Changes imported from Abseil "staging" branch:

  - 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

absl/synchronization/internal/kernel_timeout.h

@@ -39,6 +39,7 @@
 namespace absl {
 namespace synchronization_internal {
 
+class Futex;
 class Waiter;
 
 class KernelTimeout {
@@ -139,6 +140,7 @@ class KernelTimeout {
   }
 #endif
 
+  friend class Futex;
   friend class Waiter;
 };
 

absl/synchronization/internal/waiter.cc

@@ -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);
   }
 }
 

absl/synchronization/internal/waiter.h

@@ -29,6 +29,7 @@
 #endif
 
 #include <atomic>
+#include <cstdint>
 
 #include "absl/base/internal/thread_identity.h"
 #include "absl/synchronization/internal/kernel_timeout.h"
@@ -99,10 +100,10 @@ class Waiter {
 
  private:
 #if ABSL_WAITER_MODE == ABSL_WAITER_MODE_FUTEX
-  // Futexes are defined by specification to be ints.
-  // Thus std::atomic<int> must be just an int with lockfree methods.
-  std::atomic<int> futex_;
-  static_assert(sizeof(int) == sizeof(futex_), "Wrong size for futex");
+  // Futexes are defined by specification to be 32-bits.
+  // Thus std::atomic<int32_t> must be just an int32_t with lockfree methods.
+  std::atomic<int32_t> futex_;
+  static_assert(sizeof(int32_t) == sizeof(futex_), "Wrong size for futex");
 
 #elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_CONDVAR
   pthread_mutex_t mu_;

absl/time/time.h

@@ -506,9 +506,7 @@ std::string UnparseFlag(Duration d);
 //
 // `absl::Time` assumes there are 60 seconds in a minute, which means the
 // underlying time scales must be "smeared" to eliminate leap seconds.
-// POSIX, for example, legislates that a `time_t` value of `536457599` shall
-// correspond to "1986-12-31 23:59:59 +0000".
-//
+// See https://developers.google.com/time/smear.
 //
 // Even though `absl::Time` supports a wide range of timestamps, exercise
 // caution when using values in the distant past. `absl::Time` uses the