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    Subject[PATCH 4.19 56/78] btrfs: fix lockdep splat in add_missing_dev
    Date
    From: Josef Bacik <josef@toxicpanda.com>

    commit fccc0007b8dc952c6bc0805cdf842eb8ea06a639 upstream.

    Nikolay reported a lockdep splat in generic/476 that I could reproduce
    with btrfs/187.

    ======================================================
    WARNING: possible circular locking dependency detected
    5.9.0-rc2+ #1 Tainted: G W
    ------------------------------------------------------
    kswapd0/100 is trying to acquire lock:
    ffff9e8ef38b6268 (&delayed_node->mutex){+.+.}-{3:3}, at: __btrfs_release_delayed_node.part.0+0x3f/0x330

    but task is already holding lock:
    ffffffffa9d74700 (fs_reclaim){+.+.}-{0:0}, at: __fs_reclaim_acquire+0x5/0x30

    which lock already depends on the new lock.

    the existing dependency chain (in reverse order) is:

    -> #2 (fs_reclaim){+.+.}-{0:0}:
    fs_reclaim_acquire+0x65/0x80
    slab_pre_alloc_hook.constprop.0+0x20/0x200
    kmem_cache_alloc_trace+0x3a/0x1a0
    btrfs_alloc_device+0x43/0x210
    add_missing_dev+0x20/0x90
    read_one_chunk+0x301/0x430
    btrfs_read_sys_array+0x17b/0x1b0
    open_ctree+0xa62/0x1896
    btrfs_mount_root.cold+0x12/0xea
    legacy_get_tree+0x30/0x50
    vfs_get_tree+0x28/0xc0
    vfs_kern_mount.part.0+0x71/0xb0
    btrfs_mount+0x10d/0x379
    legacy_get_tree+0x30/0x50
    vfs_get_tree+0x28/0xc0
    path_mount+0x434/0xc00
    __x64_sys_mount+0xe3/0x120
    do_syscall_64+0x33/0x40
    entry_SYSCALL_64_after_hwframe+0x44/0xa9

    -> #1 (&fs_info->chunk_mutex){+.+.}-{3:3}:
    __mutex_lock+0x7e/0x7e0
    btrfs_chunk_alloc+0x125/0x3a0
    find_free_extent+0xdf6/0x1210
    btrfs_reserve_extent+0xb3/0x1b0
    btrfs_alloc_tree_block+0xb0/0x310
    alloc_tree_block_no_bg_flush+0x4a/0x60
    __btrfs_cow_block+0x11a/0x530
    btrfs_cow_block+0x104/0x220
    btrfs_search_slot+0x52e/0x9d0
    btrfs_lookup_inode+0x2a/0x8f
    __btrfs_update_delayed_inode+0x80/0x240
    btrfs_commit_inode_delayed_inode+0x119/0x120
    btrfs_evict_inode+0x357/0x500
    evict+0xcf/0x1f0
    vfs_rmdir.part.0+0x149/0x160
    do_rmdir+0x136/0x1a0
    do_syscall_64+0x33/0x40
    entry_SYSCALL_64_after_hwframe+0x44/0xa9

    -> #0 (&delayed_node->mutex){+.+.}-{3:3}:
    __lock_acquire+0x1184/0x1fa0
    lock_acquire+0xa4/0x3d0
    __mutex_lock+0x7e/0x7e0
    __btrfs_release_delayed_node.part.0+0x3f/0x330
    btrfs_evict_inode+0x24c/0x500
    evict+0xcf/0x1f0
    dispose_list+0x48/0x70
    prune_icache_sb+0x44/0x50
    super_cache_scan+0x161/0x1e0
    do_shrink_slab+0x178/0x3c0
    shrink_slab+0x17c/0x290
    shrink_node+0x2b2/0x6d0
    balance_pgdat+0x30a/0x670
    kswapd+0x213/0x4c0
    kthread+0x138/0x160
    ret_from_fork+0x1f/0x30

    other info that might help us debug this:

    Chain exists of:
    &delayed_node->mutex --> &fs_info->chunk_mutex --> fs_reclaim

