From: Nikolay Borisov Date: Tue, 24 Oct 2017 10:47:37 +0000 (+0300) Subject: btrfs: Fix memory barriers usage with device stats counters X-Git-Url: http://git.cdn.openwrt.org/?a=commitdiff_plain;h=9deae9689231964972a94bb56a79b669f9d47ac1;p=openwrt%2Fstaging%2Fblogic.git 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 Reviewed-by: Mathieu Desnoyers Reviewed-by: David Sterba Signed-off-by: David Sterba --- diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c index baf047b8cf99..c96635a26803 100644 --- 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 bf0d53a96b4a..38fd368ddfe7 100644 --- 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); }