Added some example tooling discussion.

CppCoreGuidelines.md

@@ -7324,9 +7324,9 @@ But heed the warning: [Avoid "naked" `union`s](#Ru-naked)
 ##### Example
 
     // Short string optimization
-    
+
     constexpr size_t buffer_size = 16; // Slightly larger than the size of a pointer
-    
+
     class Immutable_string {
     public:
         Immutable_string(const char* str) :
@@ -7339,18 +7339,18 @@ But heed the warning: [Avoid "naked" `union`s](#Ru-naked)
                 strcpy_s(string_ptr, size + 1, str);
             }
         }
-    
+
         ~Immutable_string()
         {
             if (size >= buffer_size)
                 delete string_ptr;
         }
-    
+
         const char* get_str() const
         {
             return (size < buffer_size) ? string_buffer : string_ptr;
         }
-    
+
     private:
         // If the string is short enough, we store the string itself
         // instead of a pointer to the string.
@@ -7358,7 +7358,7 @@ But heed the warning: [Avoid "naked" `union`s](#Ru-naked)
             char* string_ptr;
             char string_buffer[buffer_size];
         };
-    
+
         const size_t size;
     };
 
@@ -11635,16 +11635,40 @@ this can be a security risk.
 
 ##### Enforcement
 
-Some is possible, do at least something.
+When possible, rely on tooling enforcement, but be aware that any tooling
-There are commercial and open-source tools that try to address this problem, but static tools often have many false positives and run-time tools often have a significant cost.
+solution has costs and blind spots. Defense in depth (multiple tools, multiple
-We hope for better tools.
+approaches) is particularly valuable here.
 
-Help the tools:
+In the realm of static enforcement,
+both [clang](http://clang.llvm.org/docs/ThreadSafetyAnalysis.html) and some
+older verisons of [gcc](https://gcc.gnu.org/wiki/ThreadSafetyAnnotation) have
+some support for static annotation of thread safety properties. Consistent use
+of this technique turns many classes of thread-safety errors into compile-time
+errors. The annotations are generally local (marking a particular member
+variable as guarded by a particular mutex), and are usually easy to
+learn. However, as with many static tools, it can often present false
+negatives - cases that should have been caught but were allowed.
 
-* less global data
+Clang's [Thread Sanitizer](http://clang.llvm.org/docs/ThreadSanitizer.html) (aka
-* fewer `static` variables
+tsan) is a powerful example of dynamic tools: it changes the build and execution
-* more use of stack memory (and don't pass pointers around too much)
+of your program to add bookkeeping on memory access, absolutely identifying data
-* more immutable data (literals, `constexpr`, and `const`)
+races in a given execution of your binary. The cost for this is both memory
+(5-10x in most cases) and CPU slowdown (2-20x). Dynamic tools like this are best
+when applied to integration tests, canary pushes, or unittests that operate on
+multiple threads. Workload matters: When tsan identifies a problem, it is
+effectively always an actual data race, but it can only identify races seen in a
+given execution.
+
+There are many other tools, both commercial and open-source. Thread safety is
+challenging, often getting the better of experienced programmers: tooling is an
+important strategy to mitigate those risks.
+
+There are other ways you can mitigate the chance of data races:
+
+* Avoid global data
+* Avoid `static` variables
+* More use of value types on the stack (and don't pass pointers around too much)
+* More use of immutable data (literals, `constexpr`, and `const`)
 
 ### <a name="Rconc-data"></a>CP.3: Minimize explicit sharing of writable data
 
@@ -12647,7 +12671,7 @@ Example with thread-safe static local variables of C++11.
         static My_class my_object; // Constructor called only once
         // ...
     }
-    
+
     class My_class
     {
     public:
@@ -12670,7 +12694,7 @@ Double-checked locking is easy to mess up. If you really need to write your own
 
 ##### Example, bad
 
-Even if the following example works correctly on most hardware platforms, it is not guaranteed to work by the C++ standard. The x_init.load(memory_order_relaxed) call may see a value from outside of the lock guard. 
+Even if the following example works correctly on most hardware platforms, it is not guaranteed to work by the C++ standard. The x_init.load(memory_order_relaxed) call may see a value from outside of the lock guard.
 
     atomic<bool> x_init;
 
@@ -12687,12 +12711,12 @@ Even if the following example works correctly on most hardware platforms, it is
 One of the conventional patterns is below.
 
     std::atomic<int> state;
-    
+
     // If state == SOME_ACTION_NEEDED maybe an action is needed, maybe not, we need to
     // check again in a lock. However, if state != SOME_ACTION_NEEDED, then we can be
     // sure that an action is not needed. This is the basic assumption of double-checked
     // locking.
-    
+
     if (state == SOME_ACTION_NEEDED)
     {
         std::lock_guard<std::mutex> lock(mutex);