linux-kernel - Re: [PATCH v4 05/15] lockdep: Make check_prev_add can use a separate stack

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Date:   Tue, 17 Jan 2017 16:54:31 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Byungchul Park <byungchul.park@....com>
Cc:     mingo@...nel.org, tglx@...utronix.de, walken@...gle.com,
        boqun.feng@...il.com, kirill@...temov.name,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org,
        npiggin@...il.com
Subject: Re: [PATCH v4 05/15] lockdep: Make check_prev_add can use a separate
 stack_trace

On Fri, Jan 13, 2017 at 07:11:43PM +0900, Byungchul Park wrote:
> What do you think about the following patches doing it?

I was more thinking about something like so...

Also, I think I want to muck with struct stack_trace; the members:
max_nr_entries and skip are input arguments to save_stack_trace() and
bloat the structure for no reason.

---
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 7c38f8f3d97b..f2df300a96ee 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -430,6 +430,21 @@ static int save_trace(struct stack_trace *trace)
 	return 1;
 }
 
+static bool return_trace(struct stack_trace *trace)
+{
+	/*
+	 * If @trace is the last trace generated by save_trace(), then we can
+	 * return the entries by simply subtracting @nr_stack_trace_entries
+	 * again.
+	 */
+	if (trace->entries != stack_trace + nr_stack_trace_entries - trace->nr_entres)
+		return false;
+
+	nr_stack_trace_entries -= trace->nr_entries;
+	trace->entries = NULL;
+	return true;
+}
+
 unsigned int nr_hardirq_chains;
 unsigned int nr_softirq_chains;
 unsigned int nr_process_chains;
@@ -1797,20 +1812,12 @@ static inline void inc_chains(void)
  */
 static int
 check_prev_add(struct task_struct *curr, struct held_lock *prev,
-	       struct held_lock *next, int distance, int *stack_saved)
+	       struct held_lock *next, int distance, struct stack_trace *trace)
 {
 	struct lock_list *entry;
 	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;
 
 	/*
 	 * Prove that the new <prev> -> <next> dependency would not
@@ -1858,11 +1865,7 @@ static inline void inc_chains(void)
 		}
 	}
 
-	if (!*stack_saved) {
-		if (!save_trace(&trace))
-			return 0;
-		*stack_saved = 1;
-	}
+	trace->skip = 1; /* mark used */
 
 	/*
 	 * Ok, all validations passed, add the new lock
@@ -1870,14 +1873,14 @@ static inline void inc_chains(void)
 	 */
 	ret = add_lock_to_list(hlock_class(next),
 			       &hlock_class(prev)->locks_after,
-			       next->acquire_ip, distance, &trace);
+			       next->acquire_ip, distance, trace);
 
 	if (!ret)
 		return 0;
 
 	ret = add_lock_to_list(hlock_class(prev),
 			       &hlock_class(next)->locks_before,
-			       next->acquire_ip, distance, &trace);
+			       next->acquire_ip, distance, trace);
 	if (!ret)
 		return 0;
 
@@ -1885,8 +1888,6 @@ static inline void inc_chains(void)
 	 * Debugging printouts:
 	 */
 	if (verbose(hlock_class(prev)) || verbose(hlock_class(next))) {
-		/* We drop graph lock, so another thread can overwrite trace. */
-		*stack_saved = 0;
 		graph_unlock();
 		printk("\n new dependency: ");
 		print_lock_name(hlock_class(prev));
@@ -1908,10 +1909,15 @@ static inline void inc_chains(void)
 static int
 check_prevs_add(struct task_struct *curr, struct held_lock *next)
 {
+	struct stack_trace trace = { .nr_entries = 0, .skip = 0, };
 	int depth = curr->lockdep_depth;
-	int stack_saved = 0;
 	struct held_lock *hlock;
 
+	if (!save_trace(&trace))
+		goto out_bug;
+
+	trace.skip = 0; /* abuse to mark usage */
+
 	/*
 	 * Debugging checks.
 	 *
@@ -1936,7 +1942,7 @@ static inline void inc_chains(void)
 		 */
 		if (hlock->read != 2 && hlock->check) {
 			if (!check_prev_add(curr, hlock, next,
-						distance, &stack_saved))
+					    distance, &trace))
 				return 0;
 			/*
 			 * Stop after the first non-trylock entry,
@@ -1962,6 +1968,9 @@ static inline void inc_chains(void)
 	}
 	return 1;
 out_bug:
+	if (trace.nr_entries && !trace.skip)
+		return_trace(&trace);
+
 	if (!debug_locks_off_graph_unlock())
 		return 0;