btrfs: Fix memory barriers usage with device stats counters
commit9deae96892
upstream. Commitaddc3fa74e
("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> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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@ -7082,10 +7082,24 @@ int btrfs_run_dev_stats(struct btrfs_trans_handle *trans,
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mutex_lock(&fs_devices->device_list_mutex);
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list_for_each_entry(device, &fs_devices->devices, dev_list) {
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if (!device->dev_stats_valid || !btrfs_dev_stats_dirty(device))
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stats_cnt = atomic_read(&device->dev_stats_ccnt);
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if (!device->dev_stats_valid || stats_cnt == 0)
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continue;
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stats_cnt = atomic_read(&device->dev_stats_ccnt);
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/*
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* There is a LOAD-LOAD control dependency between the value of
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* dev_stats_ccnt and updating the on-disk values which requires
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* reading the in-memory counters. Such control dependencies
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* require explicit read memory barriers.
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*
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* This memory barriers pairs with smp_mb__before_atomic in
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* btrfs_dev_stat_inc/btrfs_dev_stat_set and with the full
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* barrier implied by atomic_xchg in
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* btrfs_dev_stats_read_and_reset
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*/
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smp_rmb();
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ret = update_dev_stat_item(trans, fs_info, device);
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if (!ret)
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atomic_sub(stats_cnt, &device->dev_stats_ccnt);
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@ -498,6 +498,12 @@ static inline void btrfs_dev_stat_inc(struct btrfs_device *dev,
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int index)
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{
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atomic_inc(dev->dev_stat_values + index);
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/*
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* This memory barrier orders stores updating statistics before stores
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* updating dev_stats_ccnt.
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*
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* It pairs with smp_rmb() in btrfs_run_dev_stats().
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*/
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smp_mb__before_atomic();
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atomic_inc(&dev->dev_stats_ccnt);
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}
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@ -523,6 +529,12 @@ static inline void btrfs_dev_stat_set(struct btrfs_device *dev,
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int index, unsigned long val)
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{
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atomic_set(dev->dev_stat_values + index, val);
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/*
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* This memory barrier orders stores updating statistics before stores
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* updating dev_stats_ccnt.
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*
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* It pairs with smp_rmb() in btrfs_run_dev_stats().
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*/
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smp_mb__before_atomic();
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atomic_inc(&dev->dev_stats_ccnt);
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}
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