art::ConditionVariable::WaitHoldingLocks(art::Thread*)

art::ConditionVariable::WaitHoldingLocks(art::Thread*)

Overview

The `art::ConditionVariable::WaitHoldingLocks(art::Thread*)` method is a crucial part of the Android Runtime (ART) synchronization mechanism, allowing threads to wait on a condition while holding locks. This ensures that the condition remains valid throughout the wait and that the thread acquires the necessary locks before proceeding.

Purpose

The primary purpose of this method is to provide a safe and efficient way for threads to wait on a condition while holding multiple locks. It accomplishes this by:

* **Releasing the locks:** Before waiting, the method releases the locks held by the calling thread.
* **Waiting on the condition:** The thread then waits on the condition variable until it is signaled.
* **Reacquiring the locks:** Upon waking up, the thread reacquires all the previously held locks.

Usage

The `WaitHoldingLocks(art::Thread*)` method takes a single argument:

* **art::Thread* thread:** A pointer to the thread object. This parameter is used to determine which thread’s locks should be released and reacquired.

**Code Example:**

“`cpp
#include
#include
#include

std::mutex mutex;
std::condition_variable cv;
bool data_ready = false;

void thread_function() {
std::unique_lock lock(mutex);
cv.wait(lock, [] { return data_ready; });
// Process data…
}

int main() {
// …
std::thread t(thread_function);
// …
data_ready = true;
cv.notify_one();
// …
t.join();
return 0;
}
“`

**Output:**

The thread will wait until `data_ready` is set to `true`.

Advantages

* **Safety:** Ensures that the condition remains valid while the thread is waiting, preventing race conditions.
* **Efficiency:** Avoids unnecessary lock contention and reduces the risk of deadlocks.
* **Flexibility:** Allows threads to wait on multiple conditions simultaneously.

Comparison with Other Methods

| Method | Description |
|——————————-|———————————————————————————————|
| `wait()` | Waits on a condition without releasing any locks. |
| `wait_for()` | Waits on a condition for a specified duration, without releasing any locks. |
| `wait_until()` | Waits on a condition until a specific time point, without releasing any locks. |
| `WaitHoldingLocks()` | Waits on a condition while releasing and reacquiring locks held by the calling thread. |

Conclusion

The `art::ConditionVariable::WaitHoldingLocks(art::Thread*)` method provides a crucial synchronization mechanism in ART, allowing threads to wait on conditions safely and efficiently while maintaining control over their locks. Its ability to release and reacquire locks ensures the integrity of shared resources and prevents potential race conditions.