+                            !atomic_read(&fs_info->balance_running)) {
+                                /* this is (3) */
+                                need_unlock = false;
+                                goto locked;
+                        }
+
+                        mutex_unlock(&fs_info->balance_mutex);
+                        mutex_unlock(&fs_info->volume_mutex);
+                        goto again;
+                } else {
+                        /* this is (2) */
+                        mutex_unlock(&fs_info->balance_mutex);
+                        ret = -EINPROGRESS;
+                        goto out;
+                }
+        } else {
+                /* this is (1) */
+                mutex_unlock(&fs_info->balance_mutex);
+                pr_info("btrfs: dev add/delete/balance/replace/resize operation in progress\n");
+                ret = -EINVAL;
+                goto out;
+        }
+
+locked:
+        BUG_ON(!atomic_read(&fs_info->mutually_exclusive_operation_running));
 
         if (arg) {
                 bargs = memdup_user(arg, sizeof(*bargs));
                 if (IS_ERR(bargs)) {
                         ret = PTR_ERR(bargs);
-                        goto out;
+                        goto out_unlock;
                 }
 
                 if (bargs->flags & BTRFS_BALANCE_RESUME) {
@@ -3477,13 +3524,10 @@ static long btrfs_ioctl_balance(struct file *file, void __user *arg)
                 bargs = NULL;
         }
 
-        if (atomic_xchg(&root->fs_info->mutually_exclusive_operation_running,
-                        1)) {
-                pr_info("btrfs: dev add/delete/balance/replace/resize operation in progress\n");
+        if (fs_info->balance_ctl) {
                 ret = -EINPROGRESS;
                 goto out_bargs;
         }
-        need_to_clear_lock = 1;
 
         bctl = kzalloc(sizeof(*bctl), GFP_NOFS);
         if (!bctl) {
@@ -3504,11 +3548,17 @@ static long btrfs_ioctl_balance(struct file *file, void __user *arg)
         }
 
 do_balance:
-        ret = btrfs_balance(bctl, bargs);
         /*
-         * bctl is freed in __cancel_balance or in free_fs_info if
-         * restriper was paused all the way until unmount
+         * Ownership of bctl and mutually_exclusive_operation_running
+         * goes to to btrfs_balance.  bctl is freed in __cancel_balance,
+         * or, if restriper was paused all the way until unmount, in
+         * free_fs_info.  mutually_exclusive_operation_running is
+         * cleared in __cancel_balance.
          */
+        need_unlock = false;
+
+        ret = btrfs_balance(bctl, bargs);
+
         if (arg) {
                 if (copy_to_user(arg, bargs, sizeof(*bargs)))
                         ret = -EFAULT;
@@ -3516,12 +3566,12 @@ do_balance:
 
 out_bargs:
         kfree(bargs);
-out:
-        if (need_to_clear_lock)
-                atomic_set(&root->fs_info->mutually_exclusive_operation_running,
-                           0);
+out_unlock:
         mutex_unlock(&fs_info->balance_mutex);
         mutex_unlock(&fs_info->volume_mutex);
+        if (need_unlock)
+                atomic_set(&fs_info->mutually_exclusive_operation_running, 0);
+out:
         mnt_drop_write_file(file);
         return ret;
 }