4.14-stable review patch. If anyone has any objections, please let me know.
------------------
From: Nikolay Borisov <nborisov@xxxxxxxx>
commit 9deae9689231964972a94bb56a79b669f9d47ac1 upstream.
Commit addc3fa74e5b ("Btrfs: Fix the problem that the dirty flag of dev
stats is cleared") reworked the way device stats changes are tracked. A
new atomic dev_stats_ccnt counter was introduced which is incremented
every time any of the device stats counters are changed. This serves as
a flag whether there are any pending stats changes. However, this patch
only partially implemented the correct memory barriers necessary:
- It only ordered the stores to the counters but not the reads e.g.
btrfs_run_dev_stats
- It completely omitted any comments documenting the intended design and
how the memory barriers pair with each-other
This patch provides the necessary comments as well as adds a missing
smp_rmb in btrfs_run_dev_stats. Furthermore since dev_stats_cnt is only
a snapshot at best there was no point in reading the counter twice -
once in btrfs_dev_stats_dirty and then again when assigning stats_cnt.
Just collapse both reads into 1.
Fixes: addc3fa74e5b ("Btrfs: Fix the problem that the dirty flag of dev stats is cleared")
Signed-off-by: Nikolay Borisov <nborisov@xxxxxxxx>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Reviewed-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
fs/btrfs/volumes.c | 18 ++++++++++++++++--
fs/btrfs/volumes.h | 12 ++++++++++++
2 files changed, 28 insertions(+), 2 deletions(-)
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -7082,10 +7082,24 @@ int btrfs_run_dev_stats(struct btrfs_tra
mutex_lock(&fs_devices->device_list_mutex);
list_for_each_entry(device, &fs_devices->devices, dev_list) {
- if (!device->dev_stats_valid || !btrfs_dev_stats_dirty(device))
+ stats_cnt = atomic_read(&device->dev_stats_ccnt);
+ if (!device->dev_stats_valid || stats_cnt == 0)
continue;
- stats_cnt = atomic_read(&device->dev_stats_ccnt);
+
+ /*
+ * There is a LOAD-LOAD control dependency between the value of
+ * dev_stats_ccnt and updating the on-disk values which requires
+ * reading the in-memory counters. Such control dependencies
+ * require explicit read memory barriers.
+ *
+ * This memory barriers pairs with smp_mb__before_atomic in
+ * btrfs_dev_stat_inc/btrfs_dev_stat_set and with the full
+ * barrier implied by atomic_xchg in
+ * btrfs_dev_stats_read_and_reset
+ */
+ smp_rmb();
+
ret = update_dev_stat_item(trans, fs_info, device);
if (!ret)
atomic_sub(stats_cnt, &device->dev_stats_ccnt);
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -498,6 +498,12 @@ static inline void btrfs_dev_stat_inc(st
int index)
{
atomic_inc(dev->dev_stat_values + index);
+ /*
+ * This memory barrier orders stores updating statistics before stores
+ * updating dev_stats_ccnt.
+ *
+ * It pairs with smp_rmb() in btrfs_run_dev_stats().
+ */
smp_mb__before_atomic();
atomic_inc(&dev->dev_stats_ccnt);
}
@@ -523,6 +529,12 @@ static inline void btrfs_dev_stat_set(st
int index, unsigned long val)
{
atomic_set(dev->dev_stat_values + index, val);
+ /*
+ * This memory barrier orders stores updating statistics before stores
+ * updating dev_stats_ccnt.
+ *
+ * It pairs with smp_rmb() in btrfs_run_dev_stats().
+ */
smp_mb__before_atomic();
atomic_inc(&dev->dev_stats_ccnt);
}