I'm studying for my systems programming examination, but am having some trouble coming up with a simple deadlock example in c. I understand deadlocks occur whenever the waiting process is still holding on to another resource that the first needs, however I am having a difficult time understanding it in terms of c code. Are there any simple examples someone can provide?
Are there any simple examples someone can provide?
Here is a simple example: a piece of code might need to acquire two locks, A and B, e.g. so it can atomically modify two pieces of data, one protected by A and the other by B. For example:
pthread_mutex_t a, b;
pthread_mutex_lock(&a);
pthread_mutex_lock(&b);
// ... process data protected by A and B
pthread_mutex_unlock(&b);
pthread_mutex_unlock(&a);
At the same time, another piece of code can have the same requirement, but merely acquire the locks in the opposite order:
pthread_mutex_lock(&b);
pthread_mutex_lock(&a);
// ... process data protected by A and B
pthread_mutex_unlock(&a);
pthread_mutex_unlock(&b);
In isolation, either approach works just fine. But when the two pieces of code execute concurrently, the following can occur:
// neither lock is held
// thread 1 // thread 2
pthread_mutex_lock(&a); pthread_mutex_lock(&b);
// thread 1 now holds A // thread 2 now holds B
// thread 1 is waiting for B // thread 2 is waiting for A
pthread_mutex_lock(&b); pthread_mutex_lock(&a);
The last line constitutes a deadlock: thread 1 is blocked waiting for lock B, which is held by thread 2. Thread 2 is blocked waiting for lock A, which is held by thread 1. As written, the threads are blocked indefinitely and neither can proceed.
Related
I'm reading this book here (official link, it's free) to understand threads and parallel programming.
Here's the question.
Why does the book say that pthread_cond_signal must be done with a lock held to prevent data race? I wasn't sure, so I referred to this question (and this question too), which basically said "no, it's not required". Why would a race condition occur?
What and where is the race condition being described?
The code and passage in question is as follows.
...
The code to wake a thread, which would run in some other thread, looks like this:
pthread_mutex_lock(&lock);
ready = 1;
pthread_cond_signal(&cond);
pthread_mutex_unlock(&lock);
A few things to note about this code sequence. First, when signaling (as well as when modifying the global variable ready), we always make sure to have the lock held. This ensures that we don’t accidentally introduce a race condition into our code.
...
(please refer to the free, official pdf to get context.)
I couldn't comment with a small question in the link-2, so here is a full question.
Edit 1: I understand the lock is to control access to the ready variable. I am wondering why there's a race condition associated with the signaling. Specifically,
First, when signaling [...] we always make sure to have the lock held. This ensures that we don’t accidentally introduce a race condition into our code
Edit 2: I've seen resources and comments (from links commented below and during my own research), sometimes within the same page that say it doesn't matter or you must put it within a lock for Predictable BehaviorTM (would be nice if this can be touched upon too, if the behavior can be other than spurious wakeups). What must I follow?
Edit 3: I'm looking for more of a 'theoretical' answer, not implementation specific so that I can understand the core idea. I understand answers to these can be platform specific, but an answer that focuses on the core ideas of lock, mutex, condition variable as all implementations must follow these semantics, perhaps adding their own little quirks. Example, wait() can wake up spuriously, and given bad timing of signaling, can happen on 'pure' implementations too. Mentioning these would help.
My apologies for so many edits, but my dearth of in-depth knowledge in this field is confusing the heck outta me.
Any insight would be really helpful, thanks. Also, please feel free to point me to books where I can read these concepts in detail, and where I can learn C++ with these concepts too. Thanks.
Why does the book say that pthread_cond_signal must be done with a lock held to prevent data race? I wasn't sure, so I referred to this
question (and this question too), which basically said "no, it's not
required". Why would a race condition occur?
The book not presenting a complete example, my best guess as to the intended meaning is that there can be a data race with the CV itself if it is signaled without the associated mutex being held. That may be the case for some CV implementations, but the book is talking specifically about pthreads, and pthreads CVs are not subject to such a limitation. Neither is C++ std::condition_variable, which is what the two other SO questions you referred to are talking about. So in that sense, the book is just wrong.
