Barriers and synchronization points with non-atomic variables - data race?
up vote
6
down vote
favorite
Consider the following program:
int i{0};
std::experimental::barrier b{2};
int main()
{
std::thread t0{ {
b.arrive_and_wait();
std::cout << i << 'n';
}};
std::thread t1{ {
i = 2;
b.arrive_and_wait();
}};
t0.join();
t1.join();
}
Is this program guaranteed to print out 2, even though i is not an atomic variable?
According to cppreference:
Calls to
arrive_and_waitsynchronizes with the start of the completion phase of the barrier. The completion of the completion phase synchronizes with the return from the call.
Calls to
arrive_and_dropandarrive_and_waitnever introduce data races with themselves or each other.
This suggests that there is a synchronization point on each arrive_and_wait call. However I am not sure if the compiler is allowed to reorder the read/write operations on i, since it is non-atomic.
c++ multithreading concurrency language-lawyer barrier
asked Nov 20 at 11:06
Vittorio Romeo
56.1k17149289
add a comment |
up vote
6
down vote
favorite
Consider the following program:
int i{0};
std::experimental::barrier b{2};
int main()
{
std::thread t0{ {
b.arrive_and_wait();
std::cout << i << 'n';
}};
std::thread t1{ {
i = 2;
b.arrive_and_wait();
}};
t0.join();
t1.join();
}
Is this program guaranteed to print out 2, even though i is not an atomic variable?
According to cppreference:
Calls to
arrive_and_waitsynchronizes with the start of the completion phase of the barrier. The completion of the completion phase synchronizes with the return from the call.
Calls to
arrive_and_dropandarrive_and_waitnever introduce data races with themselves or each other.
This suggests that there is a synchronization point on each arrive_and_wait call. However I am not sure if the compiler is allowed to reorder the read/write operations on i, since it is non-atomic.
c++ multithreading concurrency language-lawyer barrier
asked Nov 20 at 11:06
Vittorio Romeo
56.1k17149289
4
Bartop is right.. "Calls toarrive_and_wait()synchronize with the return from the call". Therein lies your answer.. If A synchronizes with B, statements sequenced before A (i = 2) inter-thead happen before statements that are sequenced after B (cout << i). Whether or not those statements are atomic is not relevant; It is the exact same mechanism that a mutex uses to synchronize non-atomic data between threads.
– LWimsey
Nov 21 at 7:55
add a comment |
up vote
6
down vote
favorite
up vote
6
down vote
favorite
Consider the following program:
int i{0};
std::experimental::barrier b{2};
int main()
{
std::thread t0{ {
b.arrive_and_wait();
std::cout << i << 'n';
}};
std::thread t1{ {
i = 2;
b.arrive_and_wait();
}};
t0.join();
t1.join();
}
Is this program guaranteed to print out 2, even though i is not an atomic variable?
According to cppreference:
Calls to
arrive_and_waitsynchronizes with the start of the completion phase of the barrier. The completion of the completion phase synchronizes with the return from the call.
Calls to
arrive_and_dropandarrive_and_waitnever introduce data races with themselves or each other.
This suggests that there is a synchronization point on each arrive_and_wait call. However I am not sure if the compiler is allowed to reorder the read/write operations on i, since it is non-atomic.
c++ multithreading concurrency language-lawyer barrier
asked Nov 20 at 11:06
Vittorio Romeo
56.1k17149289
Consider the following program:
int i{0};
std::experimental::barrier b{2};
int main()
{
std::thread t0{ {
b.arrive_and_wait();
std::cout << i << 'n';
}};
std::thread t1{ {
i = 2;
b.arrive_and_wait();
}};
t0.join();
t1.join();
}
Is this program guaranteed to print out 2, even though i is not an atomic variable?
According to cppreference:
Calls to
arrive_and_waitsynchronizes with the start of the completion phase of the barrier. The completion of the completion phase synchronizes with the return from the call.
Calls to
arrive_and_dropandarrive_and_waitnever introduce data races with themselves or each other.
This suggests that there is a synchronization point on each arrive_and_wait call. However I am not sure if the compiler is allowed to reorder the read/write operations on i, since it is non-atomic.
c++ multithreading concurrency language-lawyer barrier
c++ multithreading concurrency language-lawyer barrier
asked Nov 20 at 11:06
Vittorio Romeo
56.1k17149289
asked Nov 20 at 11:06
Vittorio Romeo
56.1k17149289
asked Nov 20 at 11:06
Vittorio Romeo
56.1k17149289
asked Nov 20 at 11:06
Vittorio Romeo
56.1k17149289
asked Nov 20 at 11:06
Vittorio Romeo
56.1k17149289
56.1k17149289
4
Bartop is right.. "Calls toarrive_and_wait()synchronize with the return from the call". Therein lies your answer.. If A synchronizes with B, statements sequenced before A (i = 2) inter-thead happen before statements that are sequenced after B (cout << i). Whether or not those statements are atomic is not relevant; It is the exact same mechanism that a mutex uses to synchronize non-atomic data between threads.
– LWimsey
Nov 21 at 7:55
add a comment |
4
Bartop is right.. "Calls toarrive_and_wait()synchronize with the return from the call". Therein lies your answer.. If A synchronizes with B, statements sequenced before A (i = 2) inter-thead happen before statements that are sequenced after B (cout << i). Whether or not those statements are atomic is not relevant; It is the exact same mechanism that a mutex uses to synchronize non-atomic data between threads.
