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    Subject[RFC][PATCH] Fix hang in posix_locks_deadlock()
    From: Armin Kuster <AKuster@mvista.com>

    We have observed hangs in posix_locks_deadlock() when multiple threads
    use fcntl(2) F_SETLKW to synchronize file accesses. The problem appears
    to be due to an error in the implementation of posix_locks_deadlock() in
    which "goto next_task" is used to break out of the list_for_each_entry()
    file_lock search after which the posix_same_owner(caller_fl, block_fl)
    test may evaluate to false and the list_for_each_entry() loop restarts
    all over again. This in turn leads to a hang where posix_locks_deadlock()
    never returns. The workaround is to change the posix_same_owner()
    test within the list_for_each_entry() loop to directly compare caller_fl
    against current fl entry.


    Signed-off-by: Armin Kuster <AKuster@mvista.com>
    Signed-off-by: George G. Davis <gdavis@mvista.com>

    ---
    Not sure if this is the correct fix but it does resolve the hangs we're
    observing in posix_locks_deadlock(). Comments greatly appreciated...

    diff --git a/fs/locks.c b/fs/locks.c
    index 7f9a3ea..7669a0c 100644
    --- a/fs/locks.c
    +++ b/fs/locks.c
    @@ -702,14 +702,11 @@ static int posix_locks_deadlock(struct file_lock *caller_fl,
    {
    struct file_lock *fl;

    -next_task:
    if (posix_same_owner(caller_fl, block_fl))
    return 1;
    list_for_each_entry(fl, &blocked_list, fl_link) {
    - if (posix_same_owner(fl, block_fl)) {
    - fl = fl->fl_next;
    - block_fl = fl;
    - goto next_task;
    + if (posix_same_owner(fl, caller_fl)) {
    + return 1;
    }
    }
    return 0;
    --
    Regards,
    George
    -
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