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    Subject[PATCH v2 5/7] VFS: Avoid read-write deadlock in try_to_writeback_inodes_sb
    Date
    From: Valerie Aurora <val@vaaconsulting.com>

    Use trylock in try_to_writeback_inodes_sb to avoid read-write
    deadlocks that could be triggered by freeze.

    BugLink: https://bugs.launchpad.net/bugs/897421
    Signed-off-by: Valerie Aurora <val@vaaconsulting.com>
    Cc: Kamal Mostafa <kamal@canonical.com>
    Tested-by: Peter M. Petrakis <peter.petrakis@canonical.com>
    [kamal@canonical.com: patch restructure]
    Signed-off-by: Kamal Mostafa <kamal@canonical.com>
    ---
    fs/fs-writeback.c | 13 ++++++++-----
    1 files changed, 8 insertions(+), 5 deletions(-)

    diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
    index ea89b3f..3a80f1b 100644
    --- a/fs/fs-writeback.c
    +++ b/fs/fs-writeback.c
    @@ -1274,8 +1274,9 @@ EXPORT_SYMBOL(writeback_inodes_sb);
    * try_to_writeback_inodes_sb - start writeback if none underway
    * @sb: the superblock
    *
    - * Invoke writeback_inodes_sb if no writeback is currently underway.
    - * Returns 1 if writeback was started, 0 if not.
    + * Invoke writeback_inodes_sb if no writeback is currently underway
    + * and no one else holds the s_umount lock. Returns 1 if writeback
    + * was started, 0 if not.
    */
    int try_to_writeback_inodes_sb(struct super_block *sb, enum wb_reason reason)
    {
    @@ -1288,15 +1289,17 @@ EXPORT_SYMBOL(try_to_writeback_inodes_sb);
    * @sb: the superblock
    * @nr: the number of pages to write
    *
    - * Invoke writeback_inodes_sb if no writeback is currently underway.
    - * Returns 1 if writeback was started, 0 if not.
    + * Invoke writeback_inodes_sb if no writeback is currently underway
    + * and no one else holds the s_umount lock. Returns 1 if writeback
    + * was started, 0 if not.
    */
    int try_to_writeback_inodes_sb_nr(struct super_block *sb,
    unsigned long nr,
    enum wb_reason reason)
    {
    if (!writeback_in_progress(sb->s_bdi)) {
    - down_read(&sb->s_umount);
    + if (!down_read_trylock(&sb->s_umount))
    + return 0;
    if (nr == 0)
    writeback_inodes_sb(sb, reason);
    else
    --
    1.7.5.4


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