Re: [PATCH V2] lockdep: reduce stack_trace usage

[Peter Zijlstra]

On Tue, 2010-05-04 at 14:57 +0800, Yong Zhang wrote:
> From 04395389820e89c0bd4bb57b939ec1882e0bb5da Mon Sep 17 00:00:00 2001
> From: Yong Zhang <yong.zhang@xxxxxxxxxxxxx>
> Date: Tue, 4 May 2010 14:16:48 +0800
> Subject: [PATCH] lockdep: Reduce stack_trace usage
> 
> When calling check_prevs_add(), if all validations passed
> add_lock_to_list() will add new lock to dependency tree and
> alloc stack_trace for each list_entry. But at this time,
> we are always on the same stack, so stack_trace for each
> list_entry has the same value. This is redundant and eats up
> lots of memory which could lead to warning on low
> MAX_STACK_TRACE_ENTRIES.
> Using one copy of stack_trace instead.

Right, this looks like it ought to do as intended.

I added the below snipped because I know I'll fail to remember and that
'static' qualifier is fairly easy to miss.

Thanks!

---

Index: linux-2.6/kernel/lockdep.c
===================================================================
--- linux-2.6.orig/kernel/lockdep.c
+++ linux-2.6/kernel/lockdep.c
@@ -1627,6 +1627,13 @@ check_prev_add(struct task_struct *curr,
 	int ret;
 	struct lock_list this;
 	struct lock_list *uninitialized_var(target_entry);
+	/*
+	 * Static variable, serialized by the graph_lock().
+	 *
+	 * We use this static variable to save the stack trace in case
+	 * we call into this function multiple times due to encountering
+	 * trylocks in the held lock stack.
+	 */
 	static struct stack_trace trace;
 
 	/*

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