Re: [PATCH v2 3/7] btrfs: do not read super look for a device path
From: Anand Jain <hidden>
Date: 2021-08-25 02:00:37
On 28/07/2021 05:01, Josef Bacik wrote:
For device removal and replace we call btrfs_find_device_by_devspec, which if we give it a device path and nothing else will call btrfs_find_device_by_path, which opens the block device and reads the super block and then looks up our device based on that. However this is completely unnecessary because we have the path stored in our device on our fsdevices. All we need to do if we're given a path is look through the fs_devices on our file system and use that device if we find it, reading the super block is just silly.
The device path as stored in our fs_devices can differ from the path provided by the user for the same device (for example, dm, lvm). btrfs-progs sanitize the device path but, others (for example, an ioctl test case) might not. And the path lookup would fail. Also, btrfs dev scan <path> can update the device path anytime, even after it is mounted. Fixing that failed the subsequent subvolume mounts (if I remember correctly).
This fixes the case where we end up with our sb write "lock" getting the dependency of the block device ->open_mutex, which resulted in the following lockdep splat
Can we do.. btrfs_exclop_start() :: find device part (read sb) :: mnt_want_write_file()? Thanks, Anand
quoted hunk ↗ jump to hunk
====================================================== WARNING: possible circular locking dependency detected 5.14.0-rc2+ #405 Not tainted ------------------------------------------------------ losetup/11576 is trying to acquire lock: ffff9bbe8cded938 ((wq_completion)loop0){+.+.}-{0:0}, at: flush_workqueue+0x67/0x5e0 but task is already holding lock: ffff9bbe88e4fc68 (&lo->lo_mutex){+.+.}-{3:3}, at: __loop_clr_fd+0x41/0x660 [loop] which lock already depends on the new lock. the existing dependency chain (in reverse order) is: -> #4 (&lo->lo_mutex){+.+.}-{3:3}: __mutex_lock+0x7d/0x750 lo_open+0x28/0x60 [loop] blkdev_get_whole+0x25/0xf0 blkdev_get_by_dev.part.0+0x168/0x3c0 blkdev_open+0xd2/0xe0 do_dentry_open+0x161/0x390 path_openat+0x3cc/0xa20 do_filp_open+0x96/0x120 do_sys_openat2+0x7b/0x130 __x64_sys_openat+0x46/0x70 do_syscall_64+0x38/0x90 entry_SYSCALL_64_after_hwframe+0x44/0xae -> #3 (&disk->open_mutex){+.+.}-{3:3}: __mutex_lock+0x7d/0x750 blkdev_get_by_dev.part.0+0x56/0x3c0 blkdev_get_by_path+0x98/0xa0 btrfs_get_bdev_and_sb+0x1b/0xb0 btrfs_find_device_by_devspec+0x12b/0x1c0 btrfs_rm_device+0x127/0x610 btrfs_ioctl+0x2a31/0x2e70 __x64_sys_ioctl+0x80/0xb0 do_syscall_64+0x38/0x90 entry_SYSCALL_64_after_hwframe+0x44/0xae -> #2 (sb_writers#12){.+.+}-{0:0}: lo_write_bvec+0xc2/0x240 [loop] loop_process_work+0x238/0xd00 [loop] process_one_work+0x26b/0x560 worker_thread+0x55/0x3c0 kthread+0x140/0x160 ret_from_fork+0x1f/0x30 -> #1 ((work_completion)(&lo->rootcg_work)){+.+.}-{0:0}: process_one_work+0x245/0x560 worker_thread+0x55/0x3c0 kthread+0x140/0x160 ret_from_fork+0x1f/0x30 -> #0 ((wq_completion)loop0){+.+.}-{0:0}: __lock_acquire+0x10ea/0x1d90 lock_acquire+0xb5/0x2b0 flush_workqueue+0x91/0x5e0 drain_workqueue+0xa0/0x110 destroy_workqueue+0x36/0x250 __loop_clr_fd+0x9a/0x660 [loop] block_ioctl+0x3f/0x50 __x64_sys_ioctl+0x80/0xb0 do_syscall_64+0x38/0x90 entry_SYSCALL_64_after_hwframe+0x44/0xae other info that might help us debug this: Chain exists of: (wq_completion)loop0 --> &disk->open_mutex --> &lo->lo_mutex Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(&lo->lo_mutex); lock(&disk->open_mutex); lock(&lo->lo_mutex); lock((wq_completion)loop0); *** DEADLOCK *** 1 lock held by losetup/11576: #0: ffff9bbe88e4fc68 (&lo->lo_mutex){+.