This is a note to let you know that I've just added the patch titled
btrfs: Fix memory barriers usage with device stats counters
to the 4.15-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...
The filename of the patch is: btrfs-fix-memory-barriers-usage-with-device-stats-counters.patch and it can be found in the queue-4.15 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.
From 9deae9689231964972a94bb56a79b669f9d47ac1 Mon Sep 17 00:00:00 2001
From: Nikolay Borisov nborisov@suse.com Date: Tue, 24 Oct 2017 13:47:37 +0300 Subject: btrfs: Fix memory barriers usage with device stats counters
From: Nikolay Borisov nborisov@suse.com
commit 9deae9689231964972a94bb56a79b669f9d47ac1 upstream.
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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/btrfs/volumes.c | 18 ++++++++++++++++-- fs/btrfs/volumes.h | 12 ++++++++++++ 2 files changed, 28 insertions(+), 2 deletions(-)
--- a/fs/btrfs/volumes.c +++ b/fs/btrfs/volumes.c @@ -7093,10 +7093,24 @@ int btrfs_run_dev_stats(struct btrfs_tra
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); --- a/fs/btrfs/volumes.h +++ b/fs/btrfs/volumes.h @@ -498,6 +498,12 @@ static inline void btrfs_dev_stat_inc(st 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); } @@ -523,6 +529,12 @@ static inline void btrfs_dev_stat_set(st 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); }
Patches currently in stable-queue which might be from nborisov@suse.com are
queue-4.15/btrfs-fix-memory-barriers-usage-with-device-stats-counters.patch queue-4.15/btrfs-fix-use-after-free-when-cleaning-up-fs_devs-with-a-single-stale-device.patch