xfs: use the CIL space used counter for emptiness checks

In the next patches we are going to make the CIL list itself
per-cpu, and so we cannot use list_empty() to check is the list is
empty. Replace the list_empty() checks with a flag in the CIL to
indicate we have committed at least one transaction to the CIL and
hence the CIL is not empty.

We need this flag to be an atomic so that we can clear it without
holding any locks in the commit fast path, but we also need to be
careful to avoid atomic operations in the fast path. Hence we use
the fact that test_bit() is not an atomic op to first check if the
flag is set and then run the atomic test_and_clear_bit() operation
to clear it and steal the initial unit reservation for the CIL
context checkpoint.

When we are switching to a new context in a push, we place the
setting of the XLOG_CIL_EMPTY flag under the xc_push_lock. THis
allows all the other places that need to check whether the CIL is
empty to use test_bit() and still be serialised correctly with the
CIL context swaps that set the bit.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

1 files changed, 4 insertions, 0 deletions

diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h
index 686c01eb3661..8fad33ea2582 100644
--- a/fs/xfs/xfs_log_priv.h
+++ b/fs/xfs/xfs_log_priv.h
@@ -248,6 +248,7 @@ struct xfs_cil_ctx {
  */
 struct xfs_cil {
 	struct xlog		*xc_log;
+	unsigned long		xc_flags;
 	struct list_head	xc_cil;
 	spinlock_t		xc_cil_lock;
 	struct workqueue_struct	*xc_push_wq;
@@ -265,6 +266,9 @@ struct xfs_cil {
 	wait_queue_head_t	xc_push_wait;	/* background push throttle */
 } ____cacheline_aligned_in_smp;
 
+/* xc_flags bit values */
+#define	XLOG_CIL_EMPTY		1
+
 /*
  * The amount of log space we allow the CIL to aggregate is difficult to size.
  * Whatever we choose, we have to make sure we can get a reservation for the