[PATCH v6 07/15] lockdep: Avoid adding redundant direct links of crosslocks

[Byungchul Park]

We can skip adding a dependency 'AX -> B', in case that we ensure 'AX ->
the previous of B in hlocks' to be created, where AX is a crosslock and
B is a typical lock. Remember that two adjacent locks in hlocks generate
a dependency like 'prev -> next', that is, 'the previous of B in hlocks
-> B' in this case.

For example:

             in hlocks[]
             ------------
          ^  A (gen_id: 4) --+
          |                  | previous gen_id
          |  B (gen_id: 3) <-+
          |  C (gen_id: 3)
          |  D (gen_id: 2)
   oldest |  E (gen_id: 1)

             in xhlocks[]
             ------------
          ^  A (gen_id: 4, prev_gen_id: 3(B's gen id))
          |  B (gen_id: 3, prev_gen_id: 3(C's gen id))
          |  C (gen_id: 3, prev_gen_id: 2(D's gen id))
          |  D (gen_id: 2, prev_gen_id: 1(E's gen id))
   oldest |  E (gen_id: 1, prev_gen_id: NA)

On commit for a crosslock AX(gen_id = 3), it's engough to add 'AX -> C',
but adding 'AX -> B' and 'AX -> A' is unnecessary since 'AX -> C', 'C ->
B' and 'B -> A' cover them, which are guaranteed to be generated.

This patch intoduces a variable, prev_gen_id, to avoid adding this kind
of redundant dependencies. In other words, the previous in hlocks will
anyway handle it if the previous's gen_id >= the crosslock's gen_id.

Signed-off-by: Byungchul Park <byungchul.park@xxxxxxx>
---
 include/linux/lockdep.h  | 11 +++++++++++
 kernel/locking/lockdep.c | 30 ++++++++++++++++++++++++++++--
 2 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index 5356f71..31c6289 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -284,6 +284,17 @@ struct held_lock {
  */
 struct hist_lock {
 	/*
+	 * We can skip adding a dependency 'a target crosslock -> this
+	 * lock', in case that we ensure 'the target crosslock -> the
+	 * previous lock in held_locks' to be created. Remember that
+	 * 'the previous lock in held_locks -> this lock' is guaranteed
+	 * to be created, and 'A -> B' and 'B -> C' cover 'A -> C'.
+	 *
+	 * Keep the previous's gen_id to make the decision.
+	 */
+	unsigned int		prev_gen_id;
+
+	/*
 	 * Each work of workqueue might run in a different context,
 	 * thanks to concurrency support of workqueue. So we have to
 	 * distinguish each work to avoid false positive.
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index ec4f6af..c78dd9d 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -4716,13 +4716,18 @@ static inline int xhlock_used(struct hist_lock *xhlock)
 /*
  * Only access local task's data, so irq disable is only required.
  */
-static void add_xhlock(struct held_lock *hlock)
+static void add_xhlock(struct held_lock *hlock, unsigned int prev_gen_id)
 {
 	unsigned int idx = current->xhlock_idx++;
 	struct hist_lock *xhlock = &xhlock(idx);
 
 	/* Initialize hist_lock's members */
 	xhlock->hlock = *hlock;
+	/*
+	 * prev_gen_id is used to skip adding redundant dependencies,
+	 * which can be covered by the previous lock in held_locks.
+	 */
+	xhlock->prev_gen_id = prev_gen_id;
 	xhlock->work_id = current->work_id;
 
 	xhlock->trace.nr_entries = 0;
@@ -4761,10 +4766,30 @@ static int same_context_xhlock(struct hist_lock *xhlock)
  */
 static void check_add_xhlock(struct held_lock *hlock)
 {
+	struct held_lock *prev;
+	struct held_lock *start;
+	unsigned int gen_id;
+	unsigned int gen_id_invalid;
+
 	if (!current->xhlocks || !depend_before(hlock))
 		return;
 
-	add_xhlock(hlock);
+	gen_id = (unsigned int)atomic_read(&cross_gen_id);
+	/*
+	 * gen_id_invalid should be old enough to be invalid.
+	 * Current gen_id - (UINIT_MAX / 4) would be a good
+	 * value to meet it.
+	 */
+	gen_id_invalid = gen_id - (UINT_MAX / 4);
+	start = current->held_locks;
+
+	for (prev = hlock - 1; prev >= start &&
+			!depend_before(prev); prev--);
+
+	if (prev < start)
+		add_xhlock(hlock, gen_id_invalid);
+	else if (prev->gen_id != gen_id)
+		add_xhlock(hlock, prev->gen_id);
 }
 
 /*
@@ -4902,6 +4927,7 @@ static int commit_xhlocks(struct cross_lock *xlock)
 				break;
 
 			if (same_context_xhlock(xhlock) &&
+			    before(xhlock->prev_gen_id, xlock->hlock.gen_id) &&
 			    !commit_xhlock(xlock, xhlock))
 				return 0;
 		}
-- 
1.9.1