Execute a memory barrier when adding a new page to a space to synchronize access with concurrent sw…

src/spaces.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
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@@ skipping 519 matching lines @@
   }
 
   area_end_ = area_start_ + requested;
   return true;
 }
 
 
 void MemoryChunk::InsertAfter(MemoryChunk* other) {
   next_chunk_ = other->next_chunk_;
   prev_chunk_ = other;
+
+  // This memory barrier is needed since concurrent sweeper threads may iterate
+  // over the list of pages while a new page is inserted. 
+  MemoryBarrier();

[Michael Starzinger, 2013/03/11 11:40:36]

As discussed in length offline: We agree that starting to spread memory barriers
through the code (outside of dedicated synchronized data structures) is a
slippery slope. But since we don't have a cleaner solution for this data
structure that is embedded into the memory chunks, I agree to this temporary
solution.

But please add a TODO here, that we should find a cleaner way to guarantee that
the page list can be expanded concurrently. You can even attribute that TODO to
my name if you like.

+
+  // The following two write operations can take effect in arbitrary order
+  // since pages are always iterated by the sweeper threads in LIFO order, i.e,
+  // the inserted page becomes visible for the sweeper threads after 
+  // other->next_chunk_ = this;
   other->next_chunk_->prev_chunk_ = this;
   other->next_chunk_ = this;
 }
 
 
 void MemoryChunk::Unlink() {
   if (!InNewSpace() && IsFlagSet(SCAN_ON_SCAVENGE)) {
     heap_->decrement_scan_on_scavenge_pages();
     ClearFlag(SCAN_ON_SCAVENGE);
   }
@@ skipping 2602 matching lines @@
     object->ShortPrint();
     PrintF("\n");
   }
   printf(" --------------------------------------\n");
   printf(" Marked: %x, LiveCount: %x\n", mark_size, LiveBytes());
 }
 
 #endif  // DEBUG
 
 } }  // namespace v8::internal