fixup! fix(arc): Per-CID locking. Map CID to lock.

arc_cache.go

@@ -20,7 +20,7 @@ type arccache struct {
 	arc *lru.TwoQueueCache
 
 	arcLks   map[cid.Cid]*sync.Mutex
-	arcLksMu sync.Mutex
+	arcLksMu sync.RWMutex
 
 	blockstore Blockstore
 
@@ -74,12 +74,12 @@ func (b *arccache) hasCachedSync(k cid.Cid) (has bool, size int, ok bool, releas
 		}
 	}
 
-	// lock the map of cid->locks.
-	b.arcLksMu.Lock()
-	defer b.arcLksMu.Unlock()
-
-	// check if we have a lock for this content.
+	// read lock the map of cid->locks.
+	// This ensures other CID's can be locked when more than one lock/waiting is held on the same CID.
+	b.arcLksMu.RLock()
 	lk, hasLk := b.arcLks[k]
+	b.arcLksMu.RUnlock()
+	// check if a lock exists for content `k`.
 	if has && hasLk {
 		// cache and lock hit.
 		lk.Lock()
@@ -89,13 +89,18 @@ func (b *arccache) hasCachedSync(k cid.Cid) (has bool, size int, ok bool, releas
 	} else if has && !hasLk {
 		// cache hit and lock miss, create the lock, lock it, and add it to the lockMap
 		lk = new(sync.Mutex)
+
+		b.arcLksMu.Lock()
 		b.arcLks[k] = lk
+		b.arcLksMu.Unlock()
 
 		lk.Lock()
 		release = func() { lk.Unlock() }
 	} else if !has && hasLk {
 		// cache miss and lock hit, remove lock from map
+		b.arcLksMu.Lock()
 		delete(b.arcLks, k)
+		b.arcLksMu.Unlock()
 	}
 	// else cache miss and lock miss, noop
 	return