It is true that one can compose examples of poor CV use, in conjunction with which signaling under protection of the associated mutex largely protects against data races, but signaling without such protection is susceptible to data races. But In such a case, the fault is not with the signaling itself, but with the waiting, and if that's what the book means then it is deceptively worded. And probably still wrong.
What and where is the race condition being described?
One can only guess what the author had in mind.
For the record, the proper usage of condition variables involves firstly determining what condition one wants to ensure holds before execution proceeds. That condition will necessarily involve shared variables, else there is no reason to expect that anything another thread does could change whether the condition is satisfied. That being the case, all access to the shared variables involved needs to be protected by a mutex if more than one thread is alive.
That mutex should then, secondly, also be the one associated with the CV, and threads must wait on the CV only while the mutex is held. This is a requirement of every CV implementation I know, and it protects against signals being missed and possible deadlock resulting from that. Consider this faulty, and somewhat contrived, example:
// BAD
int temp;
result = pthread_mutex_lock(m);
// handle failure results ...
temp = shared;
result = pthread_mutex_unlock(m);
// handle failure results ...
if (temp == 0) {
result = pthread_cond_wait(cv, m);
// handle failure results ...
}
// do something ...
Suppose that it was allowed to wait on the CV without holding the mutex, as that code does. That code supposes that at some point in the future, some other thread (T2) will update shared (under protection of the mutex) and then signal the CV to tell the waiting one (T1) that it can proceed. But what if T2 does that between when T1 unlocks the mutex and when it begins its wait? It doesn't matter whether T2 signals the CV under protection of the mutex or not -- T1 will begin a wait for a signal that has already been delivered. And CV signals do not queue.
So suppose that T1 only waits under protection of the mutex, as is in fact required. That's not enough. Consider this:
// ALSO BAD
result = pthread_mutex_lock(m);
// handle failure results ...
if (shared == 0) {
result = pthread_cond_wait(cv, m);
// handle failure results ...
}
result = pthread_mutex_unlock(m);
// handle failure results ...
// do something ...
This is still wrong, because it does not reliably prevent T1 from proceeding past the wait when the condition of interest is unsatisfied. Such a scenario can arise from
the signal being legitimately sent and received even though the particular condition of interest to T1 is not satisfied
the signal being legitimately sent and received, and the condition being satisfied when the signal is sent, but T2 or another thread modifying the shared variable again before T1 returns from its wait.
spurious return from the wait, which is very rare, but does occasionally happen in many real-world implementations.
None of that depends on T2 sending the signal without mutex protection.
The correct way to wait on a condition variable is to check the condition of interest before waiting, and afterward to loop back and check again before proceeding:
// OK
result = pthread_mutex_lock(m);
// handle failure results ...
while (shared == 0) { // <-- 'while', not 'if'
result = pthread_cond_wait(cv, m);
// handle failure results ...
}
// typically, shared = 0 at this point
result = pthread_mutex_unlock(m);
// handle failure results ...
// do something ...
It may sometimes be the case that thread T1 executing that code will return from its wait when the condition is not satisfied, but if ever it does then it will simply return to waiting instead of proceeding when it shouldn't. If other threads signal only under protection of the mutex then that should be rare, but still possible. If other threads signal without mutex protection then T1 may wake more often than strictly needed, but there is no data race involved, and no inherent risk of misbehavior.
Why does the book say that pthread_cond_signal must be done with a lock held to prevent data race? I wasn't sure, so I referred to this question (and this question too), which basically said "no, it's not required". Why would a race condition occur?
Yes, condition variable notification should generally be performed with the corresponding mutex locked. The reason is not so much to avoid a race condition but to avoid a missed or superfluous notification.
Consider the following piece of code:
std::queue< int > events;
std::mutex mutex;
std::condition_variable cond;
// Thread 1
void consume_events()
{
std::unique_lock< std::mutex > lock(mutex); // #1
while (true)
{
if (events.empty()) // #2
{
cond.wait(lock); // #3
continue;
}
// Process an event
events.pop();
}
}
// Thread 2
void produce_event(int event)
{
{
std::unique_lock< std::mutex > lock(mutex); // #4
events.push(event); // #5
} // #6
cond.notify_one(); // #7
}
This is a classical example of one producer/one consumer queue of data.
