From b4ce4d681ed0a3660be08d3d1ce881783fb018ab Mon Sep 17 00:00:00 2001
From: Tavian Barnes <tavianator@tavianator.com>
Date: Thu, 28 Nov 2024 14:56:08 -0500
Subject: ioq: Try spinning before blocking in ioq_slot_wait()

---
 src/ioq.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/src/ioq.c b/src/ioq.c
index 1280598..b845a2c 100644
--- a/src/ioq.c
+++ b/src/ioq.c
@@ -277,10 +277,26 @@ static struct ioq_monitor *ioq_slot_monitor(struct ioqq *ioqq, ioq_slot *slot) {
 /** Atomically wait for a slot to change. */
 _noinline
 static uintptr_t ioq_slot_wait(struct ioqq *ioqq, ioq_slot *slot, uintptr_t value) {
+	// Try spinning a few times before blocking
+	uintptr_t ret;
+	for (int i = 0; i < 10; ++i) {
+		// Exponential backoff
+		for (int j = 0; j < (1 << i); ++j) {
+			spin_loop();
+		}
+
+		// Check if the slot changed
+		ret = load(slot, relaxed);
+		if (ret != value) {
+			return ret;
+		}
+	}
+
+	// Nothing changed, start blocking
 	struct ioq_monitor *monitor = ioq_slot_monitor(ioqq, slot);
 	mutex_lock(&monitor->mutex);
 
-	uintptr_t ret = load(slot, relaxed);
+	ret = load(slot, relaxed);
 	if (ret != value) {
 		goto done;
 	}
-- 
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