[RFC 5/6] btrfs: replace kthread freezing with auto fs freezing

[Luis Chamberlain <mcgrof@kernel.org>]

The kernel power management now supports allowing the VFS
to handle filesystem freezing freezes and thawing. Take advantage
of that and remove the kthread freezing. This is needed so that we
properly really stop IO in flight without races after userspace
has been frozen. Without this we rely on kthread freezing and
its semantics are loose and error prone.

The filesystem therefore is in charge of properly dealing with
quiescing of the filesystem through its callbacks if it thinks
it knows better than how the VFS handles it.

The following Coccinelle rule was used as to remove the now superfluous
freezer calls:

make coccicheck MODE=patch SPFLAGS="--in-place --no-show-diff" COCCI=./fs-freeze-cleanup.cocci M=fs/btrfs

virtual patch

@ remove_set_freezable @
expression time;
statement S, S2;
expression task, current;
@@

(
-       set_freezable();
|
-       if (try_to_freeze())
-               continue;
|
-       try_to_freeze();
|
-       freezable_schedule();
+       schedule();
|
-       freezable_schedule_timeout(time);
+       schedule_timeout(time);
|
-       if (freezing(task)) { S }
|
-       if (freezing(task)) { S }
-       else
	    { S2 }
|
-       freezing(current)
)

@ remove_wq_freezable @
expression WQ_E, WQ_ARG1, WQ_ARG2, WQ_ARG3, WQ_ARG4;
identifier fs_wq_fn;
@@

(
    WQ_E = alloc_workqueue(WQ_ARG1,
-                              WQ_ARG2 | WQ_FREEZABLE,
+                              WQ_ARG2,
			   ...);
|
    WQ_E = alloc_workqueue(WQ_ARG1,
-                              WQ_ARG2 | WQ_FREEZABLE | WQ_ARG3,
+                              WQ_ARG2 | WQ_ARG3,
			   ...);
|
    WQ_E = alloc_workqueue(WQ_ARG1,
-                              WQ_ARG2 | WQ_ARG3 | WQ_FREEZABLE,
+                              WQ_ARG2 | WQ_ARG3,
			   ...);
|
    WQ_E = alloc_workqueue(WQ_ARG1,
-                              WQ_ARG2 | WQ_ARG3 | WQ_FREEZABLE | WQ_ARG4,
+                              WQ_ARG2 | WQ_ARG3 | WQ_ARG4,
			   ...);
|
	    WQ_E =
-               WQ_ARG1 | WQ_FREEZABLE
+               WQ_ARG1
|
	    WQ_E =
-               WQ_ARG1 | WQ_FREEZABLE | WQ_ARG3
+               WQ_ARG1 | WQ_ARG3
|
    fs_wq_fn(
-               WQ_FREEZABLE | WQ_ARG2 | WQ_ARG3
+               WQ_ARG2 | WQ_ARG3
    )
|
    fs_wq_fn(
-               WQ_FREEZABLE | WQ_ARG2
+               WQ_ARG2
    )
|
    fs_wq_fn(
-               WQ_FREEZABLE
+               0
    )
)

@ add_auto_flag @
expression E1;
identifier fs_type;
@@

struct file_system_type fs_type = {
	.fs_flags = E1
+                   | FS_AUTOFREEZE
	,
};

Generated-by: Coccinelle SmPL
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
---
 fs/btrfs/disk-io.c | 4 ++--
 fs/btrfs/scrub.c   | 2 +-
 fs/btrfs/super.c   | 2 +-
 3 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 3dd555db3d32..03332f914be7 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1962,8 +1962,8 @@ static void btrfs_init_qgroup(struct btrfs_fs_info *fs_info)
 static int btrfs_init_workqueues(struct btrfs_fs_info *fs_info)
 {
 	u32 max_active = fs_info->thread_pool_size;
-	unsigned int flags = WQ_MEM_RECLAIM | WQ_FREEZABLE | WQ_UNBOUND;
-	unsigned int ordered_flags = WQ_MEM_RECLAIM | WQ_FREEZABLE;
+	unsigned int flags = WQ_MEM_RECLAIM | WQ_UNBOUND;
+	unsigned int ordered_flags = WQ_MEM_RECLAIM;
 
 	fs_info->workers =
 		btrfs_alloc_workqueue(fs_info, "worker", flags, max_active, 16);
diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
index ae34353a34d9..52ef84923645 100644
--- a/fs/btrfs/scrub.c
+++ b/fs/btrfs/scrub.c
@@ -2811,7 +2811,7 @@ static void scrub_workers_put(struct btrfs_fs_info *fs_info)
 static noinline_for_stack int scrub_workers_get(struct btrfs_fs_info *fs_info)
 {
 	struct workqueue_struct *scrub_workers = NULL;
-	unsigned int flags = WQ_FREEZABLE | WQ_UNBOUND;
+	unsigned int flags = WQ_UNBOUND;
 	int max_active = fs_info->thread_pool_size;
 	int ret = -ENOMEM;
 
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 40709e2a44fc..153e8a2d7fbb 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -2178,7 +2178,7 @@ static struct file_system_type btrfs_fs_type = {
 	.parameters		= btrfs_fs_parameters,
 	.kill_sb		= btrfs_kill_super,
 	.fs_flags		= FS_REQUIRES_DEV | FS_BINARY_MOUNTDATA |
-				  FS_ALLOW_IDMAP | FS_MGTIME,
+				  FS_ALLOW_IDMAP | FS_MGTIME | FS_AUTOFREEZE,
  };
 
 MODULE_ALIAS_FS("btrfs");
-- 
2.47.2