--- libmalloc/libmalloc-374.40.6/src/magazine_medium.c
+++ libmalloc/libmalloc-792.60.6/src/magazine_medium.c
@@ -132,25 +132,6 @@
 	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
 
 static MALLOC_INLINE void
@@ -1009,11 +990,6 @@
 {
 	magazine_t *depot_ptr = &(rack->magazines[DEPOT_MAGAZINE_INDEX]);
 
-	/* FIXME: Would Uniprocessor benefit from recirc and MADV_FREE? */
-	if (rack->num_magazines == 1) { // Uniprocessor, single magazine, so no recirculation necessary
-		return 0;
-	}
-
 #if DEBUG_MALLOC
 	if (DEPOT_MAGAZINE_INDEX == mag_index) {
 		malloc_zone_error(rack->debug_flags, true, "medium_get_region_from_depot called for magazine index -1\n", NULL, NULL);
@@ -1514,11 +1490,7 @@
 	region_trailer_t *node = REGION_TRAILER_FOR_MEDIUM_REGION(region);
 	size_t bytes_used = node->bytes_used;
 
-	/* FIXME: Would Uniprocessor benefit from recirc and MADV_FREE? */
-	if (rack->num_magazines == 1) { // Uniprocessor, single magazine, so no recirculation necessary
-		/* NOTHING */
-		return TRUE; // Caller must do SZONE_MAGAZINE_PTR_UNLOCK(tiny_mag_ptr)
-	} else if (DEPOT_MAGAZINE_INDEX != mag_index) {
+	if (DEPOT_MAGAZINE_INDEX != mag_index) {
 		// Emptiness discriminant
 		if (bytes_used < DENSITY_THRESHOLD(MEDIUM_REGION_PAYLOAD_BYTES)) {
 			/* Region has crossed threshold from density to sparsity. Mark it "suitable" on the
@@ -2412,7 +2384,7 @@
 medium_malloc_should_clear(rack_t *rack, msize_t msize, boolean_t cleared_requested)
 {
 	void *ptr;
-	mag_index_t mag_index = medium_mag_get_thread_index() % rack->num_magazines;
+	mag_index_t mag_index = rack_get_thread_index(rack) % 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);
@@ -2461,16 +2433,17 @@
 
 		// 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 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) {
+		// 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
 			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,
@@ -2483,9 +2456,8 @@
 					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;
 			}
 
@@ -2494,14 +2466,20 @@
 					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);
-			yield();
+
+			// 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
 			SZONE_MAGAZINE_PTR_LOCK(medium_mag_ptr);
 		}
 	}