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    Subject[PATCH 4.19 22/73] Btrfs: fix race updating log root item during fsync
    Date
    From: Filipe Manana <fdmanana@suse.com>

    commit 06989c799f04810f6876900d4760c0edda369cf7 upstream.

    When syncing the log, the final phase of a fsync operation, we need to
    either create a log root's item or update the existing item in the log
    tree of log roots, and that depends on the current value of the log
    root's log_transid - if it's 1 we need to create the log root item,
    otherwise it must exist already and we update it. Since there is no
    synchronization between updating the log_transid and checking it for
    deciding whether the log root's item needs to be created or updated, we
    end up with a tiny race window that results in attempts to update the
    item to fail because the item was not yet created:

    CPU 1 CPU 2

    btrfs_sync_log()

    lock root->log_mutex

    set log root's log_transid to 1

    unlock root->log_mutex

    btrfs_sync_log()

    lock root->log_mutex

    sets log root's
    log_transid to 2

    unlock root->log_mutex

    update_log_root()

    sees log root's log_transid
    with a value of 2

    calls btrfs_update_root(),
    which fails with -EUCLEAN
    and causes transaction abort

    Until recently the race lead to a BUG_ON at btrfs_update_root(), but after
    the recent commit 7ac1e464c4d47 ("btrfs: Don't panic when we can't find a
    root key") we just abort the current transaction.

    A sample trace of the BUG_ON() on a SLE12 kernel:

    ------------[ cut here ]------------
    kernel BUG at ../fs/btrfs/root-tree.c:157!
    Oops: Exception in kernel mode, sig: 5 [#1]
    SMP NR_CPUS=2048 NUMA pSeries
    (...)
    Supported: Yes, External
    CPU: 78 PID: 76303 Comm: rtas_errd Tainted: G X 4.4.156-94.57-default #1
    task: c00000ffa906d010 ti: c00000ff42b08000 task.ti: c00000ff42b08000
    NIP: d000000036ae5cdc LR: d000000036ae5cd8 CTR: 0000000000000000
    REGS: c00000ff42b0b860 TRAP: 0700 Tainted: G X (4.4.156-94.57-default)
    MSR: 8000000002029033 <SF,VEC,EE,ME,IR,DR,RI,LE> CR: 22444484 XER: 20000000
    CFAR: d000000036aba66c SOFTE: 1
    GPR00: d000000036ae5cd8 c00000ff42b0bae0 d000000036bda220 0000000000000054
    GPR04: 0000000000000001 0000000000000000 c00007ffff8d37c8 0000000000000000
    GPR08: c000000000e19c00 0000000000000000 0000000000000000 3736343438312079
    GPR12: 3930373337303434 c000000007a3a800 00000000007fffff 0000000000000023
    GPR16: c00000ffa9d26028 c00000ffa9d261f8 0000000000000010 c00000ffa9d2ab28
    GPR20: c00000ff42b0bc48 0000000000000001 c00000ff9f0d9888 0000000000000001
    GPR24: c00000ffa9d26000 c00000ffa9d261e8 c00000ffa9d2a800 c00000ff9f0d9888
    GPR28: c00000ffa9d26028 c00000ffa9d2aa98 0000000000000001 c00000ffa98f5b20
    NIP [d000000036ae5cdc] btrfs_update_root+0x25c/0x4e0 [btrfs]
    LR [d000000036ae5cd8] btrfs_update_root+0x258/0x4e0 [btrfs]
    Call Trace:
    [c00000ff42b0bae0] [d000000036ae5cd8] btrfs_update_root+0x258/0x4e0 [btrfs] (unreliable)
    [c00000ff42b0bba0] [d000000036b53610] btrfs_sync_log+0x2d0/0xc60 [btrfs]
    [c00000ff42b0bce0] [d000000036b1785c] btrfs_sync_file+0x44c/0x4e0 [btrfs]
    [c00000ff42b0bd80] [c00000000032e300] vfs_fsync_range+0x70/0x120
    [c00000ff42b0bdd0] [c00000000032e44c] do_fsync+0x5c/0xb0
    [c00000ff42b0be10] [c00000000032e8dc] SyS_fdatasync+0x2c/0x40
    [c00000ff42b0be30] [c000000000009488] system_call+0x3c/0x100
    Instruction dump:
    7f43d378 4bffebb9 60000000 88d90008 3d220000 e8b90000 3b390009 e87a01f0
    e8898e08 e8f90000 4bfd48e5 60000000 <0fe00000> e95b0060 39200004 394a0ea0
    ---[ end trace 8f2dc8f919cabab8 ]---

    So fix this by doing the check of log_transid and updating or creating the
    log root's item while holding the root's log_mutex.

    Fixes: 7237f1833601d ("Btrfs: fix tree logs parallel sync")
    CC: stable@vger.kernel.org # 4.4+
    Signed-off-by: Filipe Manana <fdmanana@suse.com>
    Signed-off-by: David Sterba <dsterba@suse.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    fs/btrfs/tree-log.c | 8 ++++++--
    1 file changed, 6 insertions(+), 2 deletions(-)

    --- a/fs/btrfs/tree-log.c
    +++ b/fs/btrfs/tree-log.c
    @@ -3038,6 +3038,12 @@ int btrfs_sync_log(struct btrfs_trans_ha
    log->log_transid = root->log_transid;
    root->log_start_pid = 0;
    /*
    + * Update or create log root item under the root's log_mutex to prevent
    + * races with concurrent log syncs that can lead to failure to update
    + * log root item because it was not created yet.
    + */
    + ret = update_log_root(trans, log);
    + /*
    * IO has been started, blocks of the log tree have WRITTEN flag set
    * in their headers. new modifications of the log will be written to
    * new positions. so it's safe to allow log writers to go in.
    @@ -3056,8 +3062,6 @@ int btrfs_sync_log(struct btrfs_trans_ha

    mutex_unlock(&log_root_tree->log_mutex);

    - ret = update_log_root(trans, log);
    -
    mutex_lock(&log_root_tree->log_mutex);
    if (atomic_dec_and_test(&log_root_tree->log_writers)) {
    /* atomic_dec_and_test implies a barrier */

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