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Changes imported from Abseil "staging" branch:

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- 28631b3dbc582cb88a637cc4c70886e38a4be0cf Refactoring to support production kernel Futex implementa... by Derek Mauro <dmauro@google.com>
  - 411e7bb779c32bbc02b5fa6f516087a02fcb0812 Update comments about leap smearing. by Abseil Team <absl-team@google.com>

GitOrigin-RevId: 28631b3dbc582cb88a637cc4c70886e38a4be0cf
Change-Id: I0506aa2705212cd466460cae60182b0c2c667972
This commit is contained in:
Abseil Team 2017-11-10 06:33:50 -08:00 • committed by katzdm
parent 778abb7c27
commit 9e94e488f5
4 changed files with 55 additions and 33 deletions
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73
absl/synchronization/internal/waiter.cc
View file

@ -39,10 +39,12 @@
#include <atomic>
#include <cassert>
#include <cstdint>
#include "absl/base/internal/malloc_extension.h"
#include "absl/base/internal/raw_logging.h"
#include "absl/base/internal/thread_identity.h"
#include "absl/base/optimization.h"
#include "absl/synchronization/internal/kernel_timeout.h"
namespace absl {
@ -82,6 +84,42 @@ static void MaybeBecomeIdle() {
#define FUTEX_BITSET_MATCH_ANY 0xFFFFFFFF
#endif
#endif
class Futex {
public:
static int WaitUntil(std::atomic<int32_t> *v, int32_t val,
KernelTimeout t) {
int err = 0;
if (t.has_timeout()) {
// https://locklessinc.com/articles/futex_cheat_sheet/
// Unlike FUTEX_WAIT, FUTEX_WAIT_BITSET uses absolute time.
struct timespec abs_timeout = t.MakeAbsTimespec();
// Atomically check that the futex value is still 0, and if it
// is, sleep until abs_timeout or until woken by FUTEX_WAKE.
err = syscall(
SYS_futex, reinterpret_cast<int32_t *>(v),
FUTEX_WAIT_BITSET | FUTEX_PRIVATE_FLAG | FUTEX_CLOCK_REALTIME, val,
&abs_timeout, nullptr, FUTEX_BITSET_MATCH_ANY);
} else {
// Atomically check that the futex value is still 0, and if it
// is, sleep until woken by FUTEX_WAKE.
err = syscall(SYS_futex, reinterpret_cast<int32_t *>(v),
FUTEX_WAIT | FUTEX_PRIVATE_FLAG, val, nullptr);
}
if (err != 0) {
err = -errno;
}
return err;
}
static int Wake(std::atomic<int32_t> *v, int32_t count) {
int err = syscall(SYS_futex, reinterpret_cast<int32_t *>(v),
FUTEX_WAKE | FUTEX_PRIVATE_FLAG, count);
if (ABSL_PREDICT_FALSE(err < 0)) {
err = -errno;
}
return err;
}
};
void Waiter::Init() {
futex_.store(0, std::memory_order_relaxed);
@ -91,7 +129,7 @@ bool Waiter::Wait(KernelTimeout t) {
// Loop until we can atomically decrement futex from a positive
// value, waiting on a futex while we believe it is zero.
while (true) {
int x = futex_.load(std::memory_order_relaxed);
int32_t x = futex_.load(std::memory_order_relaxed);
if (x != 0) {
if (!futex_.compare_exchange_weak(x, x - 1,
std::memory_order_acquire,
@ -101,30 +139,14 @@ bool Waiter::Wait(KernelTimeout t) {
return true; // Consumed a wakeup, we are done.
}
int err = 0;
if (t.has_timeout()) {
// https://locklessinc.com/articles/futex_cheat_sheet/
// Unlike FUTEX_WAIT, FUTEX_WAIT_BITSET uses absolute time.
struct timespec abs_timeout = t.MakeAbsTimespec();
// Atomically check that the futex value is still 0, and if it
// is, sleep until abs_timeout or until woken by FUTEX_WAKE.
err = syscall(
SYS_futex, reinterpret_cast<int *>(&futex_),
FUTEX_WAIT_BITSET | FUTEX_PRIVATE_FLAG | FUTEX_CLOCK_REALTIME, 0,
&abs_timeout, nullptr, FUTEX_BITSET_MATCH_ANY);
} else {
// Atomically check that the futex value is still 0, and if it
// is, sleep until woken by FUTEX_WAKE.
err = syscall(SYS_futex, reinterpret_cast<int *>(&futex_),
FUTEX_WAIT | FUTEX_PRIVATE_FLAG, 0, nullptr);
}
const int err = Futex::WaitUntil(&futex_, 0, t);
if (err != 0) {
if (errno == EINTR || errno == EWOULDBLOCK) {
if (err == -EINTR || err == -EWOULDBLOCK) {
// Do nothing, the loop will retry.
} else if (errno == ETIMEDOUT) {
return false; // Timeout.
} else if (err == -ETIMEDOUT) {
return false;
} else {
ABSL_RAW_LOG(FATAL, "Futex operation failed with errno %d\n", errno);
ABSL_RAW_LOG(FATAL, "Futex operation failed with error %d\n", err);
}
}
@ -141,10 +163,9 @@ void Waiter::Post() {
void Waiter::Poke() {
// Wake one thread waiting on the futex.
int err = syscall(SYS_futex, reinterpret_cast<int *>(&futex_),
FUTEX_WAKE | FUTEX_PRIVATE_FLAG, 1);
if (err < 0) {
ABSL_RAW_LOG(FATAL, "FUTEX_WAKE failed with errno %d\n", errno);
const int err = Futex::Wake(&futex_, 1);
if (ABSL_PREDICT_FALSE(err < 0)) {
ABSL_RAW_LOG(FATAL, "Futex operation failed with error %d\n", err);
}
}