[cfe-dev] [libc++] thread safety annotations for unique_lock

[christoph@ruediger.engineering via cfe-dev]

Sorry for the incomplete mail; hit the wrong shortcut. The mail continues down below.

Am 08.11.16, 19:38 schrieb "cfe-dev im Auftrag von cfe-dev at lists.llvm.org" <cfe-dev-bounces at lists.llvm.org im Auftrag von cfe-dev at lists.llvm.org>:

>Hi everyone,
>
>this is my first post to this list, so please remind me if I’m violating your netiquette in some way.
>
>I’m trying to introduce the thread safety analysis to one of our projects and stumble upon the missing thread safety annotations for unique_lock. I thought this might be easy, so let’s provide a patch. But now I’m stuck with correctly annotating the move constructor and the lock() and unlock() methods of unique_lock.
>
>Here is a very basic implementation of my annotated unique_lock:
>
>class SCOPED_CAPABILITY unique_lock {
>  ::bluebox::mutex *mu;
>  bool lock_held;
>public:
>  unique_lock() = delete;
>  unique_lock(unique_lock &&) = delete;   // We cannot annotate move semantics
>
>  explicit unique_lock(::bluebox::mutex &m) ACQUIRE(m) : mu(&m), lock_held(true) { m.lock(); }
>  explicit unique_lock(::bluebox::mutex *m) ACQUIRE(m) : mu(m), lock_held(true) { m->lock(); }
>
>  void unlock() RELEASE(mu) NO_THREAD_SAFETY_ANALYSIS {
>    if (lock_held) {
>      lock_held = false;
>      mu->unlock();
>    }
>  }
>
>  void lock() ACQUIRE(mu) NO_THREAD_SAFETY_ANALYSIS {
>    if (!lock_held) {
>      mu->lock();
>      lock_held = true;
>    }
>  }
>
>  ~unique_lock() RELEASE() NO_THREAD_SAFETY_ANALYSIS {
>    if (lock_held)
>      mu->unlock();
>  }
>};
>
>
>And this is my test case:
>
>static std::mutex mtx;
>void someFunction() {
>  my::unique_lock lock(mtx);
>  // … Here you would normally do something meaningful
>  lock.unlock();
>  // … Call a method that might modify your protected data
>  lock.lock();
>  // … Continue with super important work
>}
>
>And here is my list of issues I can’t get around with:
>1. How can I correctly annotate the unlock and lock methods? When using unlock() in the above example, clang throws this error: fatal error: releasing mutex
>      'lock.mu' that was not held [-Wthread-safety-analysis]
>2. How can I correctly annotate the move constructor of unique_lock?

3. I couldn’t find really that much valuable information about the annotations except for the one page in the clang documentation and a talk from DeLesly Hutchins. Plus the information, that the guys at Google are making heavy use of the thread safety analyzer. But are you really limiting yourself to std::mutex and std::lock_guard? You can’t even use std::condition_variable. Maybe one of the Google engineers on this list can shade some light without revealing confidential information.

With these first tests in mind, I’m having troubles to use this really helpful feature in my projects. Anybody who wants to share some experience is appreciated. I will try to update the documentation accordingly or provide any other help.

Thanks and regards,
Christoph

-- 
rüdiger.engineering
Christoph Rüdiger
Düsseldorfer Str. 12
45145 Essen
Germany
 
phone: +49 201 458 478 58
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