C++: __sync_synchronize() still needed with std::atomic? - c++

Question

I've been running into an infrequent but re-occurring race condition.
The program has two threads and uses std::atomic. I'll simplify the critical parts of the code to look like:
std::atomic<uint64_t> b; // flag, initialized to 0
uint64_t data[100]; // shared data, initialized to 0
thread 1 (publishing):
// set various shared variables here, for example
data[5] = 10;
uint64_t a = b.exchange(1); // signal to thread 2 that data is ready
thread 2 (receiving):
if (b.load() != 0) { // signal that data is ready
// read various shared variables here, for example:
uint64_t x = data[5];
// race condition sometimes (x sometimes not consistent)
}
The odd thing is that when I add __sync_synchronize() to each thread, then the race condition goes away. I've seen this happen on two different servers.
i.e. when I change the code to look like the following, then the problem goes away:
thread 1 (publishing):
// set various shared variables here, for example
data[5] = 10;
__sync_synchronize();
uint64_t a = b.exchange(1); // signal to thread 2 that data is ready
thread 2 (receiving):
if (b.load() != 0) { // signal that data is ready
__sync_synchronize();
// read various shared variables here, for example:
uint64_t x = data[5];
}
Why is __sync_synchronize() necessary? It seems redundant as I thought both exchange and load ensured the correct sequential ordering of logic.
Architecture is x86_64 processors, linux, g++ 4.6.2

Answer 1

Whilst it is impossible to say from your simplified code what actually goes on in your actual application, the fact that __sync_synchronize helps, and the fact that this function is a memory barrier tells me that you are writing things in the one thread that the other thread is reading, in a way that isn't atomic.
An example:
thread_1:
object *p = new object;
p->x = 1;
b.exchange(p); /* give pointer p to other thread */
thread_2:
object *p = b.load();
if (p->x == 1) do_stuff();
else error("Huh?");
This may very well trigger the error-path in thread2, because the write to p->x has not actually been completed when thread 2 reads the new pointer value p.
Adding memory barrier, in this case, in the thread_1 code should fix this. Note that for THIS case, a memory barrier in thread_2 will not do anything - it may alter the timing and appear to fix the problem, but it won't be the right thing. You may need memory barriers on both sides still, if you are reading/writing memory that is shared between two threads.
I understand that this may not be precisely what your code is doing, but the concept is the same - __sync_synchronize ensures that memory reads and memory writes have completed for ALL of the instructions before that function call [which isn't a real function call, it will inline a single instruction that waits for any pending memory operations to comlete].
Noteworthy is that operations on std::atomic ONLY affect the actual data stored in the atomic object. Not reads/writes of other data.
Sometimes you also need a "compiler barrier" to avoid the compiler moving stuff from one side of an operation to another:
std::atomic<bool> flag(false);
value = 42;
flag.store(true);
....
another thread:
while(!flag.load());
print(value);
Now, there is a chance that the compiler generates the first form as:
flag.store(true);
value = 42;
Now, that wouldn't be good, would it? std::atomic is guaranteed to be a "compiler barrier", but in other cases, the compiler may well shuffle stuff around in a similar way.