| From foo@baz Fri Dec 17 03:55:51 PM CET 2021 |
| From: Anand Jain <anand.jain@oracle.com> |
| Date: Thu, 16 Dec 2021 21:04:11 +0800 |
| Subject: btrfs: use latest_dev in btrfs_show_devname |
| To: linux-kernel@vger.kernel.org, stable@vger.kernel.org |
| Cc: linux-btrfs@vger.kernel.org, Anand Jain <anand.jain@oracle.com>, Su Yue <l@damenly.su>, David Sterba <dsterba@suse.com> |
| Message-ID: <b7925cc522db365166b8ecddde9ba6f4f6a6a058.1639658429.git.anand.jain@oracle.com> |
| |
| From: Anand Jain <anand.jain@oracle.com> |
| |
| Commit 6605fd2f394bba0a0059df2b6cfc87b0b6d393a2 upstream. |
| |
| The test case btrfs/238 reports the warning below: |
| |
| WARNING: CPU: 3 PID: 481 at fs/btrfs/super.c:2509 btrfs_show_devname+0x104/0x1e8 [btrfs] |
| CPU: 2 PID: 1 Comm: systemd Tainted: G W O 5.14.0-rc1-custom #72 |
| Hardware name: QEMU QEMU Virtual Machine, BIOS 0.0.0 02/06/2015 |
| Call trace: |
| btrfs_show_devname+0x108/0x1b4 [btrfs] |
| show_mountinfo+0x234/0x2c4 |
| m_show+0x28/0x34 |
| seq_read_iter+0x12c/0x3c4 |
| vfs_read+0x29c/0x2c8 |
| ksys_read+0x80/0xec |
| __arm64_sys_read+0x28/0x34 |
| invoke_syscall+0x50/0xf8 |
| do_el0_svc+0x88/0x138 |
| el0_svc+0x2c/0x8c |
| el0t_64_sync_handler+0x84/0xe4 |
| el0t_64_sync+0x198/0x19c |
| |
| Reason: |
| While btrfs_prepare_sprout() moves the fs_devices::devices into |
| fs_devices::seed_list, the btrfs_show_devname() searches for the devices |
| and found none, leading to the warning as in above. |
| |
| Fix: |
| latest_dev is updated according to the changes to the device list. |
| That means we could use the latest_dev->name to show the device name in |
| /proc/self/mounts, the pointer will be always valid as it's assigned |
| before the device is deleted from the list in remove or replace. |
| The RCU protection is sufficient as the device structure is freed after |
| synchronization. |
| |
| Reported-by: Su Yue <l@damenly.su> |
| Tested-by: Su Yue <l@damenly.su> |
| Signed-off-by: Anand Jain <anand.jain@oracle.com> |
| Reviewed-by: David Sterba <dsterba@suse.com> |
| Signed-off-by: David Sterba <dsterba@suse.com> |
| Signed-off-by: Anand Jain <anand.jain@oracle.com> |
| Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
| --- |
| fs/btrfs/super.c | 24 +++++------------------- |
| 1 file changed, 5 insertions(+), 19 deletions(-) |
| |
| --- a/fs/btrfs/super.c |
| +++ b/fs/btrfs/super.c |
| @@ -2463,30 +2463,16 @@ static int btrfs_unfreeze(struct super_b |
| static int btrfs_show_devname(struct seq_file *m, struct dentry *root) |
| { |
| struct btrfs_fs_info *fs_info = btrfs_sb(root->d_sb); |
| - struct btrfs_device *dev, *first_dev = NULL; |
| |
| /* |
| - * Lightweight locking of the devices. We should not need |
| - * device_list_mutex here as we only read the device data and the list |
| - * is protected by RCU. Even if a device is deleted during the list |
| - * traversals, we'll get valid data, the freeing callback will wait at |
| - * least until the rcu_read_unlock. |
| + * There should be always a valid pointer in latest_dev, it may be stale |
| + * for a short moment in case it's being deleted but still valid until |
| + * the end of RCU grace period. |
| */ |
| rcu_read_lock(); |
| - list_for_each_entry_rcu(dev, &fs_info->fs_devices->devices, dev_list) { |
| - if (test_bit(BTRFS_DEV_STATE_MISSING, &dev->dev_state)) |
| - continue; |
| - if (!dev->name) |
| - continue; |
| - if (!first_dev || dev->devid < first_dev->devid) |
| - first_dev = dev; |
| - } |
| - |
| - if (first_dev) |
| - seq_escape(m, rcu_str_deref(first_dev->name), " \t\n\\"); |
| - else |
| - WARN_ON(1); |
| + seq_escape(m, rcu_str_deref(fs_info->fs_devices->latest_dev->name), " \t\n\\"); |
| rcu_read_unlock(); |
| + |
| return 0; |
| } |
| |