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    Subject[patch] lockdep: register_lock_class() fix
    Subject: [patch] lockdep: register_lock_class() fix
    From: Ingo Molnar <mingo@elte.hu>

    the hash_lock must only ever be taken with irqs disabled. This happens
    in all the important places, except one codepath: register_lock_class().
    The race should trigger rarely because register_lock_class() is quite
    rare and single-threaded (happens during init most of the time).

    The fix is to disable irqs.

    ( bug found live in -rt: there preemption is alot more agressive and
    preempting with the hash-lock held caused a lockup.)

    Signed-off-by: Ingo Molnar <mingo@elte.hu>
    ---
    kernel/lockdep.c | 6 ++++++
    1 file changed, 6 insertions(+)

    Index: linux-mm-genapic.q/kernel/lockdep.c
    ===================================================================
    --- linux-mm-genapic.q.orig/kernel/lockdep.c
    +++ linux-mm-genapic.q/kernel/lockdep.c
    @@ -1183,6 +1183,7 @@ register_lock_class(struct lockdep_map *
    struct lockdep_subclass_key *key;
    struct list_head *hash_head;
    struct lock_class *class;
    + unsigned long flags;

    class = look_up_lock_class(lock, subclass);
    if (likely(class))
    @@ -1204,6 +1205,7 @@ register_lock_class(struct lockdep_map *
    key = lock->key->subkeys + subclass;
    hash_head = classhashentry(key);

    + raw_local_irq_save(flags);
    __raw_spin_lock(&hash_lock);
    /*
    * We have to do the hash-walk again, to avoid races
    @@ -1218,6 +1220,7 @@ register_lock_class(struct lockdep_map *
    */
    if (nr_lock_classes >= MAX_LOCKDEP_KEYS) {
    __raw_spin_unlock(&hash_lock);
    + raw_local_irq_restore(flags);
    debug_locks_off();
    printk("BUG: MAX_LOCKDEP_KEYS too low!\n");
    printk("turning off the locking correctness validator.\n");
    @@ -1240,15 +1243,18 @@ register_lock_class(struct lockdep_map *

    if (verbose(class)) {
    __raw_spin_unlock(&hash_lock);
    + raw_local_irq_restore(flags);
    printk("\nnew class %p: %s", class->key, class->name);
    if (class->name_version > 1)
    printk("#%d", class->name_version);
    printk("\n");
    dump_stack();
    + raw_local_irq_save(flags);
    __raw_spin_lock(&hash_lock);
    }
    out_unlock_set:
    __raw_spin_unlock(&hash_lock);
    + raw_local_irq_restore(flags);

    if (!subclass || force)
    lock->class_cache = class;
    -
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