Re: [PATCH v7 06/16] lockdep: Detect and handle hist_lock ring buffer overwrite

[Peter Zijlstra]

On Thu, Jul 13, 2017 at 11:07:45AM +0900, Byungchul Park wrote:
> Does my approach have problems, rewinding to 'original idx' on exit and
> deciding whether overwrite or not? I think, this way, no need to do the
> drastic work. Or.. does my one get more overhead in usual case?

So I think that invalidating just the one entry doesn't work; the moment
you fill that up the iteration in commit_xhlocks() will again use the
next one etc.. even though you wanted it not to.

So we need to wipe the _entire_ history.

So I _think_ the below should work, but its not been near a compiler.


--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -822,6 +822,7 @@ struct task_struct {
 	unsigned int xhlock_idx_soft; /* For restoring at softirq exit */
 	unsigned int xhlock_idx_hard; /* For restoring at hardirq exit */
 	unsigned int xhlock_idx_hist; /* For restoring at history boundaries */
+	unsigned int xhlock_idX_max;
 #endif
 #ifdef CONFIG_UBSAN
 	unsigned int			in_ubsan;
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -4746,6 +4746,14 @@ EXPORT_SYMBOL_GPL(lockdep_rcu_suspicious
 static atomic_t cross_gen_id; /* Can be wrapped */
 
 /*
+ * make xhlock_valid() false.
+ */
+static inline void invalidate_xhlock(struct hist_lock *xhlock)
+{
+	xhlock->hlock.instance = NULL;
+}
+
+/*
  * Lock history stacks; we have 3 nested lock history stacks:
  *
  *   Hard IRQ
@@ -4764,28 +4772,58 @@ static atomic_t cross_gen_id; /* Can be
  * MAX_XHLOCKS_NR ? Possibly re-instroduce hist_gen_id ?
  */
 
-void crossrelease_hardirq_start(void)
+static inline void __crossrelease_start(unsigned int *stamp)
 {
 	if (current->xhlocks)
-		current->xhlock_idx_hard = current->xhlock_idx;
+		*stamp = current->xhlock_idx;
+}
+
+static void __crossrelease_end(unsigned int *stamp)
+{
+	int i;
+
+	if (!current->xhlocks)
+		return;
+
+	current->xhlock_idx = *stamp;
+
+	/*
+	 * If we rewind past the tail; all of history is lost.
+	 */
+	if ((current->xhlock_idx_max - *stamp) < MAX_XHLOCKS_NR)
+		return;
+
+	/*
+	 * Invalidate the entire history..
+	 */
+	for (i = 0; i < MAX_XHLOCKS_NR; i++)
+		invalidate_xhlock(&xhlock(i));
+
+	current->xhlock_idx = 0;
+	current->xhlock_idx_hard = 0;
+	current->xhlock_idx_soft = 0;
+	current->xhlock_idx_hist = 0;
+	current->xhlock_idx_max = 0;
+}
+
+void crossrelease_hardirq_start(void)
+{
+	__crossrelease_start(&current->xhlock_idx_hard);
 }
 
 void crossrelease_hardirq_end(void)
 {
-	if (current->xhlocks)
-		current->xhlock_idx = current->xhlock_idx_hard;
+	__crossrelease_end(&current->xhlock_idx_hard);
 }
 
 void crossrelease_softirq_start(void)
 {
-	if (current->xhlocks)
-		current->xhlock_idx_soft = current->xhlock_idx;
+	__crossrelease_start(&current->xhlock_idx_soft);
 }
 
 void crossrelease_softirq_end(void)
 {
-	if (current->xhlocks)
-		current->xhlock_idx = current->xhlock_idx_soft;
+	__crossrelease_end(&current->xhlock_idx_soft);
 }
 
 /*
@@ -4806,14 +4844,12 @@ void crossrelease_softirq_end(void)
  */
 void crossrelease_hist_start(void)
 {
-	if (current->xhlocks)
-		current->xhlock_idx_hist = current->xhlock_idx;
+	__crossrelease_start(&current->xhlock_idx_hist);
 }
 
 void crossrelease_hist_end(void)
 {
-	if (current->xhlocks)
-		current->xhlock_idx = current->xhlock_idx_hist;
+	__crossrelease_end(&current->xhlock_idx_hist);
 }
 
 static int cross_lock(struct lockdep_map *lock)
@@ -4880,6 +4916,9 @@ static void add_xhlock(struct held_lock
 	unsigned int idx = ++current->xhlock_idx;
 	struct hist_lock *xhlock = &xhlock(idx);
 
+	if ((int)(current->xhlock_idx_max - idx) < 0)
+		current->xhlock_idx_max = idx;
+
 #ifdef CONFIG_DEBUG_LOCKDEP
 	/*
 	 * This can be done locklessly because they are all task-local