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    Subject[PATCH 1/7] lockdep: Print a nice description of an irq locking issue
    From: Steven Rostedt <srostedt@redhat.com>

    Locking order inversion due to interrupts is a subtle problem.
    When a locking inversion due to interrupts is discovered by lockdep,
    it currently reports something like this:

    [ INFO: HARDIRQ-safe -> HARDIRQ-unsafe lock order detected ]

    And then writes the locks that are involved as well as back traces.
    But several developers are confused by what a HARDIRQ->safe to unsafe
    issue is all about, and sometimes even blow it off as a bug in lockdep.
    As it is not obvious when lockdep describes this about a lock that
    is never taken in interrupt context.

    After explaining the problems that lockdep is reporting, I decided
    to add a description of the problem in visual form. Now the following
    is shown:

    ---
    other info that might help us debug this:

    Possible interrupt unsafe locking scenario:

    CPU0 CPU1
    ---- ----
    lock(lockA);
    local_irq_disable();
    lock(&rq->lock);
    lock(lockA);
    <Interrupt>
    lock(&rq->lock);

    *** DEADLOCK ***

    ---

    The above is the case when the unsafe lock is taken while holding
    a lock taken in irq context. But when a lock is taken that also
    grabs a unsafe lock, the call chain is shown:

    ---
    other info that might help us debug this:

    Chain exists of:
    &rq->lock --> lockA --> lockC

    Possible interrupt unsafe locking scenario:

    CPU0 CPU1
    ---- ----
    lock(lockC);
    local_irq_disable();
    lock(&rq->lock);
    lock(lockA);
    <Interrupt>
    lock(&rq->lock);

    *** DEADLOCK ***

    Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
    ---
    kernel/lockdep.c | 70 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
    1 files changed, 70 insertions(+), 0 deletions(-)

    diff --git a/kernel/lockdep.c b/kernel/lockdep.c
    index 0d2058d..bb77c030 100644
    --- a/kernel/lockdep.c
    +++ b/kernel/lockdep.c
    @@ -490,6 +490,18 @@ void get_usage_chars(struct lock_class *class, char usage[LOCK_USAGE_CHARS])
    usage[i] = '\0';
    }

    +static int __print_lock_name(struct lock_class *class)
    +{
    + char str[KSYM_NAME_LEN];
    + const char *name;
    +
    + name = class->name;
    + if (!name)
    + name = __get_key_name(class->key, str);
    +
    + return printk("%s", name);
    +}
    +
    static void print_lock_name(struct lock_class *class)
    {
    char str[KSYM_NAME_LEN], usage[LOCK_USAGE_CHARS];
    @@ -1325,6 +1337,62 @@ print_shortest_lock_dependencies(struct lock_list *leaf,
    return;
    }

    +static void
    +print_irq_lock_scenario(struct lock_list *safe_entry,
    + struct lock_list *unsafe_entry,
    + struct held_lock *prev,
    + struct held_lock *next)
    +{
    + struct lock_class *safe_class = safe_entry->class;
    + struct lock_class *unsafe_class = unsafe_entry->class;
    + struct lock_class *middle_class = hlock_class(prev);
    +
    + if (middle_class == safe_class)
    + middle_class = hlock_class(next);
    +
    + /*
    + * A direct locking problem where unsafe_class lock is taken
    + * directly by safe_class lock, then all we need to show
    + * is the deadlock scenario, as it is obvious that the
    + * unsafe lock is taken under the safe lock.
    + *
    + * But if there is a chain instead, where the safe lock takes
    + * an intermediate lock (middle_class) where this lock is
    + * not the same as the safe lock, then the lock chain is
    + * used to describe the problem. Otherwise we would need
    + * to show a different CPU case for each link in the chain
    + * from the safe_class lock to the unsafe_class lock.
    + */
    + if (middle_class != unsafe_class) {
    + printk("Chain exists of:\n ");
    + __print_lock_name(safe_class);
    + printk(" --> ");
    + __print_lock_name(middle_class);
    + printk(" --> ");
    + __print_lock_name(unsafe_class);
    + printk("\n\n");
    + }
    +
    + printk(" Possible interrupt unsafe locking scenario:\n\n");
    + printk(" CPU0 CPU1\n");
    + printk(" ---- ----\n");
    + printk(" lock(");
    + __print_lock_name(unsafe_class);
    + printk(");\n");
    + printk(" local_irq_disable();\n");
    + printk(" lock(");
    + __print_lock_name(safe_class);
    + printk(");\n");
    + printk(" lock(");
    + __print_lock_name(middle_class);
    + printk(");\n");
    + printk(" <Interrupt>\n");
    + printk(" lock(");
    + __print_lock_name(safe_class);
    + printk(");\n");
    + printk("\n *** DEADLOCK ***\n\n");
    +}
    +
    static int
    print_bad_irq_dependency(struct task_struct *curr,
    struct lock_list *prev_root,
    @@ -1376,6 +1444,8 @@ print_bad_irq_dependency(struct task_struct *curr,
    print_stack_trace(forwards_entry->class->usage_traces + bit2, 1);

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

    printk("\nthe dependencies between %s-irq-safe lock", irqclass);
    --
    1.7.2.3



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