Making code thread-safe Kyle J. Knoepfel 25 June 2019 LArSoft - - PowerPoint PPT Presentation

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Making code thread-safe Kyle J. Knoepfel 25 June 2019 LArSoft - - PowerPoint PPT Presentation

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Making code thread-safe Kyle J. Knoepfel 25 June 2019 LArSoft Workshop 2019 So youre going to make your code thread-safe 2 6/25/19 Kyle J. Knoepfel | LArSoft Workshop 2019 So youre going to make your code thread-safe The

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Making code thread-safe

Kyle J. Knoepfel 25 June 2019 LArSoft Workshop 2019

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So you’re going to make your code thread-safe…

6/25/19 Kyle J. Knoepfel | LArSoft Workshop 2019 2

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  • The difficulty of this task depends on the context

So you’re going to make your code thread-safe…

6/25/19 Kyle J. Knoepfel | LArSoft Workshop 2019 3

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  • The difficulty of this task depends on the context
  • What language are you using?

– Multi-threading in (e.g.) C++ is harder – Multi-threading in (e.g.) Go, Rust, Haskell is easier

  • Are you starting from scratch or retrofitting code?
  • Does it make sense for the code in question to be multi-threaded?

6/25/19 Kyle J. Knoepfel | LArSoft Workshop 2019 4

So you’re going to make your code thread-safe…

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  • The difficulty of this task depends on the context
  • What language are you using?

– Multi-threading in (e.g.) C++ is harder – Multi-threading in (e.g.) Go, Rust, Haskell is easier

  • Are you starting from scratch or retrofitting code?
  • Does it make sense for the code in question to be multi-threaded?

So you’re going to make your code thread-safe…

6/25/19 Kyle J. Knoepfel | LArSoft Workshop 2019 5

When writing multi-threaded code, you should always ask:

What’s the context in which this function will be called?

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Thread-safety matrix

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Thread-safety matrix

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Is the object shared across threads?

Yes No

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Thread-safety matrix

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Is the object shared across threads? Is the object mutable?

Yes No Yes No

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Thread-safety matrix

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OK OK OK

Is the object shared across threads? Is the object mutable?

Yes No Yes No

Data races possible

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Thread-safety matrix

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OK OK OK OK

Is the object shared across threads? Is the object mutable?

Yes No Yes No

Data races guaranteed

Synchronous access Asynchronous access

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Thread-safety matrix

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OK OK OK OK

Is the object shared across threads? Is the object mutable?

Yes No Yes No

Data races guaranteed

Synchronous access Asynchronous access

You must get out of that box!

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Thread-safety matrix

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OK

Sometimes the easiest solution. Requires more memory.

OK OK

Is the object shared across threads? Is the object mutable?

Yes No Yes No

Data races guaranteed

Synchronous access Asynchronous access

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Thread-safety matrix

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OK

Sometimes the easiest solution. Requires more memory. Memory savings. Can be difficult to make things immutable.

OK

Is the object shared across threads? Is the object mutable?

Yes No Yes No

Data races guaranteed

Synchronous access Asynchronous access

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Thread-safety matrix

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Awkward. Will not discuss. Sometimes the easiest solution. Requires more memory. Memory savings. Can be difficult to make things immutable.

OK

Is the object shared across threads? Is the object mutable?

Yes No Yes No

Data races guaranteed

Synchronous access Asynchronous access

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Thread-safety matrix

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Awkward. Will not discuss. Sometimes the easiest solution. Requires more memory. Memory savings. Can be difficult to make things immutable.

OK

Is the object shared across threads? Is the object mutable?

Yes No Yes No

Data races guaranteed

Synchronous access Asynchronous access

Making your code thread-safe often requires a combination of methods.

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  • You must know what is shared among threads, and the contexts in which the

sharing happens.

To make your code thread-safe…

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  • You must know what is shared among threads, and the contexts in which the

sharing happens.

  • Game – Part 1

– Thread-safety and free-functions

  • Game – Part 2

– Thread-safety and class member functions

To make your code thread-safe…

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Part 1: Is it thread-safe?

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void test(...) { // ... } int main() { execute_with_10_threads(test, ...); }

The pattern we’ll follow

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Part 1: Is it thread-safe?

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void test() {} int main() { execute_with_10_threads(test); }

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Part 1: Is it thread-safe?

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void test() {} int main() { execute_with_10_threads(test); }

Yes

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Part 1: Is it thread-safe?

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void test() { auto i = 42; } int main() { execute_with_10_threads(test); }

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Part 1: Is it thread-safe?

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void test() { auto i = 42; } int main() { execute_with_10_threads(test); }

Yes

Each thread gets its

  • wn stack memory.
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Part 1: Is it thread-safe?

