[RFC 1/3] xfs: Re-introduce xg_active_wq field in struct xfs_group

["Nirjhar Roy (IBM)" <nirjhar.roy.lists@gmail.com>]

pag_active_wq was removed in
commit 9943b4573290
	("xfs: remove the unused pag_active_wq field in struct xfs_perag")
because it was not waited upon. Re-introducing this in struct xfs_group.
This patch also replaces atomic_dec() in xfs_group_rele() with

if (atomic_dec_and_test(&xg->xg_active_ref))
	wake_up(&xg->xg_active_wq);

The reason for this change is that the online shrink code will wait
for all the active references to come down to zero before actually
starting the shrink process (only if the number of blocks that
we are trying to remove is worth 1 or more AGs).

Signed-off-by: Nirjhar Roy (IBM) <nirjhar.roy.lists@gmail.com>
---
 fs/xfs/libxfs/xfs_group.c | 4 +++-
 fs/xfs/libxfs/xfs_group.h | 1 +
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/libxfs/xfs_group.c b/fs/xfs/libxfs/xfs_group.c
index e9d76bcdc820..1fe5319de338 100644
--- a/fs/xfs/libxfs/xfs_group.c
+++ b/fs/xfs/libxfs/xfs_group.c
@@ -147,7 +147,8 @@ xfs_group_rele(
 	struct xfs_group	*xg)
 {
 	trace_xfs_group_rele(xg, _RET_IP_);
-	atomic_dec(&xg->xg_active_ref);
+	if (atomic_dec_and_test(&xg->xg_active_ref))
+		wake_up(&xg->xg_active_wq);
 }
 
 void
@@ -198,6 +199,7 @@ xfs_group_insert(
 	xfs_defer_drain_init(&xg->xg_intents_drain);
 
 	/* Active ref owned by mount indicates group is online. */
+	init_waitqueue_head(&xg->xg_active_wq);
 	atomic_set(&xg->xg_active_ref, 1);
 
 	error = xa_insert(&mp->m_groups[type].xa, index, xg, GFP_KERNEL);
diff --git a/fs/xfs/libxfs/xfs_group.h b/fs/xfs/libxfs/xfs_group.h
index 4423932a2313..dbef389ef838 100644
--- a/fs/xfs/libxfs/xfs_group.h
+++ b/fs/xfs/libxfs/xfs_group.h
@@ -11,6 +11,7 @@ struct xfs_group {
 	enum xfs_group_type	xg_type;
 	atomic_t		xg_ref;		/* passive reference count */
 	atomic_t		xg_active_ref;	/* active reference count */
+	wait_queue_head_t xg_active_wq; /* woken active_ref falls to zero */
 
 	/* Precalculated geometry info */
 	uint32_t		xg_block_count;	/* max usable gbno */
-- 
2.43.5