From d73068018419c5999f594a52998621947dc1f7d0 Mon Sep 17 00:00:00 2001 From: Filipe David Borba Manana Date: Fri, 9 Aug 2013 15:41:36 +0100 Subject: [PATCH] Btrfs: fix race between removing a dev and writing sbs This change fixes an issue when removing a device and writing all super blocks run simultaneously. Here's the steps necessary for the issue to happen: 1) disk-io.c:write_all_supers() gets a number of N devices from the super_copy, so it will not panic if it fails to write super blocks for N - 1 devices; 2) Then it tries to acquire the device_list_mutex, but blocks because volumes.c:btrfs_rm_device() got it first; 3) btrfs_rm_device() removes the device from the list, then unlocks the mutex and after the unlock it updates the number of devices in super_copy to N - 1. 4) write_all_supers() finally acquires the mutex, iterates over all the devices in the list and gets N - 1 errors, that is, it failed to write super blocks to all the devices; 5) Because write_all_supers() thinks there are a total of N devices, it considers N - 1 errors to be ok, and therefore won't panic. So this change just makes sure that write_all_supers() reads the number of devices from super_copy after it acquires the device_list_mutex. Conversely, it changes btrfs_rm_device() to update the number of devices in super_copy before it releases the device list mutex. The code path to add a new device (volumes.c:btrfs_init_new_device), already has the right behaviour: it updates the number of devices in super_copy while holding the device_list_mutex. The only code path that doesn't lock the device list mutex before updating the number of devices in the super copy is disk-io.c:next_root_backup(), called by open_ctree() during mount time where concurrency issues can't happen. Signed-off-by: Filipe David Borba Manana Signed-off-by: Josef Bacik Signed-off-by: Chris Mason --- fs/btrfs/disk-io.c | 2 +- fs/btrfs/volumes.c | 8 ++++++-- 2 files changed, 7 insertions(+), 3 deletions(-) diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c index 940bc48..6d642e4 100644 --- a/fs/btrfs/disk-io.c +++ b/fs/btrfs/disk-io.c @@ -3365,7 +3365,6 @@ static int write_all_supers(struct btrfs_root *root, int max_mirrors) int total_errors = 0; u64 flags; - max_errors = btrfs_super_num_devices(root->fs_info->super_copy) - 1; do_barriers = !btrfs_test_opt(root, NOBARRIER); backup_super_roots(root->fs_info); @@ -3374,6 +3373,7 @@ static int write_all_supers(struct btrfs_root *root, int max_mirrors) mutex_lock(&root->fs_info->fs_devices->device_list_mutex); head = &root->fs_info->fs_devices->devices; + max_errors = btrfs_super_num_devices(root->fs_info->super_copy) - 1; if (do_barriers) { ret = barrier_all_devices(root->fs_info); diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c index 34068b8..3f1c2c2 100644 --- a/fs/btrfs/volumes.c +++ b/fs/btrfs/volumes.c @@ -1620,7 +1620,11 @@ int btrfs_rm_device(struct btrfs_root *root, char *device_path) /* * the device list mutex makes sure that we don't change * the device list while someone else is writing out all - * the device supers. + * the device supers. Whoever is writing all supers, should + * lock the device list mutex before getting the number of + * devices in the super block (super_copy). Conversely, + * whoever updates the number of devices in the super block + * (super_copy) should hold the device list mutex. */ cur_devices = device->fs_devices; @@ -1644,10 +1648,10 @@ int btrfs_rm_device(struct btrfs_root *root, char *device_path) device->fs_devices->open_devices--; call_rcu(&device->rcu, free_device); - mutex_unlock(&root->fs_info->fs_devices->device_list_mutex); num_devices = btrfs_super_num_devices(root->fs_info->super_copy) - 1; btrfs_set_super_num_devices(root->fs_info->super_copy, num_devices); + mutex_unlock(&root->fs_info->fs_devices->device_list_mutex); if (cur_devices->open_devices == 0) { struct btrfs_fs_devices *fs_devices; -- 2.7.4