Add a mutex to GrContext::readSurfacePixels to protect against multiple CPU raster threads accessing the same GrContext to read back GPU input data

BUG=chromium:524717

TBR=reed@google.com

Committed: https://skia.googlesource.com/skia/+/eb662bc407cec0585a821946fef123102cae64db

Review URL: https://codereview.chromium.org/1329313002

1 files changed, 19 insertions, 2 deletions

diff --git a/include/gpu/GrContext.h b/include/gpu/GrContext.h
index 7f1e4109fa..64f4f6ce30 100644
--- a/include/gpu/GrContext.h
+++ b/include/gpu/GrContext.h
@@ -15,6 +15,7 @@
 #include "GrRenderTarget.h"
 #include "GrTextureProvider.h"
 #include "SkMatrix.h"
+#include "SkMutex.h"
 #include "SkPathEffect.h"
 #include "SkTypes.h"
 
@@ -380,6 +381,18 @@ private:
     bool                            fDidTestPMConversions;
     int                             fPMToUPMConversion;
     int                             fUPMToPMConversion;
+    // The sw backend may call GrContext::readSurfacePixels on multiple threads
+    // We may transfer the responsibilty for using a mutex to the sw backend
+    // when there are fewer code paths that lead to a readSurfacePixels call
+    // from the sw backend. readSurfacePixels is reentrant in one case - when performing
+    // the PM conversions test. To handle this we do the PM conversions test outside
+    // of fReadPixelsMutex and use a separate mutex to guard it. When it re-enters
+    // readSurfacePixels it will grab fReadPixelsMutex and release it before the outer
+    // readSurfacePixels proceeds to grab it.
+    // TODO: Stop pretending to make GrContext thread-safe for sw rasterization and provide
+    // a mechanism to make a SkPicture safe for multithreaded sw rasterization.
+    SkMutex                         fReadPixelsMutex;
+    SkMutex                         fTestPMConversionsMutex;
 
     struct CleanUpData {
         PFCleanUpFunc fFunc;
@@ -446,9 +459,13 @@ private:
      * return NULL.
      */
     const GrFragmentProcessor* createPMToUPMEffect(GrProcessorDataManager*, GrTexture*,
-                                                   bool swapRAndB, const SkMatrix&);
+                                                   bool swapRAndB, const SkMatrix&) const;
     const GrFragmentProcessor* createUPMToPMEffect(GrProcessorDataManager*, GrTexture*,
-                                                   bool swapRAndB, const SkMatrix&);
+                                                   bool swapRAndB, const SkMatrix&) const;
+    /** Called before either of the above two functions to determine the appropriate fragment
+        processors for conversions. This must be called by readSurfacePixels befor a mutex is taken,
+        since testingvPM conversions itself will call readSurfacePixels */
+    void testPMConversionsIfNecessary(uint32_t flags);
     /** Returns true if we've already determined that createPMtoUPMEffect and createUPMToPMEffect
         will fail. In such cases fall back to SW conversion. */
     bool didFailPMUPMConversionTest() const;