Every pixel ref gets its own mutex.

include/core/SkPixelRef.h

Index: include/core/SkPixelRef.h
diff --git a/include/core/SkPixelRef.h b/include/core/SkPixelRef.h
index 0ed3099126de3481c384996cc9c55cd0d0306891..194c74f5efdc4012dd5e5165af04396ad788b960 100644
--- a/include/core/SkPixelRef.h
+++ b/include/core/SkPixelRef.h
@@ -36,7 +36,6 @@ class GrTexture;
 class SK_API SkPixelRef : public SkRefCnt {
 public:
     explicit SkPixelRef(const SkImageInfo&);
-    SkPixelRef(const SkImageInfo&, SkBaseMutex* mutex);
     virtual ~SkPixelRef();
 
     const SkImageInfo& info() const {
@@ -319,7 +318,7 @@ protected:
     /** Return the mutex associated with this pixelref. This value is assigned
         in the constructor, and cannot change during the lifetime of the object.
     */
-    SkBaseMutex* mutex() const { return fMutex; }
+    SkBaseMutex* mutex() const { return &fMutex; }
 
     // only call from constructor. Flags this to always be locked, removing
     // the need to grab the mutex and call onLockPixels/onUnlockPixels.
@@ -327,7 +326,7 @@ protected:
     void setPreLocked(void*, size_t rowBytes, SkColorTable*);
 
 private:
-    SkBaseMutex*    fMutex; // must remain in scope for the life of this object
+    mutable SkMutex fMutex;
 
     // mostly const. fInfo.fAlpahType can be changed at runtime.
     const SkImageInfo fInfo;
@@ -365,8 +364,6 @@ private:
     void needsNewGenID();
     void callGenIDChangeListeners();
 
-    void setMutex(SkBaseMutex* mutex);
-
     void setTemporarilyImmutable();
     void restoreMutability();
     friend class SkSurface_Raster;   // For the two methods above.