btrfs: Fix memory barriers usage with device stats counters (9deae968) · Commits · 戴 / test

Commit ("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: ("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>

fs/btrfs/volumes.c

+16 −2

Original line number	Diff line number	Diff line
		@@ -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);

fs/btrfs/volumes.h

+12 −0

Original line number	Diff line number	Diff line
		@@ -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);
		}

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