On Tue, Dec 19, 2006 at 10:50:47AM +0100, Ingo Molnar wrote:
...
> moving the graph unlock back, and by leaving the max_lockdep_depth
> variable update possibly racy. (we dont care, it's just statistics)
I would agree if it were not the lockdep.
I mean it's like the "father figure"!
> also add some minimal debugging code to graph_unlock()/graph_lock(),
> which caught this locking bug.
>
> Signed-off-by: Ingo Molnar <[email protected]>
> ---
> kernel/lockdep.c | 10 ++++++++--
> 1 file changed, 8 insertions(+), 2 deletions(-)
>
> Index: linux/kernel/lockdep.c
> ===================================================================
> --- linux.orig/kernel/lockdep.c
> +++ linux/kernel/lockdep.c
> @@ -70,6 +70,9 @@ static int graph_lock(void)
>
> static inline int graph_unlock(void)
> {
> + if (debug_locks && !__raw_spin_is_locked(&lockdep_lock))
> + return DEBUG_LOCKS_WARN_ON(1);
> +
> __raw_spin_unlock(&lockdep_lock);
> return 0;
> }
> @@ -716,6 +719,9 @@ find_usage_backwards(struct lock_class *
> struct lock_list *entry;
> int ret;
>
> + if (!__raw_spin_is_locked(&lockdep_lock))
> + return DEBUG_LOCKS_WARN_ON(1);
> +
> if (depth > max_recursion_depth)
> max_recursion_depth = depth;
> if (depth >= RECURSION_LIMIT)
> @@ -2208,6 +2214,7 @@ out_calc_hash:
> if (!chain_head && ret != 2)
> if (!check_prevs_add(curr, hlock))
> return 0;
> + graph_unlock();
> } else
> /* after lookup_chain_cache(): */
> if (unlikely(!debug_locks))
Probably similar changes should be done in
debug_locks_off_graph_unlock() etc.
I think it's going slightly complicated - there is
hard to say where and when the lock is really on.
Maybe graph_lock needs some rethinking?
My proposal is to do unconditional locking in
graph_lock() and always check its return value e.g.:
if (!graph_lock()) {
graph_unlock();
return 0;
}
It is clear and gives some place for exceptions.
Jarek P.
PS: thanks for this followup_to info!
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