--- libmalloc/libmalloc-715.100.22/src/magazine_medium.c
+++ libmalloc/libmalloc-409.81.2/src/magazine_medium.c
@@ -130,6 +130,25 @@
 medium_meta_header_set_middle(msize_t *meta_headers, msize_t index)
 {
 	meta_headers[index] = 0;
+}
+
+static MALLOC_INLINE MALLOC_ALWAYS_INLINE
+mag_index_t
+medium_mag_get_thread_index(void)
+{
+#if CONFIG_MEDIUM_USES_HYPER_SHIFT
+	if (os_likely(_os_cpu_number_override == -1)) {
+		return _malloc_cpu_number() >> hyper_shift;
+	} else {
+		return _os_cpu_number_override >> hyper_shift;
+	}
+#else // CONFIG_MEDIUM_USES_HYPER_SHIFT
+	if (os_likely(_os_cpu_number_override == -1)) {
+		return _malloc_cpu_number();
+	} else {
+		return _os_cpu_number_override;
+	}
+#endif // CONFIG_MEDIUM_USES_HYPER_SHIFT
 }
 
 #pragma mark in-place free list
@@ -2384,7 +2403,7 @@
 medium_malloc_should_clear(rack_t *rack, msize_t msize, boolean_t cleared_requested)
 {
 	void *ptr;
-	mag_index_t mag_index = rack_get_thread_index(rack) % rack->num_magazines;
+	mag_index_t mag_index = medium_mag_get_thread_index() % rack->num_magazines;
 	magazine_t *medium_mag_ptr = &(rack->magazines[mag_index]);
 
 	MALLOC_TRACE(TRACE_medium_malloc, (uintptr_t)rack, MEDIUM_BYTES_FOR_MSIZE(msize), (uintptr_t)medium_mag_ptr, cleared_requested);
@@ -2433,17 +2452,16 @@
 
 		// The magazine is exhausted. A new region (heap) must be allocated to satisfy this call to malloc().
 		// The allocation, an mmap() system call, will be performed outside the magazine spin locks by the first
-		// thread that suffers the exhaustion. That thread accquires the magazine_alloc_lock, then drops the
-		// magazine lock to allow freeing threads to proceed. Allocating thrads that arrive later  are excluded
-		// from the critial section by the alloc lock. When those are unblocked, they succeed in the code above.
-		//
-		// Note that we need to trylock the alloc lock to avoid a deadlock, since we can't block on the alloc
-		// lock while holding the magazine lock
-		if (os_likely(_malloc_lock_trylock(&medium_mag_ptr->magazine_alloc_lock))) {
-			// We got the alloc lock, so we are the thread that should allocate a new region
+		// thread that suffers the exhaustion. That thread sets "alloc_underway" and enters a critical section.
+		// Threads arriving here later are excluded from the critical section, yield the CPU, and then retry the
+		// allocation. After some time the magazine is resupplied, the original thread leaves with its allocation,
+		// and retry-ing threads succeed in the code just above.
+		if (!medium_mag_ptr->alloc_underway) {
 			void *fresh_region;
 
 			// time to create a new region (do this outside the magazine lock)
+			medium_mag_ptr->alloc_underway = TRUE;
+			OSMemoryBarrier();
 			SZONE_MAGAZINE_PTR_UNLOCK(medium_mag_ptr);
 			fresh_region = mvm_allocate_pages(MEDIUM_REGION_SIZE,
 					MEDIUM_BLOCKS_ALIGN,
@@ -2456,8 +2474,9 @@
 					fresh_region, MEDIUM_REGION_SIZE);
 
 			if (!fresh_region) { // out of memory!
+				medium_mag_ptr->alloc_underway = FALSE;
+				OSMemoryBarrier();
 				SZONE_MAGAZINE_PTR_UNLOCK(medium_mag_ptr);
-				_malloc_lock_unlock(&medium_mag_ptr->magazine_alloc_lock);
 				return NULL;
 			}
 
@@ -2466,20 +2485,14 @@
 					mag_index, msize, fresh_region);
 
 			// we don't clear because this freshly allocated space is pristine
+			medium_mag_ptr->alloc_underway = FALSE;
+			OSMemoryBarrier();
 			SZONE_MAGAZINE_PTR_UNLOCK(medium_mag_ptr);
-			_malloc_lock_unlock(&medium_mag_ptr->magazine_alloc_lock);
 			CHECK(szone, __PRETTY_FUNCTION__);
 			return ptr;
 		} else {
-			// We failed to get the alloc lock, so someone else is allocating.
-			// Drop the magazine lock...
 			SZONE_MAGAZINE_PTR_UNLOCK(medium_mag_ptr);
-
-			// Wait for the other thread on the alloc lock
-			_malloc_lock_lock(&medium_mag_ptr->magazine_alloc_lock);
-			_malloc_lock_unlock(&medium_mag_ptr->magazine_alloc_lock);
-
-			// Reacquire the magazine lock to go around the loop again
+			yield();
 			SZONE_MAGAZINE_PTR_LOCK(medium_mag_ptr);
 		}
 	}