+.}-{3:3}, at: __loop_clr_fd+0x41/0x660 [loop] stack backtrace: CPU: 0 PID: 11576 Comm: losetup Not tainted 5.14.0-rc2+ #405 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-2.fc32 04/01/2014 Call Trace: dump_stack_lvl+0x57/0x72 check_noncircular+0xcf/0xf0 ? stack_trace_save+0x3b/0x50 __lock_acquire+0x10ea/0x1d90 lock_acquire+0xb5/0x2b0 ? flush_workqueue+0x67/0x5e0 ? lockdep_init_map_type+0x47/0x220 flush_workqueue+0x91/0x5e0 ? flush_workqueue+0x67/0x5e0 ? verify_cpu+0xf0/0x100 drain_workqueue+0xa0/0x110 destroy_workqueue+0x36/0x250 __loop_clr_fd+0x9a/0x660 [loop] ? blkdev_ioctl+0x8d/0x2a0 block_ioctl+0x3f/0x50 __x64_sys_ioctl+0x80/0xb0 do_syscall_64+0x38/0x90 entry_SYSCALL_64_after_hwframe+0x44/0xae RIP: 0033:0x7f31b02404cb Signed-off-by: Josef Bacik <josef@toxicpanda.com> --- fs/btrfs/volumes.c | 61 +++++++++++++++++----------------------------- 1 file changed, 23 insertions(+), 38 deletions(-)diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c index 0e7372f637eb..bf2449cdb2ab 100644 --- a/fs/btrfs/volumes.c +++ b/fs/btrfs/volumes.c@@ -2313,37 +2313,22 @@ void btrfs_destroy_dev_replace_tgtdev(struct btrfs_device *tgtdev) btrfs_free_device(tgtdev); } -static struct btrfs_device *btrfs_find_device_by_path( - struct btrfs_fs_info *fs_info, const char *device_path) +static struct btrfs_device *find_device_by_path( + struct btrfs_fs_devices *fs_devices, + const char *path) { - int ret = 0; - struct btrfs_super_block *disk_super; - u64 devid; - u8 *dev_uuid; - struct block_device *bdev; struct btrfs_device *device; + bool missing = !strcmp(path, "missing"); - ret = btrfs_get_bdev_and_sb(device_path, FMODE_READ, - fs_info->bdev_holder, 0, &bdev, &disk_super); - if (ret) - return ERR_PTR(ret); - - devid = btrfs_stack_device_id(&disk_super->dev_item); - dev_uuid = disk_super->dev_item.uuid; - if (btrfs_fs_incompat(fs_info, METADATA_UUID)) - device = btrfs_find_device(fs_info->fs_devices, devid, dev_uuid, - disk_super->metadata_uuid); - else - device = btrfs_find_device(fs_info->fs_devices, devid, dev_uuid, - disk_super->fsid); - - btrfs_release_disk_super(disk_super); - if (!device) - device = ERR_PTR(-ENOENT); - blkdev_put(bdev, FMODE_READ); - return device; + list_for_each_entry(device, &fs_devices->devices, dev_list) { + if (missing && test_bit(BTRFS_DEV_STATE_IN_FS_METADATA, + &device->dev_state) && !device->bdev) + return device; + if (!missing && device_path_matched(path, device)) + return device; + } + return NULL; } - /* * Lookup a device given by device id, or the path if the id is 0. */@@ -2351,6 +2336,7 @@ struct btrfs_device *btrfs_find_device_by_devspec( struct btrfs_fs_info *fs_info, u64 devid, const char *device_path) { + struct btrfs_fs_devices *seed_devs; struct btrfs_device *device; if (devid) {@@ -2364,18 +2350,17 @@ struct btrfs_device *btrfs_find_device_by_devspec( if (!device_path || !device_path[0]) return ERR_PTR(-EINVAL); - if (strcmp(device_path, "missing") == 0) { - /* Find first missing device */ - list_for_each_entry(device, &fs_info->fs_devices->devices, - dev_list) { - if (test_bit(BTRFS_DEV_STATE_IN_FS_METADATA, - &device->dev_state) && !device->bdev) - return device; - } - return ERR_PTR(-ENOENT); - } + device = find_device_by_path(fs_info->fs_devices, device_path); + if (device) + return device; - return btrfs_find_device_by_path(fs_info, device_path); + list_for_each_entry(seed_devs, &fs_info->fs_devices->seed_list, + seed_list) { + device = find_device_by_path(seed_devs, device_path); + if (device) + return device; + } + return ERR_PTR(-ENOENT); } /*