On Wed, Jan 18, 2017 at 10:17:32PM +0900, Byungchul Park wrote:

+	/*
+	 * If the previous in held_locks can create a proper dependency
+	 * with a target crosslock, then we can skip commiting this,
+	 * since "the target crosslock -> the previous lock" and
+	 * "the previous lock -> this lock" can cover the case. So we
+	 * keep the previous's gen_id to make the decision.
+	 */
+	unsigned int		prev_gen_id;

+static void add_xhlock(struct held_lock *hlock, unsigned int prev_gen_id)
+{
+	struct hist_lock *xhlock;
+
+	xhlock = alloc_xhlock();
+
+	/* Initialize hist_lock's members */
+	xhlock->hlock = *hlock;
+	xhlock->nmi = !!(preempt_count() & NMI_MASK);
+	/*
+	 * prev_gen_id is used to skip adding dependency at commit step,
+	 * when the previous lock in held_locks can do that instead.
+	 */
+	xhlock->prev_gen_id = prev_gen_id;
+	xhlock->work_id = current->work_id;
+
+	xhlock->trace.nr_entries = 0;
+	xhlock->trace.max_entries = MAX_XHLOCK_TRACE_ENTRIES;
+	xhlock->trace.entries = xhlock->trace_entries;
+	xhlock->trace.skip = 3;
+	save_stack_trace(&xhlock->trace);
+}

+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;
+
+	gen_id = (unsigned int)atomic_read(&cross_gen_id);
+	/*
+	 * gen_id_invalid must be too old to be valid. That means
+	 * current hlock should not be skipped but should be
+	 * considered at commit step.
+	 */
+	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);
+}

+static int commit_xhlocks(struct cross_lock *xlock)
+{
+	struct task_struct *curr = current;
+	struct hist_lock *xhlock_c = xhlock_curr(curr);
+	struct hist_lock *xhlock = xhlock_c;
+
+	do {
+		xhlock = xhlock_prev(curr, xhlock);
+
+		if (!xhlock_used(xhlock))
+			break;
+
+		if (before(xhlock->hlock.gen_id, xlock->hlock.gen_id))
+			break;
+
+		if (same_context_xhlock(xhlock) &&
+		    before(xhlock->prev_gen_id, xlock->hlock.gen_id) &&
+		    !commit_xhlock(xlock, xhlock))
+			return 0;
+	} while (xhlock_c != xhlock);
+
+	return 1;
+}

So I'm still struggling with prev_gen_id; is it an optimization or is it
required for correctness?

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>