Don't use os_unfair_lock on 32-bit platforms

submodules/AsyncDisplayKit/Source/ASRecursiveUnfairLock.mm

@@ -31,7 +31,8 @@ void ASRecursiveUnfairLockLock(ASRecursiveUnfairLock *l)
   // because only we are `self`. And if it's already `self` then we already have
   // the lock, because we reset it to NULL before we unlock. So (thread == self) is
   // invariant.
-  
+
+#if AS_USE_OS_LOCK
   const pthread_t s = pthread_self();
   if (os_unfair_lock_trylock(&l->_lock)) {
     // Owned by nobody. We now have the lock. Assign self.
@@ -43,14 +44,28 @@ void ASRecursiveUnfairLockLock(ASRecursiveUnfairLock *l)
     os_unfair_lock_lock(&l->_lock);
     rul_set_thread(l, s);
   }
+#else
+    const pthread_t s = pthread_self();
+    if (OSSpinLockTry(&l->_lock)) {
+        // Owned by nobody. We now have the lock. Assign self.
+        rul_set_thread(l, s);
+    } else if (rul_get_thread(l) == s) {
+        // Owned by self (recursive lock). nop.
+    } else {
+        // Owned by other thread. Block and then set thread to self.
+        OSSpinLockLock(&l->_lock);
+        rul_set_thread(l, s);
+    }
+#endif
   
   l->_count++;
 }
 
 BOOL ASRecursiveUnfairLockTryLock(ASRecursiveUnfairLock *l)
 {
-  // Same as Lock above. See comments there.
-  
+    // Same as Lock above. See comments there.
+    
+#if AS_USE_OS_LOCK
   const pthread_t s = pthread_self();
   if (os_unfair_lock_trylock(&l->_lock)) {
     // Owned by nobody. We now have the lock. Assign self.
@@ -61,6 +76,18 @@ BOOL ASRecursiveUnfairLockTryLock(ASRecursiveUnfairLock *l)
     // Owned by other thread. Fail.
     return NO;
   }
+#else
+    const pthread_t s = pthread_self();
+    if (OSSpinLockTry(&l->_lock)) {
+      // Owned by nobody. We now have the lock. Assign self.
+      rul_set_thread(l, s);
+    } else if (rul_get_thread(l) == s) {
+      // Owned by self (recursive lock). nop.
+    } else {
+      // Owned by other thread. Fail.
+      return NO;
+    }
+#endif
   
   l->_count++;
   return YES;
@@ -78,6 +105,10 @@ void ASRecursiveUnfairLockUnlock(ASRecursiveUnfairLock *l)
     // try to re-lock, and fail the -tryLock, and read _thread, then we'll mistakenly
     // think that we still own the lock and proceed without blocking.
     rul_set_thread(l, NULL);
+#if AS_USE_OS_LOCK
     os_unfair_lock_unlock(&l->_lock);
+#else
+    OSSpinLockUnlock(&l->_lock);
+#endif
   }
 }

submodules/AsyncDisplayKit/Source/PublicHeaders/AsyncDisplayKit/ASRecursiveUnfairLock.h

@@ -10,6 +10,15 @@
 #import <UIKit/UIKit.h>
 #import <AsyncDisplayKit/ASBaseDefines.h>
 #import <pthread/pthread.h>
+
+#if defined(__aarch64__)
+    #define AS_USE_OS_LOCK true
+#else
+    #define AS_USE_OS_LOCK false
+#endif
+
+#if AS_USE_OS_LOCK
+
 #import <os/lock.h>
 
 // Note: We don't use ATOMIC_VAR_INIT here because C++ compilers don't like it,
@@ -46,3 +55,43 @@ AS_EXTERN OS_UNFAIR_LOCK_AVAILABILITY
 void ASRecursiveUnfairLockUnlock(ASRecursiveUnfairLock *l);
 
 NS_ASSUME_NONNULL_END
+
+#else
+
+#import <libkern/OSAtomic.h>
+
+// Note: We don't use ATOMIC_VAR_INIT here because C++ compilers don't like it,
+// and it literally does absolutely nothing.
+#define AS_RECURSIVE_UNFAIR_LOCK_INIT ((ASRecursiveUnfairLock){ OS_SPINLOCK_INIT, NULL, 0})
+
+NS_ASSUME_NONNULL_BEGIN
+
+typedef struct {
+  OSSpinLock _lock;
+  _Atomic(pthread_t) _thread;  // Write-protected by lock
+  int _count;                  // Protected by lock
+} ASRecursiveUnfairLock;
+
+/**
+ * Lock, blocking if needed.
+ */
+AS_EXTERN
+void ASRecursiveUnfairLockLock(ASRecursiveUnfairLock *l);
+
+/**
+ * Try to lock without blocking. Returns whether we took the lock.
+ */
+AS_EXTERN
+BOOL ASRecursiveUnfairLockTryLock(ASRecursiveUnfairLock *l);
+
+/**
+ * Unlock. Calling this on a thread that does not own
+ * the lock will result in an assertion failure, and undefined
+ * behavior if foundation assertions are disabled.
+ */
+AS_EXTERN
+void ASRecursiveUnfairLockUnlock(ASRecursiveUnfairLock *l);
+
+NS_ASSUME_NONNULL_END
+
+#endif

submodules/AsyncDisplayKit/Source/PublicHeaders/AsyncDisplayKit/ASThread.h

@@ -9,7 +9,6 @@
 
 #import <Foundation/Foundation.h>
 
-#import <os/lock.h>
 #import <pthread.h>
 
 #import <AsyncDisplayKit/ASAssert.h>
@@ -148,7 +147,11 @@ namespace AS {
           success = _recursive.try_lock();
           break;
         case Unfair:
+#if AS_USE_OS_LOCK
           success = os_unfair_lock_trylock(&_unfair);
+#else
+          success = OSSpinLockTry(&_unfair);
+#endif
           break;
         case RecursiveUnfair:
           success = ASRecursiveUnfairLockTryLock(&_runfair);
@@ -169,7 +172,11 @@ namespace AS {
           _recursive.lock();
           break;
         case Unfair:
+#if AS_USE_OS_LOCK
           os_unfair_lock_lock(&_unfair);
+#else
+          OSSpinLockLock(&_unfair);
+#endif
           break;
         case RecursiveUnfair:
           ASRecursiveUnfairLockLock(&_runfair);
@@ -188,7 +195,11 @@ namespace AS {
           _recursive.unlock();
           break;
         case Unfair:
+#if AS_USE_OS_LOCK
           os_unfair_lock_unlock(&_unfair);
+#else
+          OSSpinLockUnlock(&_unfair);
+#endif
           break;
         case RecursiveUnfair:
           ASRecursiveUnfairLockUnlock(&_runfair);
@@ -226,7 +237,11 @@ namespace AS {
       } else {
         if (gMutex_unfair) {
           _type = Unfair;
+#if AS_USE_OS_LOCK
           _unfair = OS_UNFAIR_LOCK_INIT;
+#else
+          _unfair = OS_SPINLOCK_INIT;
+#endif
         } else {
           _type = Plain;
           new (&_plain) std::mutex();
@@ -261,7 +276,11 @@ namespace AS {
     
     Type _type;
     union {
-      os_unfair_lock _unfair;
+#if AS_USE_OS_LOCK
+    os_unfair_lock _unfair;
+#else
+    OSSpinLock _unfair;
+#endif
       ASRecursiveUnfairLock _runfair;
       std::mutex _plain;
       std::recursive_mutex _recursive;