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    Subject[PATCH 3/7] lockdep: Print a nice description of simple deadlock
    From: Steven Rostedt <srostedt@redhat.com>

    The lockdep output can be pretty cryptic, having a nice output
    can save a lot of head scratching. When a simple deadlock scenario
    is detected by lockdep (lock A -> lock A) we get the following output:

    other info that might help us debug this:
    Possible unsafe locking scenario:

    CPU0
    ----
    lock(&(lock)->rlock);
    lock(&(lock)->rlock);

    *** DEADLOCK ***

    Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
    ---
    kernel/lockdep.c | 21 +++++++++++++++++++++
    1 files changed, 21 insertions(+), 0 deletions(-)

    diff --git a/kernel/lockdep.c b/kernel/lockdep.c
    index 1039008..a0f253b 100644
    --- a/kernel/lockdep.c
    +++ b/kernel/lockdep.c
    @@ -1664,6 +1664,26 @@ static inline void inc_chains(void)

    #endif

    +static void
    +print_deadlock_scenario(struct held_lock *nxt,
    + struct held_lock *prv)
    +{
    + struct lock_class *next = hlock_class(nxt);
    + struct lock_class *prev = hlock_class(prv);
    +
    + printk(" Possible unsafe locking scenario:\n\n");
    + printk(" CPU0\n");
    + printk(" ----\n");
    + printk(" lock(");
    + __print_lock_name(prev);
    + printk(");\n");
    + printk(" lock(");
    + __print_lock_name(next);
    + printk(");\n");
    + printk("\n *** DEADLOCK ***\n\n");
    + printk(" May be due to missing lock nesting notation\n\n");
    +}
    +
    static int
    print_deadlock_bug(struct task_struct *curr, struct held_lock *prev,
    struct held_lock *next)
    @@ -1682,6 +1702,7 @@ print_deadlock_bug(struct task_struct *curr, struct held_lock *prev,
    print_lock(prev);

    printk("\nother info that might help us debug this:\n");
    + print_deadlock_scenario(next, prev);
    lockdep_print_held_locks(curr);

    printk("\nstack backtrace:\n");
    --
    1.7.2.3



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