when a thread1 already acquired a lock on mutex object, if thread2 tries to acquire a lock on the same mutex object, thread2 will be blocked.
here are my questions:
1. how will thread2 come to know that mutex object is unlocked?
2. will thread2 try to acquire lock at predifined intervals of time?
I sense a misunderstanding of how a mutex works. When thread 2 tries to acquire a mutex that is already owned by thread 1, the call that tries to take the mutex will not return until the mutex becomes available (unless you have a timeout with trylock() variant).
So thread 2 does not need to loop there and keep trying to take the mutex (unless you're using a timeout so you can abort trying to take the mutex based on some other condition like a cancel condition).
This is really OS dependant, but what usually happens is that thread2 gets suspended and put on a wait list maintained by the mutex. When the mutex becomes available, a thread on the mutex's waitlist gets removed from the list and put back on the list of active threads. The OS can then schedule it like it usually would. thread2 is totally quiescent until it can acquire the mutex.
Related
Wak Server 10.187175
OSX 10.10.5
One of our threads uses Mutex.
Users complain that it sometimes seems to remain locked after thread execution completes.
Please verify that Mutex always unlocks when the thread either ends, or .unlock() is applied to the Mutex object.
If the thread crashes does Mutex unlock?
thread 1:
lock mutex1
long time operation
unlock mutex1
thread2:
lock mutex1
...
thread3:
lock mutex1
...
thread4:
lock mutex1
...
threadn:
lock mutex1
...
When thread1 unlocks mutex1, which thread will be woken up? Is there a standard specification for it?
When you unlock a posix mutex using pthread_mutex_unlock, if several threads are waiting on the mutex only one of them will wake up.
The documentation states:
If there are threads blocked on the mutex object referenced by mutex
when pthread_mutex_unlock() is called, resulting in the mutex becoming
available, the scheduling policy shall determine which thread shall
acquire the mutex.
when thread1 unlock mutex1, which thread will be woken up?
One of the other threads that are currently blocked on the mutex. It is unspecified which of them.
Is there a standard specification for it?
No. Different systems may implement it in different ways. (Some might use a simple fifo order for waking the threads, others might use heuristics for deciding which thread to wake up).
Imagine I have a mutex locked. There is unlimited number of other threads waiting to lock the mutex. When I unlock the mutex, one of those threads will be chosen to enter the critical section. However I have no control over which one. What if I want specific thread to enter the critical section?
I am prety sure this cannot be done using POSIX mutex, however, can I emulate the behaviour using different synchronisation object(s)?
You can use a mutex, a condition variable and a thread id to achive that.
Before unlocking the mutex the thread sets the target thread id, broadcasts the condition variable and releases the mutex. The waiting threads wake up, lock the mutex and check whether the target thread id equals to this thread id. If not the thread goes back to wait.
An optimization to this method to avoid waking up all waiting threads just to check the target thread id and then go back to wait would be to use a separate condition variable for each waiting thread. This way the signaling thread would notify the condition variable of the particular target thread.
Another option is to use signals sent to a particular thread. Let's say we use SIGRTMINfor this purpose. First, all threads block this signal at the start, so that the signal becomes pending and doesn't get lost when the thread isn't waiting for it. When a thread wants to lock the mutex it first calls sigwait() which atomically unblocks SIGRTMIN and waits for it or delivers an already pending one. Once the thread received the signal it can proceed and lock the mutex. The signaling thread uses pthread_kill(target_thread_id, SIGRTMIN) to wake up a particular thread.
Singularity - If a thread managed to lock a mutex, it is assured that no other thread will be able to lock the thread until the original thread releases the lock.
Non-Busy Wait - If a thread attempts to lock a thread that was locked by a second thread, the first thread will be suspended (and will not consume any CPU resources) until the lock is freed by the second thread. At this time, the first thread will wake up and continue execution, having the mutex locked by it.
From: Multi-Threaded Programming With POSIX Threads
Question: I thought threads lock the mutex variables. Threads don't lock other threads?
What do the bold statements above mean? How can one thread lock other thread?
Corrections:
If a thread managed to lock a mutex, it is assured that no other thread will be able to lock the mutex until the original thread releases the lock.
Non-Busy Wait - If a thread attempts to lock a mutex that was locked by a second thread, the first thread will be suspended (and will not consume any CPU resources) until the lock is freed by the second thread. At this time, the first thread will wake up and continue execution, having the mutex locked by it.
It's a good thing you don't take for granted whatever you read on the internet, also I give you thumbs up for paying attention to what you read.
pthread_cond_broadcast is used to wake up several threads waiting on a condition variable. But, at the same time it is also said that we should place a mutex before the condition variable to avoid race conditions.
So, if a mutex is there, and one thread already holds it and thus waits on the variable, how can any other thread hold the same mutex (to enter to the waiting part)?
When a thread calls pthread_cond_wait() it needs to hold the associated mutex. The API will release the mutex while it blocks the thread. Once the API decides the thread needs to be released, it will acquire the mutex before returning from the API.
Holding the mutex is required because checking the condition then calling pthread_cond_wait() isn't an atomic operation. The mutex (and the proper use of the mutex) prevents the thread from missing the condition becoming true in the short period between checking it and calling the wait.
But the short answer to the specific question (how can another thread obtain the mutex) is that while the thread is blocked in pthread_cond_wait(), the mutex is not held.