Suppose 10 processes are waiting on a semaphore using sem_wait().
and an 11th process calls sem_post on that semaphore.
which of the 10 processes will enter the critical block?
Is it like random? All the process will wake up and strive to achieve a lock.
and CPU will provide a lock to one of the processes and the rest will go back to waiting for state
The POSIX standard doesn't specify which thread will be woken up. Moreover, without artificial delays it's impossible for threads to start waiting on a semaphore in a well-defined order.
In practice, it's likely to be the thread which has been waiting the longest, as a queue structure is used to record threads waiting on a synchronization object. It definitely won't be a 'random' thread. But it's also not something you should depend on for the correctness of your code.
I have a prioritization problem in my homework. Some threads have higher priorities than the others and the other threads have to wait to access a file until all those threads have finished their job. We are not allowed to use busy waiting. Is there another solution to solve this. Thanks.
Yeah, OK, like my comment.
The low-priority threads must wait on the, (initialized to zero), semaphore before accessing the file. The high-priority threads, when they have finished with tbe file, must acquire the mutex, count down the high count check for zero and release the mutex. The last high-thread that finds zero should then issue [low count] units to the semaphore, so releasing all the low threads.
Actually, a mutex-protected struct with state data can handle just about any weird locking/sequencing/prioritizing scheme that your prof can come up with, no matter how much tequila it consumes first:)
I have a thread where I want to wait (at a particular line of code) for three callback events from another thread. Only after these three events are received then I want to proceed forward.
I am trying to use semaphores. I am aware that a semaphore can be locked at a point and it keeps waiting till it is released by some other thread.
Now, the thing is that I want to wait for three callbacks and not just one before i release the semaphore.
I thought of having a counter but I am not sure whether just have a separate counter would be thread safe.
So is there a way to have a semaphore with a thread safe counter?
This is for both Linux and Windows.
Thanks.
If the threads can have assignable numbers, you maybe can have just a boolean variable per controlling thread and then check if all are set before the suspended thread is released. Writing a byte is probably atomic.
Normal semaphores would have atomic counters, however.
Suppose I have multiple threads blocking on a call to pthread_mutex_lock(). When the mutex becomes available, does the first thread that called pthread_mutex_lock() get the lock? That is, are calls to pthread_mutex_lock() in FIFO order? If not, what, if any, order are they in? Thanks!
When the mutex becomes available, does the first thread that called pthread_mutex_lock() get the lock?
No. One of the waiting threads gets a lock, but which one gets it is not determined.
FIFO order?
FIFO mutex is rather a pattern already. See Implementing a FIFO mutex in pthreads
"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."
Aside from that, the answer to your question isn't specified by the POSIX standard. It may be random, or it may be in FIFO or LIFO or any other order, according to the choices made by the implementation.
FIFO ordering is about the least efficient mutex wake order possible. Only a truly awful implementation would use it. The thread that ran the most recently may be able to run again without a context switch and the more recently a thread ran, more of its data and code will be hot in the cache. Reasonable implementations try to give the mutex to the thread that held it the most recently most of the time.
Consider two threads that do this:
Acquire a mutex.
Adjust some data.
Release the mutex.
Go to step 1.
Now imagine two threads running this code on a single core CPU. It should be clear that FIFO mutex behavior would result in one "adjust some data" per context switch -- the worst possible outcome.
Of course, reasonable implementations generally do give some nod to fairness. We don't want one thread to make no forward progress. But that hardly justifies a FIFO implementation!
In a past question, I asked about implementing pthread barriers without destruction races:
How can barriers be destroyable as soon as pthread_barrier_wait returns?
and received from Michael Burr with a perfect solution for process-local barriers, but which fails for process-shared barriers. We later worked through some ideas, but never reached a satisfactory conclusion, and didn't even begin to get into resource failure cases.
Is it possible on Linux to make a barrier that meets these conditions:
Process-shared (can be created in any shared memory).
Safe to unmap or destroy the barrier from any thread immediately after the barrier wait function returns.
Cannot fail due to resource allocation failure.
Michael's attempt at solving the process-shared case (see the linked question) has the unfortunate property that some kind of system resource must be allocated at wait time, meaning the wait can fail. And it's unclear what a caller could reasonably do when a barrier wait fails, since the whole point of the barrier is that it's unsafe to proceed until the remaining N-1 threads have reached it...
A kernel-space solution might be the only way, but even that's difficult due to the possibility of a signal interrupting the wait with no reliable way to resume it...
This is not possible with the Linux futex API, and I think this can be proven as well.
We have here essentially a scenario in which N processes must be reliably awoken by one final process, and further no process may touch any shared memory after the final awakening (as it may be destroyed or reused asynchronously). While we can awaken all processes easily enough, the fundamental race condition is between the wakeup and the wait; if we issue the wakeup before the wait, the straggler never wakes up.