– LWimsey
Nov 21 at 7:55
4
4
Bartop is right.. "Calls to
arrive_and_wait() synchronize with the return from the call". Therein lies your answer.. If A synchronizes with B, statements sequenced before A (i = 2) inter-thead happen before statements that are sequenced after B (cout << i). Whether or not those statements are atomic is not relevant; It is the exact same mechanism that a mutex uses to synchronize non-atomic data between threads.– LWimsey
Nov 21 at 7:55
Bartop is right.. "Calls to
arrive_and_wait() synchronize with the return from the call". Therein lies your answer.. If A synchronizes with B, statements sequenced before A (i = 2) inter-thead happen before statements that are sequenced after B (cout << i). Whether or not those statements are atomic is not relevant; It is the exact same mechanism that a mutex uses to synchronize non-atomic data between threads.– LWimsey
Nov 21 at 7:55
add a comment |
1 Answer
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From what I understand from std::barrier reference(my emphasis) :
A barrier has a completion phase, which is executed by one of the participating threads once all threads in the set of participating threads arrive at the synchronization point. The arrive_and_wait and arrive_and_drop calls synchronize with the start of the completion phase; the end of the completion phase synchronizes with the returns from all calls blocked by its completion.
you can assume that all changes made before barrier in different threads are visible in other threads, even if they are not not atomic. As this reference points out (my emphasis):
Between threads, evaluation A inter-thread happens before evaluation B if any of the following is true
1) A synchronizes-with B
2) A is dependency-ordered before B
3) A synchronizes-with some evaluation X, and X is sequenced-before B
4) A is sequenced-before some evaluation X, and X inter-thread happens-before B
5) A inter-thread happens-before some evaluation X, and X inter-thread happens-before B
answered Nov 20 at 11:31
bartop
2,526825
add a comment |
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active
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up vote
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down vote
From what I understand from std::barrier reference(my emphasis) :
A barrier has a completion phase, which is executed by one of the participating threads once all threads in the set of participating threads arrive at the synchronization point. The arrive_and_wait and arrive_and_drop calls synchronize with the start of the completion phase; the end of the completion phase synchronizes with the returns from all calls blocked by its completion.
you can assume that all changes made before barrier in different threads are visible in other threads, even if they are not not atomic. As this reference points out (my emphasis):
Between threads, evaluation A inter-thread happens before evaluation B if any of the following is true
1) A synchronizes-with B
2) A is dependency-ordered before B
3) A synchronizes-with some evaluation X, and X is sequenced-before B
4) A is sequenced-before some evaluation X, and X inter-thread happens-before B
5) A inter-thread happens-before some evaluation X, and X inter-thread happens-before B
answered Nov 20 at 11:31
bartop
2,526825
add a comment |
up vote
3
down vote
From what I understand from std::barrier reference(my emphasis) :
A barrier has a completion phase, which is executed by one of the participating threads once all threads in the set of participating threads arrive at the synchronization point. The arrive_and_wait and arrive_and_drop calls synchronize with the start of the completion phase; the end of the completion phase synchronizes with the returns from all calls blocked by its completion.
you can assume that all changes made before barrier in different threads are visible in other threads, even if they are not not atomic. As this reference points out (my emphasis):
Between threads, evaluation A inter-thread happens before evaluation B if any of the following is true
1) A synchronizes-with B
2) A is dependency-ordered before B
3) A synchronizes-with some evaluation X, and X is sequenced-before B
4) A is sequenced-before some evaluation X, and X inter-thread happens-before B
5) A inter-thread happens-before some evaluation X, and X inter-thread happens-before B
answered Nov 20 at 11:31
bartop
2,526825
add a comment |
up vote
3
down vote
up vote
3
down vote
From what I understand from std::barrier reference(my emphasis) :
A barrier has a completion phase, which is executed by one of the participating threads once all threads in the set of participating threads arrive at the synchronization point. The arrive_and_wait and arrive_and_drop calls synchronize with the start of the completion phase; the end of the completion phase synchronizes with the returns from all calls blocked by its completion.
you can assume that all changes made before barrier in different threads are visible in other threads, even if they are not not atomic. As this reference points out (my emphasis):
Between threads, evaluation A inter-thread happens before evaluation B if any of the following is true
1) A synchronizes-with B
2) A is dependency-ordered before B
3) A synchronizes-with some evaluation X, and X is sequenced-before B
4) A is sequenced-before some evaluation X, and X inter-thread happens-before B
5) A inter-thread happens-before some evaluation X, and X inter-thread happens-before B
answered Nov 20 at 11:31
bartop
2,526825
From what I understand from std::barrier reference(my emphasis) :
A barrier has a completion phase, which is executed by one of the participating threads once all threads in the set of participating threads arrive at the synchronization point. The arrive_and_wait and arrive_and_drop calls synchronize with the start of the completion phase; the end of the completion phase synchronizes with the returns from all calls blocked by its completion.
you can assume that all changes made before barrier in different threads are visible in other threads, even if they are not not atomic. As this reference points out (my emphasis):
Between threads, evaluation A inter-thread happens before evaluation B if any of the following is true
1) A synchronizes-with B
2) A is dependency-ordered before B
3) A synchronizes-with some evaluation X, and X is sequenced-before B
4) A is sequenced-before some evaluation X, and X inter-thread happens-before B
5) A inter-thread happens-before some evaluation X, and X inter-thread happens-before B
answered Nov 20 at 11:31
bartop
2,526825
answered Nov 20 at 11:31
bartop
2,526825
answered Nov 20 at 11:31
bartop
2,526825
answered Nov 20 at 11:31
bartop
2,526825
2,526825
add a comment |
add a comment |
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4
Bartop is right.. "Calls to
arrive_and_wait()synchronize with the return from the call". Therein lies your answer.. If A synchronizes with B, statements sequenced before A (i = 2) inter-thead happen before statements that are sequenced after B (cout << i). Whether or not those statements are atomic is not relevant; It is the exact same mechanism that a mutex uses to synchronize non-atomic data between threads.– LWimsey
Nov 21 at 7:55