aos/common/mutex.h

#ifndef AOS_COMMON_MUTEX_H_
#define AOS_COMMON_MUTEX_H_

#include "aos/common/macros.h"
#include "aos/linux_code/ipc_lib/aos_sync.h"
#include "aos/common/die.h"

namespace aos {

class Condition;

// An abstraction of a mutex that is easy to implement for environments other
// than Linux too.
// If there are multiple threads or processes contending for the mutex,
// higher priority ones will succeed in locking first,
// and tasks of equal priorities have the same chance of getting the lock.
// To deal with priority inversion, the linux implementation does priority
// inheritance.
// Before destroying a mutex, it is important to make sure it isn't locked.
// Otherwise, the destructor will LOG(FATAL).
class Mutex {
 public:
  enum class State {
    kLocked, kUnlocked
  };

  // Creates an unlocked mutex.
  Mutex();
  // Verifies that it isn't locked.
  //
  // This is important because freeing a locked mutex means there is freed
  // memory in the middle of the robust list, which breaks things horribly.
  ~Mutex();

  // Locks the mutex. If it fails, it calls LOG(FATAL).
  // Returns false.
  bool Lock() __attribute__((warn_unused_result));
  // Unlocks the mutex. Fails like Lock.
  // Multiple unlocking is undefined.
  void Unlock();
  // Locks the mutex unless it is already locked.
  // Returns the new state of the mutex.
  // Doesn't wait for the mutex to be unlocked if it is locked.
  State TryLock() __attribute__((warn_unused_result));

 private:
  aos_mutex impl_;

  friend class Condition;  // for access to impl_
};

// A class that locks a Mutex when constructed and unlocks it when destructed.
// Designed to be used as a local variable so that
// the mutex will be unlocked when the scope is exited.
// This one immediately Dies if the previous owner died. This makes it a good
// choice for mutexes that are only used within a single process, but NOT for
// mutexes shared by multiple processes. For those, use IPCMutexLocker.
class MutexLocker {
 public:
  explicit MutexLocker(Mutex *mutex) : mutex_(mutex) {
    if (__builtin_expect(mutex_->Lock(), false)) {
      ::aos::Die("previous owner of mutex %p died but it shouldn't be able to",
                 this);
    }
  }
  ~MutexLocker() {
    mutex_->Unlock();
  }

 private:
  Mutex *const mutex_;

  DISALLOW_COPY_AND_ASSIGN(MutexLocker);
};

// A version of MutexLocker which reports the previous owner dying instead of
// immediately LOG(FATAL)ing.
class IPCMutexLocker {
 public:
  explicit IPCMutexLocker(Mutex *mutex)
      : mutex_(mutex), owner_died_(mutex_->Lock()) {}
  ~IPCMutexLocker() {
    if (__builtin_expect(!owner_died_checked_, false)) {
      ::aos::Die("nobody checked if the previous owner of mutex %p died", this);
    }
    mutex_->Unlock();
  }

  // Whether or not the previous owner died. If this is not called at least
  // once, the destructor will ::aos::Die.
  __attribute__((warn_unused_result)) bool owner_died() {
    owner_died_checked_ = true;
    return __builtin_expect(owner_died_, false);
  }

 private:
  Mutex *const mutex_;
  const bool owner_died_;
  bool owner_died_checked_ = false;

  DISALLOW_COPY_AND_ASSIGN(IPCMutexLocker);
};

}  // namespace aos

#endif  // AOS_COMMON_MUTEX_H_