From ec2db4e40e9ab05ff697e0d9b37209aa70179137 Mon Sep 17 00:00:00 2001
From: Justine Tunney <jtunney@gmail.com>
Date: Tue, 24 Dec 2024 10:30:11 -0800
Subject: [PATCH] Avoid pthread_rwlock_wrlock() starvation

---
 libc/thread/pthread_rwlock_rdlock.c | 3 +++
 libc/thread/pthread_rwlock_wrlock.c | 2 ++
 libc/thread/thread.h                | 1 +
 3 files changed, 6 insertions(+)

diff --git a/libc/thread/pthread_rwlock_rdlock.c b/libc/thread/pthread_rwlock_rdlock.c
index 743c84924..e097bb0ef 100644
--- a/libc/thread/pthread_rwlock_rdlock.c
+++ b/libc/thread/pthread_rwlock_rdlock.c
@@ -42,6 +42,9 @@ errno_t pthread_rwlock_rdlock(pthread_rwlock_t *lk) {
       for (;;)
         if (~(w = atomic_load_explicit(&lk->_word, memory_order_relaxed)) & 1)
           break;
+    // xxx: avoid writer starvation in pthread_rwlock_rdlock_test
+    while (atomic_load(&lk->_waiters))
+      pthread_yield_np();
     if (atomic_compare_exchange_weak_explicit(
             &lk->_word, &w, w + 2, memory_order_acquire, memory_order_relaxed))
       return 0;
diff --git a/libc/thread/pthread_rwlock_wrlock.c b/libc/thread/pthread_rwlock_wrlock.c
index 0120a80a0..382eba828 100644
--- a/libc/thread/pthread_rwlock_wrlock.c
+++ b/libc/thread/pthread_rwlock_wrlock.c
@@ -42,8 +42,10 @@ errno_t pthread_rwlock_wrlock(pthread_rwlock_t *rwlock) {
     if (atomic_compare_exchange_weak_explicit(
             &rwlock->_word, &w, 1, memory_order_acquire, memory_order_relaxed))
       return 0;
+    atomic_fetch_add(&rwlock->_waiters, 1);
     for (;;)
       if (!(w = atomic_load_explicit(&rwlock->_word, memory_order_relaxed)))
         break;
+    atomic_fetch_sub(&rwlock->_waiters, 1);
   }
 }
diff --git a/libc/thread/thread.h b/libc/thread/thread.h
index 15952c2a7..f45d88095 100644
--- a/libc/thread/thread.h
+++ b/libc/thread/thread.h
@@ -111,6 +111,7 @@ typedef struct pthread_rwlock_s {
       char _pshared;
       char _iswrite;
       _PTHREAD_ATOMIC(uint32_t) _word;
+      _PTHREAD_ATOMIC(uint32_t) _waiters;
     };
   };
 } pthread_rwlock_t;