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void test(int j) { ++j; } int main() { auto i = 42; execute_with_10_threads(test, i); }

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Part 1: Is it thread-safe?

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void test(int j) { ++j; } int main() { auto i = 42; execute_with_10_threads(test, i); }

Yes

The value 42 is copied into j for each thread.

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Part 1: Is it thread-safe?

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void test() { static int j{0}; ++j; } int main() { execute_with_10_threads(test); }

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Part 1: Is it thread-safe?

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void test() { static int j{0}; ++j; } int main() { execute_with_10_threads(test); }

No

j is shared across all threads that call test

  • perator++ requires

a read and then write

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Part 1: Is it thread-safe?

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void test() { static int j{0}; int k = j; ++k; } int main() { execute_with_10_threads(test); }

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Part 1: Is it thread-safe?

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void test() { static int j{0}; int k = j; ++k; } int main() { execute_with_10_threads(test); }

Yes

Although j is shared, its value never changes k is a stack variable. This is fragile, though!

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Part 1: Is it thread-safe?

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void test() { static int j{0}; int& k = j; ++k; } int main() { execute_with_10_threads(test); }

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Part 1: Is it thread-safe?

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void test() { static int j{0}; int& k = j; ++k; } int main() { execute_with_10_threads(test); }

No

k now refers to a shared object!

  • perator++ requires

a read and then write

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Part 1: Is it thread-safe?

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void test() { static int j{0}; int& k = j; ++k; } int main() { execute_with_10_threads(test); }

No

k now refers to a shared object!

  • perator++ requires

a read and then write

One character can break a program.

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Part 1: Is it thread-safe?

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void test(std::string const& sentence) { auto pos = sentence.find("C++17"); } int main() { std::string sentence{"I love C++17."}; execute_with_10_threads(test, sentence); }

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Part 1: Is it thread-safe?

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void test(std::string const& sentence) { auto pos = sentence.find("C++17"); } int main() { std::string sentence{"I love C++17."}; execute_with_10_threads(test, sentence); }

Yes

In general, calling const-qualified C++ STL member functions is thread-safe… assuming another thread isn’t adjusting the object.

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Part 1: Is it thread-safe?

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void test() { MyArbitraryType t; } int main() { execute_with_10_threads(test); }

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Part 1: Is it thread-safe?

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void test() { MyArbitraryType t; } int main() { execute_with_10_threads(test); }

It depends

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Part 1: Is it thread-safe?

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void test() { MyArbitraryType t; } int main() { execute_with_10_threads(test); } using MyArbitraryType = int;

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Part 1: Is it thread-safe?

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void test() { MyArbitraryType t; } int main() { execute_with_10_threads(test); } using MyArbitraryType = int;

Yes

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struct MyArbitraryType { MyArbitraryType() { static int counter; ++counter; } };

Part 1: Is it thread-safe?

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void test() { MyArbitraryType t; } int main() { execute_with_10_threads(test); }

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struct MyArbitraryType { MyArbitraryType() { static int counter; ++counter; } };

Part 1: Is it thread-safe?

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void test() { MyArbitraryType t; } int main() { execute_with_10_threads(test); }

No

Although there is one ‘t’ per thread, the constructor accesses shared memory.

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Part 2: Is it thread-safe?

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class MyClass { public: void test(...) { // ... } }; int main() { MyClass obj; execute_with_10_threads(&MyClass::test, obj, ...); }

The pattern we’ll follow

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Part 2: Is it thread-safe?

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class MyClass { public: void test() {} }; int main() { MyClass obj; execute_with_10_threads(&MyClass::test, obj); }

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Part 2: Is it thread-safe?

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class MyClass { public: void test() {} }; int main() { MyClass obj; execute_with_10_threads(&MyClass::test, obj); }

Yes

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Part 2: Is it thread-safe?

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class MyClass { int count_{0}; public: void test() { ++count_; } }; int main() { MyClass obj; execute_with_10_threads(&MyClass::test, obj); }

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Part 2: Is it thread-safe?

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class MyClass { int count_{0}; public: void test() { ++count_; } }; int main() { MyClass obj; execute_with_10_threads(&MyClass::test, obj); }

No

The class data for obj (count_) is shared across threads.

  • perator++ requires

a read and then write

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Part 2: Is it thread-safe?

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class MyClass { public: void test(MyOtherClass& oc) { auto const h = oc.getSomething(); } }; int main() { MyClass obj; MyOtherClass oc; execute_with_10_threads(&MyClass::test, obj, oc); }

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Part 2: Is it thread-safe?

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class MyClass { public: void test(MyOtherClass& oc) { auto const h = oc.getSomething(); } }; int main() { MyClass obj; MyOtherClass oc; execute_with_10_threads(&MyClass::test, obj, oc); }

It depends

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Part 2: Is it thread-safe?