In the line #1 the consumer (Thread 1) locks the mutex. Then, in line #2, it tests if there are any events in the queue and, if there are none, in line #3 unlocks mutex and blocks. When the notification on the condition variable happens, the thread unblocks, immediately locks mutex and continues execution past line #3 (which is to go to line #2 again).
In the line #4 the producer (Thread 2) locks the mutex and in line #5 it enqueues a new event. Because the mutex is locked, event queue modification is safe (line #5 cannot be executed concurrently with line #2), so there is no data race. Then, in line #6, the mutex is unlocked and in line #7 the condition variable is notified.
It is possible that the following happens:
Thread 2 acquires the mutex in line #4.
Thread 1 attempts to acquire the mutex in line #1 or #3 (upon being unblocked by a previous notification). Since the mutex is locked by Thread 2, Thread 1 blocks.
Thread 2 enqueues the event in line #5 and unlocks the mutex in line #6.
Thread 1 unblocks and acquires the mutex. In line #2 it sees that the event queue is not empty and processes the event. On the next loop iteration the queue is empty and the thread blocks in line #3.
Thread 2 notifies Thread 1 in line #7. But there are no queued events, and Thread 1 wakes up in vain.
Though in this particular example, the extra wake up is benign, depending on the loop contents, it may be detrimental. The correct code should call notify_one before unlocking the mutex.
Another example is when one thread is used to initiate some work in the other thread without an explicit queue of events:
std::mutex mutex;
std::condition_variable cond;
// Thread 1
void process_work()
{
std::unique_lock< std::mutex > lock(mutex); // #1
while (true)
{
cond.wait(lock); // #2
// Do some processing // #3
}
}
// Thread 2
void initiate_work_processing()
{
cond.notify_one(); // #4
}
In this case Thread 1 waits until it is time to perform some activity (e.g. render a frame in a video game). Thread 2 periodically initiates that activity by notifying Thread 1 via condition variable.
The problem is that the condition variable does not buffer notifications and acts only on the threads that are actually blocked on it at the point of notification. If there are no threads blocked then the notification does nothing. This means that the following sequence of events is possible:
Thread 1 acquires the mutex in line #1 and blocks in line #2.
Thread 2 decides it is time to perform the periodic activity and notifies Thread 1 in line #4.
Thread 1 unblocks and goes to perform the activities (e.g. render a frame).
It turns out that this frame is a lot of work, and when Thread 2 comes to notify Thread 1 about the next frame in line #2, Thread 1 is still busy with the previous one. This notification gets missed.
Thread 1 is finally done with the frame and blocks in line #2. The user observes a frame dropped.
The above wouldn't have happened if Thread 2 locked mutex before notifying Thread 1 in line #4. If Thread 1 is still busy rendering a frame, Thread 2 would block until Thread 1 is done and only then issue the notification.
However, the correct solution for the above task is to introduce a flag or some other data protected by the mutex that Thread 2 can use to signal Thread 1 that it is time to perform its activities. Aside from fixing the missed notification problem, this also takes care of spurious wakeups.
What and where is the race condition being described?
Definition of a data race depends on the memory model used in the particular environment. This means primarily your programming language memory model and may include the underlying hardware memory model (if the programming language relies on the hardware memory model, which is the case with e.g. Assembler).
C++ defines data races as follows:
When an evaluation of an expression writes to a memory location and another evaluation reads or modifies the same memory location, the expressions are said to conflict. A program that has two conflicting evaluations has a data race unless
both evaluations execute on the same thread or in the same signal handler, or
both conflicting evaluations are atomic operations (see std::atomic), or
one of the conflicting evaluations happens-before another (see std::memory_order)
If a data race occurs, the behavior of the program is undefined.
So basically, when multiple threads access the same memory location concurrently (by means other than std::atomic) and at least one of the threads is modifying the data at that location, that is a data race.
For example, we have 5 pieces of data.(assume we have a lot of space, different version of data will not overlap each others.)
DATA0, DATA1, DATA2, DATA3, DATA4.
We have 3 threads(less than 5) working on those data.
Thread 1, working on DATA1 (version 0), has accessed some data from both DATA0(version 0) and DATA2(version 0), and create DATA1(version 1).
Thread 2, working on DATA3 (version 0), has accessed some data from both DATA2(version 0) and DATA4(version 0), and create DATA3(version 1).