    Possible unsafe locking scenario:

    CPU0 CPU1
    ---- ----
    lock(fs_reclaim);
    lock(&fs_info->chunk_mutex);
    lock(fs_reclaim);
    lock(&delayed_node->mutex);

    *** DEADLOCK ***

    3 locks held by kswapd0/100:
    #0: ffffffffa9d74700 (fs_reclaim){+.+.}-{0:0}, at: __fs_reclaim_acquire+0x5/0x30
    #1: ffffffffa9d65c50 (shrinker_rwsem){++++}-{3:3}, at: shrink_slab+0x115/0x290
    #2: ffff9e8e9da260e0 (&type->s_umount_key#48){++++}-{3:3}, at: super_cache_scan+0x38/0x1e0

    stack backtrace:
    CPU: 1 PID: 100 Comm: kswapd0 Tainted: G W 5.9.0-rc2+ #1
    Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-2.fc32 04/01/2014
    Call Trace:
    dump_stack+0x92/0xc8
    check_noncircular+0x12d/0x150
    __lock_acquire+0x1184/0x1fa0
    lock_acquire+0xa4/0x3d0
    ? __btrfs_release_delayed_node.part.0+0x3f/0x330
    __mutex_lock+0x7e/0x7e0
    ? __btrfs_release_delayed_node.part.0+0x3f/0x330
    ? __btrfs_release_delayed_node.part.0+0x3f/0x330
    ? lock_acquire+0xa4/0x3d0
    ? btrfs_evict_inode+0x11e/0x500
    ? find_held_lock+0x2b/0x80
    __btrfs_release_delayed_node.part.0+0x3f/0x330
    btrfs_evict_inode+0x24c/0x500
    evict+0xcf/0x1f0
    dispose_list+0x48/0x70
    prune_icache_sb+0x44/0x50
    super_cache_scan+0x161/0x1e0
    do_shrink_slab+0x178/0x3c0
    shrink_slab+0x17c/0x290
    shrink_node+0x2b2/0x6d0
    balance_pgdat+0x30a/0x670
    kswapd+0x213/0x4c0
    ? _raw_spin_unlock_irqrestore+0x46/0x60
    ? add_wait_queue_exclusive+0x70/0x70
    ? balance_pgdat+0x670/0x670
    kthread+0x138/0x160
    ? kthread_create_worker_on_cpu+0x40/0x40
    ret_from_fork+0x1f/0x30

    This is because we are holding the chunk_mutex when we call
    btrfs_alloc_device, which does a GFP_KERNEL allocation. We don't want
    to switch that to a GFP_NOFS lock because this is the only place where
    it matters. So instead use memalloc_nofs_save() around the allocation
    in order to avoid the lockdep splat.

    Reported-by: Nikolay Borisov <nborisov@suse.com>
    CC: stable@vger.kernel.org # 4.4+
    Reviewed-by: Anand Jain <anand.jain@oracle.com>
    Signed-off-by: Josef Bacik <josef@toxicpanda.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 | 10 ++++++++++
    1 file changed, 10 insertions(+)

    --- a/fs/btrfs/volumes.c
    +++ b/fs/btrfs/volumes.c
    @@ -4,6 +4,7 @@
    */

    #include <linux/sched.h>
    +#include <linux/sched/mm.h>
    #include <linux/bio.h>
    #include <linux/slab.h>
    #include <linux/buffer_head.h>
    @@ -6292,8 +6293,17 @@ static struct btrfs_device *add_missing_
    u64 devid, u8 *dev_uuid)
    {
    struct btrfs_device *device;
    + unsigned int nofs_flag;

    + /*
    + * We call this under the chunk_mutex, so we want to use NOFS for this
    + * allocation, however we don't want to change btrfs_alloc_device() to
    + * always do NOFS because we use it in a lot of other GFP_KERNEL safe
    + * places.
    + */
    + nofs_flag = memalloc_nofs_save();
    device = btrfs_alloc_device(NULL, &devid, dev_uuid);
    + memalloc_nofs_restore(nofs_flag);
    if (IS_ERR(device))
    return device;


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