Implementing Dekker's algorithm with Fences

Tuesday, 27 July 2010

Dekker's algorithm is one of the most basic algorithms for mutual exclusion, alongside Peterson's algorithm and Lamport's bakery algorithm. It has the nice property that it only requires load and store operations rather than exchange or test-and-set, but it still requires some level of ordering between the operations. On a weakly-ordered architecture such as PowerPC or SPARC, a correct implementation of Dekker's algorithm thus requires the use of fences or memory barriers in order to ensure correct operation.

The code

For those of you who just want the code: here it is — Dekker's algorithm in C++, with explicit fences.

std::atomic<bool> flag0(false),flag1(false);
std::atomic<int> turn(0);

void p0()
{
    flag0.store(true,std::memory_order_relaxed);
    std::atomic_thread_fence(std::memory_order_seq_cst);

    while (flag1.load(std::memory_order_relaxed))
    {
        if (turn.load(std::memory_order_relaxed) != 0)
        {
            flag0.store(false,std::memory_order_relaxed);
            while (turn.load(std::memory_order_relaxed) != 0)
            {
            }
            flag0.store(true,std::memory_order_relaxed);
            std::atomic_thread_fence(std::memory_order_seq_cst);
        }
    }
    std::atomic_thread_fence(std::memory_order_acquire);
 
    // critical section


    turn.store(1,std::memory_order_relaxed);
    std::atomic_thread_fence(std::memory_order_release);
    flag0.store(false,std::memory_order_relaxed);
}

void p1()
{
    flag1.store(true,std::memory_order_relaxed);
    std::atomic_thread_fence(std::memory_order_seq_cst);

    while (flag0.load(std::memory_order_relaxed))
    {
        if (turn.load(std::memory_order_relaxed) != 1)
        {
            flag1.store(false,std::memory_order_relaxed);
            while (turn.load(std::memory_order_relaxed) != 1)
            {
            }
            flag1.store(true,std::memory_order_relaxed);
            std::atomic_thread_fence(std::memory_order_seq_cst);
        }
    }
    std::atomic_thread_fence(std::memory_order_acquire);
 
    // critical section


    turn.store(0,std::memory_order_relaxed);
    std::atomic_thread_fence(std::memory_order_release);
    flag1.store(false,std::memory_order_relaxed);
}

The analysis

If you're like me then you'll be interested in why stuff works, rather than just taking the code. Here is my analysis of the required orderings, and how the fences guarantee those orderings.

Suppose thread 0 and thread 1 enter p0 and p1 respectively at the same time. They both set their respective flags to true, execute the fence and then read the other flag at the start of the while loop.

If both threads read false then both will enter the critical section, and the algorithm doesn't work. It is the job of the fences to ensure that this doesn't happen.

The fences are marked with memory_order_seq_cst, so either the fence in p0 is before the fence in p1 in the global ordering of memory_order_seq_cst operations, or vice-versa. Without loss of generality, we can assume that the fence in p0 comes before the fence in p1, since the code is symmetric. The store to flag0 is sequenced before the fence in p0, and the fence in p1 is sequenced before the read from flag0. Therefore the read from flag0 must see the value stored (true), so p1 will enter the while loop.

On the other side, there is no such guarantee for the read from flag1 in p0, so p0 may or may not enter the while loop. If p0 reads the value of false for flag1, it will not enter the while loop, and will instead enter the critical section, but that is OK since p1 has entered the while loop.

Though flag0 is not set to false until p0 has finished the critical section, we need to ensure that p1 does not see this until the values modified in the critical section are also visible to p1, and this is the purpose of the release fence prior to the store to flag0 and the acquire fence after the while loop. When p1 reads the value false from flag0 in order to exit the while loop, it must be reading the value store by p0 after the release fence at the end of the critical section. The acquire fence after the load guarantees that all the values written before the release fence prior to the store are visible, which is exactly what we need here.

If p0 reads true for flag1, it will enter the while loop rather than the critical section. Both threads are now looping, so we need a way to ensure that exactly one of them breaks out. This is the purpose of the turn variable. Initially, turn is 0, so p1 will enter the if and set flag1 to false, whilst p1 will not enter the if. Because p1 set flag1 to false, eventually p0 will read flag1 as false and exit the outer while loop to enter the critical section. On the other hand, p1 is now stuck in the inner while loop because turn is 0. When p0 exits the critical section it sets turn to 1. This will eventually be seen by p1, allowing it to exit the inner while loop. When the store to flag0 becomes visible p1 can then exit the outer while loop too.

If turn had been 1 initially (because p0 was the last thread to enter the critical section) then the inverse logic would apply, and p0 would enter the inner loop, allowing p1 to enter the critical section first.

Second time around

If p0 is called a second time whilst p1 is still in the inner loop then we have a similar situation to the start of the function — p1 may exit the inner loop and store true in flag1 whilst p0 stores true in flag0. We therefore need the second memory_order_seq_cst fence after the store to the flag in the inner loop. This guarantees that at least one of the threads will see the flag from the other thread set when it executes the check in the outer loop. Without this fence then both threads can read false, and both can enter the critical section.

Alternatives

You could put the ordering constraints on the loads and stores themselves rather than using fences. Indeed, the default memory ordering for atomic operations in C++ is memory_order_seq_cst, so the algorithm would "just work" with plain loads and stores to atomic variables. However, by using memory_order_relaxed on the loads and stores we can add fences to ensure we have exactly the ordering constraints required.

Posted by Anthony Williams
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Tags: concurrency, synchronization, Dekker, fences, cplusplus
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