Thread 3, working on DATA2 (version 0), has accessed some data from both DATA1(version 0) and DATA3(version 0), and create DATA2(version 1).
Now, if thread 1 finishes first. It has several choices, it can work on DATA0 (to create DATA0 version 1) since DATA1(version 0) and DATA4 (version 0) is available (Assume DATA0 & DATA4 are neighbors). It can also work on DATA 2 if it finds out that both DATA1(version1) and DATA3(version1) are available and create DATA2(version 2).
The requirement is the next version of data can be processed once it's neighbor data is ready(in 1 lower version).
At last, I want all threads to exit when all data arrive at version 10.
Question: How to implement this scheme using pthread library.
Note: I want to have data in different versions at the same time, so to create a barrier and make sure all data reach the same version is not an option.
Lets discuss the implementation. To have all versions (0~10) stored we would need 5*11*sizeof(data) space. Let us create two arrays of size 5 x 11. First array is DATA such that DATA[i][j] is the j th version of data i. Second array is an 'Access Matrix' - A, it denotes the state of an index, it could be:
Not started
In Progress
Completed
Algorithm: Each thread would search for an index [i][j] in the matrix such that, index [i-1][j-1] and [i+1][j-1] is 'Completed'. It would set A[i][j] to 'In Progress' while working on it. In case i=0, i-1 refers to n-1, if i=n-1, i+1 refers to 0. (like a circular queue). When all entries in the last column are 'Completed', the thread terminates. Otherwise it searches for a new data which is not completed.
Using pthread library to realize this:
Important variables: mutex, conditional variables.
pthread_mutex_t mutex= PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t condvar= PTHREAD_COND_INITIALIZER;
mutex is a 'lock'. We use it when we need to make an operation atomic. Atomic operation refers to an operation that needs to be done in 1 step without breaking execution. 'condvar' is a condition variable. Using it a thread can sleep until a condition is reached, when it is reached, the thread is woken up. This avoids busy waiting using a loop.
Here, our atomic operation is updating A. Reason: If the threads simultaneously update A, it may lead to race conditions such as more than 1 thread working on a Data in parallel.
To realize this, we search and set A inside the lock. Once A is set, we release the lock and work on the data. But if no available data was found which could be worked on, we wait on the conditional variable - condvar. When we call wait on condvar, we also pass mutex. While inside the lock, wait function releases the mutex lock and waits for the conditional variable to be signaled. Once it is signaled, it requires the lock and proceeds with execution. While waiting process is in sleeping state and hence does not waste CPU time.
Whenever any thread finishes working on a piece of data, it may prepare 1 or more other samples for being worked on. Hence after a thread finishes work, it signals all other threads to check for a 'workable' Data before continuing the algorithm. Pseudo code for this is as follows:
Read the comments and function names. They describe in detail the working of pthread library. While compilation with gcc add -lpthread flag and for further details of the library looking up the man pages of these functions is more than sufficient.
void thread(void)
{
//Note: If there are various threads in the line pthread_mutex_lock(&mutex)
// each will wait till the lock is released and acquired. Until then it will sleep.
pthread_mutex_lock(&mutex); //Do the searching inside the lock
while(lastColumnNotDone){ //This will avoid previously searched indices being updated
//Search for a workable index
if(found)
{ //As A has been updated and set to in progress, no need to hold lock. As we start work on the thread we release the lock so other process might use it.
pthread_mutex_unlock(&mutex); //Note:
//WORK ON DATA
pthread_mutex_lock(&mutex); //Restore lock to in order to continue thread's execution safely.
pthread_cond_broadcast(&condvar); //Sends a wake up signal to all threads which are waiting for the conditional variable 'condvar'.
}
else //No executable data found
pthread_cond_wait(&condvar,&mutex); //While waiting, we pass the address of mutex as second parameter to wait function. This releases the lock on mutex while this function is waiting and tries to reacquire it once condvar is signaled.
}
pthread_mutex_unlock(&mutex);
}
Search and checking if all data is completed in the while loop condition can be optimized but that is a different algorithms question. Key idea here is use of pthread library and thread concept.
A is a common access matrix. Do NOT update it outside of lock.