The usual solution to something like this is to have the straggler check a status variable atomically with the wait; this allows it to avoid sleeping at all if the wakeup has already occurred. However, we cannot do this here - as soon as the wakeup becomes possible, it is unsafe to touch shared memory!
One other approach is to actually check if all processes have gone to sleep yet. However, this is not possible with the Linux futex API; the only indication of number of waiters is the return value from FUTEX_WAKE; if it returns less than the number of waiters you expected, you know some weren't asleep yet. However, even if we find out we haven't woken enough waiters, it's too late to do anything - one of the processes that did wake up may have destroyed the barrier already!
So, unfortunately, this kind of immediately-destroyable primitive cannot be constructed with the Linux futex API.
Note that in the specific case of one waiter, one waker, it may be possible to work around the problem; if FUTEX_WAKE returns zero, we know nobody has actually been awoken yet, so you have a chance to recover. Making this into an efficient algorithm, however, is quite tricky.
It's tricky to add a robust extension to the futex model that would fix this. The basic problem is, we need to know when N threads have successfully entered their wait, and atomically awaken them all. However, any of those threads may leave the wait to run a signal handler at any time - indeed, the waker thread may also leave the wait for signal handlers as well.
One possible way that may work, however, is an extension to the keyed event model in the NT API. With keyed events, threads are released from the lock in pairs; if you have a 'release' without a 'wait', the 'release' call blocks for the 'wait'.
This in itself isn't enough due to the issues with signal handlers; however, if we allow for the 'release' call to specify a number of threads to be awoken atomically, this works. You simply have each thread in the barrier decrement a count, then 'wait' on a keyed event on that address. The last thread 'releases' N - 1 threads. The kernel doesn't allow any wake event to be processed until all N-1 threads have entered this keyed event state; if any thread leaves the futex call due to signals (including the releasing thread), this prevents any wakeups at all until all threads are back.
After a long discussion with bdonlan on SO chat, I think I have a solution. Basically, we break the problem down into the two self-synchronized deallocation issues: the destroy operation and unmapping.
Handling destruction is easy: Simply make the pthread_barrier_destroy function wait for all waiters to stop inspecting the barrier. This can be done by having a usage count in the barrier, atomically incremented/decremented on entry/exit to the wait function, and having the destroy function spin waiting for the count to reach zero. (It's also possible to use a futex here, rather than just spinning, if you stick a waiter flag in the high bit of the usage count or similar.)
Handling unmapping is also easy, but non-local: ensure that munmap or mmap with the MAP_FIXED flag cannot occur while barrier waiters are in the process of exiting, by adding locking to the syscall wrappers. This requires a specialized sort of reader-writer lock. The last waiter to reach the barrier should grab a read lock on the munmap rw-lock, which will be released when the final waiter exits (when decrementing the user count results in a count of 0). munmap and mmap can be made reentrant (as some programs might expect, even though POSIX doesn't require it) by making the writer lock recursive. Actually, a sort of lock where readers and writers are entirely symmetric, and each type of lock excludes the opposite type of lock but not the same type, should work best.
Well, I think I can do it with a clumsy approach...
Have the "barrier" be its own process listening on a socket. Implement barrier_wait as:
open connection to barrier process
send message telling barrier process I am waiting
block in read() waiting for reply
Once N threads are waiting, the barrier process tells all of them to proceed. Each waiter then closes its connection to the barrier process and continues.
Implement barrier_destroy as:
open connection to barrier process
send message telling barrier process to go away
close connection
Once all connections are closed and the barrier process has been told to go away, it exits.
[Edit: Granted, this allocates and destroys a socket as part of the wait and release operations. But I think you can implement the same protocol without doing so; see below.]
First question: Does this protocol actually work? I think it does, but maybe I do not understand the requirements.
Second question: If it does work, can it be simulated without the overhead of an extra process?
I believe the answer is "yes". You can have each thread "take the role of" the barrier process at the appropriate time. You just need a master mutex, held by whichever thread is currently "taking the role" of the barrier process. Details, details... OK, so the barrier_wait might look like:
lock(master_mutex);
++waiter_count;
if (waiter_count < N)
cond_wait(master_condition_variable, master_mutex);
else
cond_broadcast(master_condition_variable);
--waiter_count;
bool do_release = time_to_die && waiter_count == 0;
unlock(master_mutex);
if (do_release)
release_resources();
Here master_mutex (a mutex), master_condition_variable (a condition variable), waiter_count (an unsigned integer), N (another unsigned integer), and time_to_die (a Boolean) are all shared state allocated and initialized by barrier_init. waiter_count is initialiazed to zero, time_to_die to false, and N to the number of threads the barrier is waiting for.
Then barrier_destroy would be:
lock(master_mutex);
time_to_die = true;
bool do_release = waiter_count == 0;
unlock(master_mutex);
if (do_release)
release_resources();
Not sure about all the details concerning signal handling etc... But the basic idea of "last one out turns off the lights" is workable, I think.