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btrfs: Fix memory barriers usage with device stats counters 

Commit addc3fa74e ("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: addc3fa74e ("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>
This commit is contained in:
Nikolay Borisov 2017-10-24 13:47:37 +03:00 committed by David Sterba
parent 1cb34c8ecd
commit 9deae96892
2 changed files with 28 additions and 2 deletions
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18
fs/btrfs/volumes.c
View File

@ -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);

12
fs/btrfs/volumes.h
View File

@ -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);
}