btrfs: Fix memory barriers usage with device stats counters

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@suse.com>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index baf047b..c96635a 100644 (file)
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -7080,10 +7080,24 @@ int btrfs_run_dev_stats(struct btrfs_trans_handle *trans,
 
        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);

diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index bf0d53a..38fd368 100644 (file)
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -497,6 +497,12 @@ static inline void btrfs_dev_stat_inc(struct btrfs_device *dev,
                                      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);
 }
@@ -522,6 +528,12 @@ static inline void btrfs_dev_stat_set(struct btrfs_device *dev,
                                      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);
 }