From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx2.suse.de ([195.135.220.15]:37602 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751825AbdJXKrl (ORCPT ); Tue, 24 Oct 2017 06:47:41 -0400 From: Nikolay Borisov To: linux-btrfs@vger.kernel.org Cc: mathieu.desnoyers@efficios.com, Nikolay Borisov Subject: [PATCH v2] btrfs: Fix memory barriers usage with device stats counters Date: Tue, 24 Oct 2017 13:47:37 +0300 Message-Id: <1508842057-28901-1-git-send-email-nborisov@suse.com> In-Reply-To: <1508512259-16867-1-git-send-email-nborisov@suse.com> References: <1508512259-16867-1-git-send-email-nborisov@suse.com> Sender: linux-btrfs-owner@vger.kernel.org List-ID: 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 --- V2: Reworded the comment in btrfs_run_dev_stats according to Mathieu Desnoyers' feedback fs/btrfs/volumes.c | 18 ++++++++++++++++-- fs/btrfs/volumes.h | 12 ++++++++++++ 2 files changed, 28 insertions(+), 2 deletions(-) diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c index 4de498817e8a..7f8ed3e08ece 100644 --- a/fs/btrfs/volumes.c +++ b/fs/btrfs/volumes.c @@ -7084,10 +7084,24 @@ 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 6108fdfec67f..c5dd48eb7b3d 100644 --- a/fs/btrfs/volumes.h +++ b/fs/btrfs/volumes.h @@ -498,6 +498,12 @@ 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); } @@ -523,6 +529,12 @@ 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); } -- 2.7.4