std::atomic_thread_fence (3) Linux Manual Page
std::atomic_thread_fence – std::atomic_thread_fence
Synopsis
Defined in header <atomic>
extern "C" void atomic_thread_fence( std::memory_order order ) noexcept; (since C++11)
Establishes memory_synchronization_ordering of non-atomic and relaxed atomic accesses, as instructed by order, without an associated atomic operation.
Fence-atomic synchronization
A release fence F in thread A synchronizes-with atomic acquire_operation Y in thread B, if
* there exists an atomic store X (with any memory order)
* Y reads the value written by X (or the value would be written by release_sequence_headed_by_X if X were a release operation)
* F is sequenced-before X in thread A
In this case, all non-atomic and relaxed atomic stores that are sequenced-before F in thread A will happen-before all non-atomic and relaxed atomic loads from the same locations made in thread B after Y.
Atomic-fence synchronization
An atomic release_operation X in thread A synchronizes-with an acquire fence F in thread B, if
* there exists an atomic read Y (with any memory order)
* Y reads the value written by X (or by the release_sequence_headed_by_X)
* Y is sequenced-before F in thread B
In this case, all non-atomic and relaxed atomic stores that are sequenced-before X in thread A will happen-before all non-atomic and relaxed atomic loads from the same locations made in thread B after F.
Fence-fence synchronization
A release fence FA in thread A synchronizes-with an acquire fence FB in thread B, if
* There exists an atomic object M,
* There exists an atomic write X (with any memory order) that modifies M in thread A
* FA is sequenced-before X in thread A
* There exists an atomic read Y (with any memory order) in thread B
* Y reads the value written by X (or the value would be written by release_sequence_headed_by_X if X were a release operation)
* Y is sequenced-before FB in thread B
In this case, all non-atomic and relaxed atomic stores that are sequenced-before FA in thread A will happen-before all non-atomic and relaxed atomic loads from the same locations made in thread B after FB
Parameters
order – the memory ordering executed by this fence
Return value
(none)
Notes
atomic_thread_fence imposes stronger synchronization constraints than an atomic store operation with the same std::memory_order. While an atomic store-release operation prevents all preceding writes from moving past the store-release, an atomic_thread_fence with memory_order_release ordering prevents all preceding writes from moving past all subsequent stores.
Fence-fence synchronization can be used to add synchronization to a sequence of several relaxed atomic operations, for example
// Global
std::string computation(int);
void print(std::string);
std::atomic<int> arr[3] = {-1, -1, -1};
std::string data[1000] // non-atomic data
// Thread A, compute 3 values
void
ThreadA(int v0, int v1, int v2)
{
// assert( 0 <= v0, v1, v2 < 1000 );
data[v0] = computation(v0);
data[v1] = computation(v1);
data[v2] = computation(v2);
std::atomic_thread_fence(std::memory_order_release);
std::atomic_store_explicit(&arr[0], v0, std::memory_order_relaxed);
std::atomic_store_explicit(&arr[1], v1, std::memory_order_relaxed);
std::atomic_store_explicit(&arr[2], v2, std::memory_order_relaxed);
}
// Thread B, prints between 0 and 3 values already computed.
void ThreadB()
{
int v0 = std::atomic_load_explicit(&arr[0], std::memory_order_relaxed);
int v1 = std::atomic_load_explicit(&arr[1], std::memory_order_relaxed);
int v2 = std::atomic_load_explicit(&arr[2], std::memory_order_relaxed);
std::atomic_thread_fence(std::memory_order_acquire);
// v0, v1, v2 might turn out to be -1, some or all of them.
// otherwise it is safe to read the non-atomic data because of the fences:
if (v0 != -1) {
print(data[v0]);
}
if (v1 != -1) {
print(data[v1]);
}
if (v2 != -1) {
print(data[v2]);
}
}
Examples
Scan an array of mailboxes, and process only the ones intended for us, without unnecessary synchronization. This example uses atomic-fence synchronization.
// Run this code
const int num_mailboxes = 32;
std::atomic<int> mailbox_receiver[num_mailboxes];
std::string mailbox_data[num_mailboxes];
// The writer threads update non-atomic shared data
// and then update mailbox_receiver[i] as follows
mailbox_data[i] = ...;
std::atomic_store_explicit(&mailbox_receiver[i], receiver_id, std::memory_order_release);
// Reader thread needs to check all mailbox[i], but only needs to sync with one
for (int i = 0; i < num_mailboxes; ++i) {
if (std::atomic_load_explicit(&mailbox_receiver[i], std::memory_order_relaxed) == my_id) {
std::atomic_thread_fence(std::memory_order_acquire); // synchronize with just one writer
do_work(mailbox_data[i]); // guaranteed to observe everything done in the writer thread before
// the atomic_store_explicit()
}
}
See also
memory_order defines memory ordering constraints for the given atomic operation
(C++11)
atomic_signal_fence fence between a thread and a signal handler executed in the same thread
(C++11)