Replace @synchronized lock with dispatch_semaphore lock for SDWebImageCombinedOperation

@@ -347,10 +347,24 @@
 @end
 @end
 
 
 
 
+@interface SDWebImageCombinedOperation ()
+
+@property (strong, nonatomic, nonnull) dispatch_semaphore_t cancelLock; // a lock to make the `cancel` method thread-safe
+
+@end
+
 @implementation SDWebImageCombinedOperation
 @implementation SDWebImageCombinedOperation
 
 
+- (instancetype)init {
+    self = [super init];
+    if (self) {
+        _cancelLock = dispatch_semaphore_create(1);
+    }
+    return self;
+}
+
 - (void)cancel {
 - (void)cancel {
-    @synchronized(self) {
+    LOCK(self.cancelLock);
     self.cancelled = YES;
     self.cancelled = YES;
     if (self.cacheOperation) {
     if (self.cacheOperation) {
         [self.cacheOperation cancel];
         [self.cacheOperation cancel];
@@ -360,7 +374,7 @@
         [self.manager.imageDownloader cancel:self.downloadToken];
         [self.manager.imageDownloader cancel:self.downloadToken];
     }
     }
     [self.manager safelyRemoveOperationFromRunning:self];
     [self.manager safelyRemoveOperationFromRunning:self];
-    }
+    UNLOCK(self.cancelLock);
 }
 }
 
 
 @end
 @end