On 06.12.2016 16:36, Peter Zijlstra wrote:
On Thu, Dec 01, 2016 at 03:06:48PM +0100, Nicolai Hähnle wrote:
+static inline int __sched
+__ww_mutex_add_waiter(struct mutex_waiter *waiter,
+ struct mutex *lock,
+ struct ww_acquire_ctx *ww_ctx)
+{
+ struct mutex_waiter *cur;
+
+ if (!ww_ctx) {
+ list_add_tail(&waiter->list, &lock->wait_list);
+ return 0;
+ }
+
+ /*
+ * Add the waiter before the first waiter with a higher stamp.
+ * Waiters without a context are skipped to avoid starving
+ * them.
+ */
+ list_for_each_entry(cur, &lock->wait_list, list) {
+ if (!cur->ww_ctx)
+ continue;
+
+ if (__ww_mutex_stamp_after(ww_ctx, cur->ww_ctx)) {
+ /* Back off immediately if necessary. */
+ if (ww_ctx->acquired > 0) {
+#ifdef CONFIG_DEBUG_MUTEXES
+ struct ww_mutex *ww;
+
+ ww = container_of(lock, struct ww_mutex, base);
+ DEBUG_LOCKS_WARN_ON(ww_ctx->contending_lock);
+ ww_ctx->contending_lock = ww;
+#endif
+ return -EDEADLK;
+ }
+
+ continue;
+ }
+
+ list_add_tail(&waiter->list, &cur->list);
+ return 0;
+ }
+
+ list_add_tail(&waiter->list, &lock->wait_list);
+ return 0;
+}
So you keep the list in order of stamp, and in general stamps come in,
in-order. That is, barring races on concurrent ww_mutex_lock(), things
are already ordered.
>
So it doesn't make sense to scan the entire list forwards, that's almost
guarantees you scan the entire list every single time.
Or am I reading this wrong? Which in itself is a hint a comment might be
in place.
No, it's a reasonable question. Some things to keep in mind:
1. Each ww_acquire_ctx may be used with hundreds of locks. It's not that
clear that things will be ordered in a contention scenario, especially
since the old stamp is re-used when a context backs off and goes into
the slow path (with ww_ctx->acquired == 0).
2. We want to add waiters directly before the first waiter with a higher
stamp. Keeping in mind that there may be non-ww_ctx waiters in between,
and we don't want to starve them, traversing the list backwards would
require keeping the best insertion point around in a separate variable.
Clearly possible, but it seemed more awkward.
In hindsight, backwards iteration may not be so bad.
Nicolai
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