While checking anything with respect to A, such as finding a process or checking if all data is done, lock must be held. Otherwise A can be changed by a different thread at the same time a thread is reading it.
We acquire and release locks using the functions pthread_mutex_lock and pthread_mutex_unlock. Remember, these functions take pointers of the mutex and not it's value. It is a variable that needs to be accessed and updated.
Avoid holding the lock for long amounts of time. This will cause the threads to wait for a long time for small access needs.
When calling wait, be sure that lock is held. Wait unlocks the mutex held passed as the second parameter during the duration of it's wait. After receiving the signal to wake up it tries to acquire the lock once again.
I have the following problem to solve:
Consider an application where there are three types of threads: Calculus-A,Calculus-B and Finalization. Whenever a thread type Calculus-A ends, it calls the routine endA(), which returns immediately. Whenever a thread type Calculus-B ends, it calls the routine endB(), which returns immediately. Threads like Finalization routine call wait(),
which returns only if they have already completed two Calculation-A threads and 2 Calculation-B threads. In other words, for exactly 2 conclusions of Calculus-A and 2 conclusions of Calculus-B one thread Finalization is allowed to continue.
There is an undetermined number of threads of the 3 types. It is not known the order of the routines called by threads. Threads Completion are answered in the order of arrival.
Implement routines endA(), endB() and wait() using semaphores. Besides the variables initialization, the only possible operations are P and V. Solutions with busy-waiting are not acceptable.
Here's is my solution:
semaphore calcA = 2;
semaphore calcB = 2;
semaphore wait = -3;
void endA()
{
P(calcA);
V(wait);
}
void endB()
{
P(calcB);
V(wait);
}
void wait()
{
P(wait);
P(wait);
P(wait);
P(wait);
V(calcA);
V(calcA);
V(calcB);
V(calcB);
}
I believe that there will be a deadlock due to the wait's initialization and if and wait() executes before endA() and endB(). Is there any other solution for this?
I tend to view semaphore problems as problems where one must identify "sources of waiting" and define for each a semaphore and a protocol for their access.
With that in mind, the "sources of waiting" are
Completions of CalcA
Completions of CalcB
Maybe, if I understood this right, a wait on whole completion groups, consisting of two CalcAs and two CalcBs. I say maybe because I'm not sure what "Threads Completion are answered in the order of arrival." means.
Completions of CalcA and CalcB should therefore increment their respective counters. At the other end, one Finalization thread gains exclusive access to the counters and waits in any order for the needed number of completions to constitute a completion group. It then unlocks access to the next group.
My code is below, although since I'm unfamiliar with the Dutch V and P I will use take()/give().
semaphore calcA = 0;
semaphore calcB = 0;
semaphore groupSem = 1;
void endA(){
give(calcA);
}
void endB(){
give(calcB);
}
void wait(){
take(groupSem);
take(calcA);
take(calcA);
take(calcB);
take(calcB);
give(groupSem);
}
The groupSem semaphore ensures all-or-nothing: the thread that enters the critical section will get the next two completions of each of CalcA and CalcB. If groupSem wasn't there, the first thread to enter wait could take two As and block, then be taken over by another thread that grabs two As and two B and then run away.
A worse problem that exists if the groupSem isn't there is if this second thread takes two As, one B and then blocks, and then the first thread grabs the second B. If somehow the result of the finalization allows more runs of CalculationA and CalculationB, then you may have a deadlock, because there may be no more opportunity for instances of calculation A and B to complete, therefore leaving the finalization threads hanging, unable to produce more calculation instances.
I was reading a text on semaphores and their operations. The author emphasized that the wait() and post() operations of semaphores should be executed atomically, otherwise mutual exclusion of the threads may be violated. Can anybody, please explain to me what he means? I am new to multi-threading by the way
The operation of context switch, where a task / process is replaced by another by the kernel is asynchronous and undeterministic.
Let's examine the following code:
x++;
seems simple, hah ?
However this code is prone to synchronization errors, if x is shared among different tasks / process.
To understand that, you must understand the concept of atomic operation.
Atomic operation are instructions that the processor can execute on a single clock.
It usually involves reading a register, writing a register, etc.
back to the code example:
What actually happens behind the scenes (assembly) when incrementing a variable is
that the cpu reads the value of the variable into a register.
then, it increments it.
and then it saves it back to the original place it tool it from (memory).
