Re: [PATCH] btrfs: avoid blocking open_ctree from cleaner_kthread

2016-06-15 Thread David Sterba 
On Sun, Jun 12, 2016 at 11:39:58PM -0400, Zygo Blaxell wrote:
> This fixes a problem introduced in commit 
> 2f3165ecf103599f82bf0ea254039db335fb5005
> "btrfs: don't force mounts to wait for cleaner_kthread to delete one or more 
> subvolumes".
> 
> open_ctree eventually calls btrfs_replay_log which in turn calls
> btrfs_commit_super which tries to lock the cleaner_mutex, causing a
> recursive mutex deadlock during mount.
> 
> Instead of playing whack-a-mole trying to keep up with all the
> functions that may want to lock cleaner_mutex, put all the cleaner_mutex
> lockers back where they were, and attack the problem more directly:
> keep cleaner_kthread asleep until the filesystem is mounted.

This approach looks good to me.

> When filesystems are mounted read-only and later remounted read-write,
> open_ctree did not set fs_info->open and neither does anything else.
> Set this flag in btrfs_remount so that neither btrfs_delete_unused_bgs
> nor cleaner_kthread get confused by the common case of "/" filesystem
> read-only mount followed by read-write remount.
> 
> Signed-off-by: Zygo Blaxell 

Reviewed-by: David Sterba 
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[PATCH] btrfs: avoid blocking open_ctree from cleaner_kthread

2016-06-12 Thread Zygo Blaxell 
This fixes a problem introduced in commit 
2f3165ecf103599f82bf0ea254039db335fb5005
"btrfs: don't force mounts to wait for cleaner_kthread to delete one or more 
subvolumes".

open_ctree eventually calls btrfs_replay_log which in turn calls
btrfs_commit_super which tries to lock the cleaner_mutex, causing a
recursive mutex deadlock during mount.

Instead of playing whack-a-mole trying to keep up with all the
functions that may want to lock cleaner_mutex, put all the cleaner_mutex
lockers back where they were, and attack the problem more directly:
keep cleaner_kthread asleep until the filesystem is mounted.

When filesystems are mounted read-only and later remounted read-write,
open_ctree did not set fs_info->open and neither does anything else.
Set this flag in btrfs_remount so that neither btrfs_delete_unused_bgs
nor cleaner_kthread get confused by the common case of "/" filesystem
read-only mount followed by read-write remount.

Signed-off-by: Zygo Blaxell 
---
 fs/btrfs/disk-io.c | 25 ++---
 fs/btrfs/super.c   |  2 ++
 2 files changed, 12 insertions(+), 15 deletions(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 1142127..190a5e0 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1806,6 +1806,13 @@ static int cleaner_kthread(void *arg)
if (btrfs_need_cleaner_sleep(root))
goto sleep;
 
+   /*
+* Do not do anything if we might cause open_ctree() to block
+* before we have finished mounting the filesystem.
+*/
+   if (!root->fs_info->open)
+   goto sleep;
+
if (!mutex_trylock(>fs_info->cleaner_mutex))
goto sleep;
 
@@ -2520,7 +2527,6 @@ int open_ctree(struct super_block *sb,
int num_backups_tried = 0;
int backup_index = 0;
int max_active;
-   bool cleaner_mutex_locked = false;
 
tree_root = fs_info->tree_root = btrfs_alloc_root(fs_info, GFP_KERNEL);
chunk_root = fs_info->chunk_root = btrfs_alloc_root(fs_info, 
GFP_KERNEL);
@@ -2999,13 +3005,6 @@ retry_root_backup:
goto fail_sysfs;
}
 
-   /*
-* Hold the cleaner_mutex thread here so that we don't block
-* for a long time on btrfs_recover_relocation.  cleaner_kthread
-* will wait for us to finish mounting the filesystem.
-*/
-   mutex_lock(_info->cleaner_mutex);
-   cleaner_mutex_locked = true;
fs_info->cleaner_kthread = kthread_run(cleaner_kthread, tree_root,
   "btrfs-cleaner");
if (IS_ERR(fs_info->cleaner_kthread))
@@ -3065,8 +3064,10 @@ retry_root_backup:
ret = btrfs_cleanup_fs_roots(fs_info);
if (ret)
goto fail_qgroup;
-   /* We locked cleaner_mutex before creating cleaner_kthread. */
+
+   mutex_lock(_info->cleaner_mutex);
ret = btrfs_recover_relocation(tree_root);
+   mutex_unlock(_info->cleaner_mutex);
if (ret < 0) {
btrfs_warn(fs_info, "failed to recover relocation: %d",
ret);
@@ -3074,8 +3075,6 @@ retry_root_backup:
goto fail_qgroup;
}
}
-   mutex_unlock(_info->cleaner_mutex);
-   cleaner_mutex_locked = false;
 
location.objectid = BTRFS_FS_TREE_OBJECTID;
location.type = BTRFS_ROOT_ITEM_KEY;
@@ -3189,10 +3188,6 @@ fail_cleaner:
filemap_write_and_wait(fs_info->btree_inode->i_mapping);
 
 fail_sysfs:
-   if (cleaner_mutex_locked) {
-   mutex_unlock(_info->cleaner_mutex);
-   cleaner_mutex_locked = false;
-   }
btrfs_sysfs_remove_mounted(fs_info);
 
 fail_fsdev_sysfs:
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 4339b66..9934519 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1807,6 +1807,8 @@ static int btrfs_remount(struct super_block *sb, int 
*flags, char *data)
}
}
sb->s_flags &= ~MS_RDONLY;
+
+   fs_info->open = 1;
}
 out:
wake_up_process(fs_info->transaction_kthread);
-- 
2.1.4

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