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class MyClass { public: void test(MyOtherClass& oc) { auto const h = oc.getSomething(); } }; int main() { MyClass obj; MyOtherClass oc; execute_with_10_threads(&MyClass::test, obj, oc); } class MyOtherClass { public: int getSomething() const { return 42; } };

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Part 2: Is it thread-safe?

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class MyClass { public: void test(MyOtherClass& oc) { auto const h = oc.getSomething(); } }; int main() { MyClass obj; MyOtherClass oc; execute_with_10_threads(&MyClass::test, obj, oc); } class MyOtherClass { public: int getSomething() const { return 42; } };

Yes

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Part 2: Is it thread-safe?

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class MyClass { public: void test(MyOtherClass& oc) { auto const h = oc.getSomething(); } }; int main() { MyClass obj; MyOtherClass oc; execute_with_10_threads(&MyClass::test, obj, oc); } class MyOtherClass { public: int getSomething() const { static int i{42}; ++i; return i; } };

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Part 2: Is it thread-safe?

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class MyClass { public: void test(MyOtherClass& oc) { auto const h = oc.getSomething(); } }; int main() { MyClass obj; MyOtherClass oc; execute_with_10_threads(&MyClass::test, obj, oc); } class MyOtherClass { public: int getSomething() const { static int i{42}; ++i; return i; } };

No!

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Part 2: Is it thread-safe?

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class MyClass { public: void test(MyOtherClass& oc) { auto const h = oc.getSomething(); } }; int main() { MyClass obj; MyOtherClass oc; execute_with_10_threads(&MyClass::test, obj, oc); }

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Part 2: Is it thread-safe?

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class MyClass { public: void produce(art::Event& e) { auto const h = e.getValidHandle(); } }; int main() { MyClass obj; art::Event e; execute_with_10_threads(&MyClass::produce, obj, e); }

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  • Takes analysis!
  • Know what objects are shared.
  • Know when they are shared.

Determining thread-safety …

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  • Who owns your module?

Considerations for art

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  • Who owns your module?
  • art owns the module objects, which are created at run-time based on the

configuration you provide.

  • You provide the definition of the module class:

– art knows very little of your module’s definition – art calls module functions via C++ polymorphism

  • Suppose art were to call your produce function concurrently on multiple events.

Considerations for art

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Module example

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  • We want to create a track from a collection of hits

void TrackMaker::produce(art::Event& e) { auto const& hits = e.getValidHandle<Hits>(tag_); unique_ptr<Track> track = trackFromHits(*hits); e.put(move(track)); }

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Module example

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  • We want to create a track from a collection of hits
  • Assuming trackFromHits does not update any state, then this produce function

is thread-safe—i.e. it can be called concurrently with different art::Event

  • bjects.
  • Why?

– art guarantees that product retrieval and insertion is thread-safe – the produce function above modifies no state of the TrackMaker object

void TrackMaker::produce(art::Event& e) { auto const& hits = e.getValidHandle<Hits>(tag_); unique_ptr<Track> track = trackFromHits(*hits); e.put(move(track)); }

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  • Functions (free or member) which access a global object whose state can change,

including non-const function-scope static data.

  • Functions (free or member) which change the state of objects which were passed

as const function arguments (e.g. casting away const on an argument).

  • const non-static member functions which modify the state of the object on

which they are called (e.g. mutable members, or casting away const on this).

  • Pointer member data or data held by member data being passed as a non-const

argument to functions.

  • const member functions returning values of member variables which are pointers

to non-const items.

Indications of thread-unsafe C++ code

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  • Apply const liberally
  • Avoid using non-const static variables in functions/classes
  • To the extent possible, do not use the mutable keyword
  • Use as few global objects as possible
  • If you must use a global object, provide only const-qualified interface
  • Don’t use output arguments

General guidelines

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  • Apply const liberally
  • Avoid using non-const static variables in functions/classes
  • To the extent possible, do not use the mutable keyword
  • Use as few global objects as possible
  • If you must use a global object, provide only const-qualified interface
  • Don’t use output arguments

General guidelines

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void fillInts(std::vector<int>& ints) { for (size_t i{0}; i < 42; ++i) ints.push_back(some_calculation(i)); } std::vector<int> ints; fillInts(ints); auto getInts() { std::vector<int> ints; for (size_t i{0}; i < 42; ++i) ints.push_back(some_calculation(i)); return ints; } auto const ints = getInts();

Discouraged Encouraged

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  • Apply const liberally
  • Avoid using non-const static variables in functions/classes
  • To the extent possible, do not use the mutable keyword
  • Use as few global objects as possible
  • If you must use a global object, provide only const-qualified interface
  • Don’t use output arguments

General guidelines

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All of these are good ideas for single-threaded code. You can do this now!