As you can see, a simple operation like this involves 3 cpu steps.
context switch can occur between these 3 steps.
Let's take an example of two threads that needs to increment the same variable x.
Let's examine the pseudo assembly code of an imaginary (yet possible) scenario
read the value to register (thread 1)
increment the value (thread 1)
CONTEXT SWITCH
read the value to register (thread 2)
increment the value (thread 2)
save the value (thread 2)
CONTEXT SWITCH
save the value (thread 1)
if x was 3, it appears that it needs to be 5 now, but it will be 4.
Now, let's refer to your original question.
a semaphore / mutex is actually a variable.
and when a process wants to to take it it increments it.
Yes, if wait() and post() operations on semaphore are not executed atomically mutual execution of thread can be violated.
For example, consider a semaphore with value S = 1 and processes P1 and P2 try to to execute wait() simultaneously as below,
At time T0, T1 process P1, P2 finds the value of semaphore as S = 1 respectively followed by decrementing the semaphore to acquire the lock and enter the critical section simultaneously violating the mutual execution of threads.
To employ atomicity between wait() and post() spin-locking until the lock is obtained is advised.
I am programming using pthreads in C.
I have a parent thread which needs to create 4 child threads with id 0, 1, 2, 3.
When the parent thread gets data, it will set split the data and assign it to 4 seperate context variables - one for each sub-thread.
The sub-threads have to process this data and in the mean time the parent thread should wait on these threads.
Once these sub-threads have done executing, they will set the output in their corresponding context variables and wait(for reuse).
Once the parent thread knows that all these sub-threads have completed this round, it computes the global output and prints it out.
Now it waits for new data(the sub-threads are not killed yet, they are just waiting).
If the parent thread gets more data the above process is repeated - albeit with the already created 4 threads.
If the parent thread receives a kill command (assume a specific kind of data), it indicates to all the sub-threads and they terminate themselves. Now the parent thread can terminate.
I am a Masters research student and I am encountering the need for the above scenario. I know that this can be done using pthread_cond_wait, pthread_Cond_signal. I have written the code but it is just running indefinitely and I cannot figure out why.
My guess is that, the way I have coded it, I have over-complicated the scenario. It will be very helpful to know how this can be implemented. If there is a need, I can post a simplified version of my code to show what I am trying to do(even though I think that my approach is flawed!)...
Can you please give me any insights into how this scenario can be implemented using pthreads?
As far what can be seen from your description, there seems to be nothing wrong with the principle.
What you are trying to implement is a worker pool, I guess, there should be a lot of implementations out there. If the work that your threads are doing is a substantial computation (say at least a CPU second or so) such a scheme is a complete overkill. Mondern implementations of POSIX threads are efficient enough that they support the creation of a lot of threads, really a lot, and the overhead is not prohibitive.
The only thing that would be important if you have your workers communicate through shared variables, mutexes etc (and not via the return value of the thread) is that you start your threads detached, by using the attribute parameter to pthread_create.
Once you have such an implementation for your task, measure. Only then, if your profiler tells you that you spend a substantial amount of time in the pthread routines, start thinking of implementing (or using) a worker pool to recycle your threads.
One producer-consumer thread with 4 threads hanging off it. The thread that wants to queue the four tasks assembles the four context structs containing, as well as all the other data stuff, a function pointer to an 'OnComplete' func. Then it submits all four contexts to the queue, atomically incrementing a a taskCount up to 4 as it does so, and waits on an event/condvar/semaphore.
The four threads get a context from the P-C queue and work away.
When done, the threads call the 'OnComplete' function pointer.
In OnComplete, the threads atomically count down taskCount. If a thread decrements it to zero, is signals the the event/condvar/semaphore and the originating thread runs on, knowing that all the tasks are done.
It's not that difficult to arrange it so that the assembly of the contexts and the synchro waiting is done in a task as well, so allowing the pool to process multiple 'ForkAndWait' operations at once for multiple requesting threads.
I have to add that operations like this are a huge pile easier in an OO language. The latest Java, for example, has a 'ForkAndWait' threadpool class that should do exactly this kind of stuff, but C++, (or even C#, if you're into serfdom), is better than plain C.