From 14830bfd0c8ed9fadb8bb31ff4c654810138664e Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Sun, 30 Nov 2025 12:58:01 +0100
Subject: [PATCH] ThreadId generation fallback path: avoid spurious yields

---
 library/std/src/thread/id.rs | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/library/std/src/thread/id.rs b/library/std/src/thread/id.rs
index ba7024327881b..3da0825db604d 100644
--- a/library/std/src/thread/id.rs
+++ b/library/std/src/thread/id.rs
@@ -70,7 +70,9 @@ impl ThreadId {
 
                 // Acquire lock.
                 let mut spin = 0;
-                while COUNTER_LOCKED.compare_exchange_weak(false, true, Ordering::Acquire, Ordering::Relaxed).is_err() {
+                // Miri doesn't like it when we yield here as it interferes with deterministically
+                // scheduling threads, so avoid `compare_exchange_weak` to avoid spurious yields.
+                while COUNTER_LOCKED.swap(true, Ordering::Acquire) {
                     if spin <= 3 {
                         for _ in 0..(1 << spin) {
                             spin_loop();
@@ -80,6 +82,7 @@ impl ThreadId {
                     }
                     spin += 1;
                 }
+                // This was `false` before the swap, so we got the lock.
 
                 // SAFETY: we have an exclusive lock on the counter.
                 unsafe {