Changes imported from Abseil "staging" branch:

  - fbff677ef850865ea67ed6771a8ed348be181e8e Modify sysinfo.cc to support GetTID on Akaros. by Abseil Team <absl-team@google.com>
  - f1c2929e08a3d4181e08cb5014c4a569306fd922 Two functions that did not refer to their arguments unles... by Abseil Team <absl-team@google.com>
  - ee43cc3bfdb4d84d40eee31fb25ecdc1aa060f47 Support Akaros (https://akaros.org) in the ABSL spinlock_... by Abseil Team <absl-team@google.com>
  - 6869c8c5253126459d6c7f0aa708d8612c8e5963 Make sure vdso_base_ is constant-intialized. by Abseil Team <absl-team@google.com>
  - d54e0366efc8d44cd5da5fd157734da966dc45e8 Add missing include for assert used by ABSL_ASSERT. by Derek Mauro <dmauro@google.com>
  - a5139775f3917bb5201e7fc838135766daa05b8d When building against GLIBC-2.16 or newer, use getauxval(... by Abseil Team <absl-team@google.com>

GitOrigin-RevId: fbff677ef850865ea67ed6771a8ed348be181e8e
Change-Id: Ie3549f6ef054783dd104304d2faf8d9800c16b83

9 files changed, 119 insertions, 33 deletions

diff --git a/absl/base/BUILD.bazel b/absl/base/BUILD.bazel
index d68448daa5d2..e68c4500e561 100644
--- a/absl/base/BUILD.bazel
+++ b/absl/base/BUILD.bazel
@@ -28,6 +28,7 @@ licenses(["notice"])  # Apache 2.0
 cc_library(
     name = "spinlock_wait",
     srcs = [
+        "internal/spinlock_akaros.inc",
         "internal/spinlock_posix.inc",
         "internal/spinlock_wait.cc",
         "internal/spinlock_win32.inc",
diff --git a/absl/base/internal/spinlock_akaros.inc b/absl/base/internal/spinlock_akaros.inc
new file mode 100644
index 000000000000..051c8cf87fb6
--- /dev/null
+++ b/absl/base/internal/spinlock_akaros.inc
@@ -0,0 +1,35 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// This file is an Akaros-specific part of spinlock_wait.cc
+
+#include <atomic>
+
+#include "absl/base/internal/scheduling_mode.h"
+
+extern "C" {
+
+ABSL_ATTRIBUTE_WEAK void AbslInternalSpinLockDelay(
+    std::atomic<uint32_t>* /* lock_word */, uint32_t /* value */,
+    int /* loop */, absl::base_internal::SchedulingMode /* mode */) {
+  // In Akaros, one must take care not to call anything that could cause a
+  // malloc(), a blocking system call, or a uthread_yield() while holding a
+  // spinlock. Our callers assume will not call into libraries or other
+  // arbitrary code.
+}
+
+ABSL_ATTRIBUTE_WEAK void AbslInternalSpinLockWake(
+    std::atomic<uint32_t>* /* lock_word */, bool /* all */) {}
+
+}  // extern "C"
diff --git a/absl/base/internal/spinlock_wait.cc b/absl/base/internal/spinlock_wait.cc
index 0fd36286629d..8f951b66dc36 100644
--- a/absl/base/internal/spinlock_wait.cc
+++ b/absl/base/internal/spinlock_wait.cc
@@ -23,6 +23,8 @@
 
 #if defined(_WIN32)
 #include "absl/base/internal/spinlock_win32.inc"
+#elif defined(__akaros__)
+#include "absl/base/internal/spinlock_akaros.inc"
 #else
 #include "absl/base/internal/spinlock_posix.inc"
 #endif
diff --git a/absl/base/internal/sysinfo.cc b/absl/base/internal/sysinfo.cc
index 9e0140fadb1f..00e98b662cc6 100644
--- a/absl/base/internal/sysinfo.cc
+++ b/absl/base/internal/sysinfo.cc
@@ -284,6 +284,30 @@ pid_t GetTID() {
   return syscall(SYS_gettid);
 }
 
+#elif defined(__akaros__)
+
+pid_t GetTID() {
+  // Akaros has a concept of "vcore context", which is the state the program
+  // is forced into when we need to make a user-level scheduling decision, or
+  // run a signal handler.  This is analogous to the interrupt context that a
+  // CPU might enter if it encounters some kind of exception.
+  //
+  // There is no current thread context in vcore context, but we need to give
+  // a reasonable answer if asked for a thread ID (e.g., in a signal handler).
+  // Thread 0 always exists, so if we are in vcore context, we return that.
+  //
+  // Otherwise, we know (since we are using pthreads) that the uthread struct
+  // current_uthread is pointing to is the first element of a
+  // struct pthread_tcb, so we extract and return the thread ID from that.
+  //
+  // TODO(dcross): Akaros anticipates moving the thread ID to the uthread
+  // structure at some point. We should modify this code to remove the cast
+  // when that happens.
+  if (in_vcore_context())
+    return 0;
+  return reinterpret_cast<struct pthread_tcb *>(current_uthread)->id;
+}
+
 #else
 
 // Fallback implementation of GetTID using pthread_getspecific.
diff --git a/absl/base/macros.h b/absl/base/macros.h
index 31d1c02ecfa6..d41408727731 100644
--- a/absl/base/macros.h
+++ b/absl/base/macros.h
@@ -29,6 +29,7 @@
 #ifndef ABSL_BASE_MACROS_H_
 #define ABSL_BASE_MACROS_H_
 
+#include <cassert>
 #include <cstddef>
 
 #include "absl/base/port.h"
diff --git a/absl/debugging/internal/elf_mem_image.cc b/absl/debugging/internal/elf_mem_image.cc
index f6c6bc07d511..3dfef5e83e57 100644
--- a/absl/debugging/internal/elf_mem_image.cc
+++ b/absl/debugging/internal/elf_mem_image.cc
@@ -75,8 +75,9 @@ const T *GetTableElement(const ElfW(Ehdr) * ehdr, ElfW(Off) table_offset,
 
 }  // namespace
 
-const void *const ElfMemImage::kInvalidBase =
-    reinterpret_cast<const void *>(~0L);
+// The value of this variable doesn't matter; it's used only for its
+// unique address.
+const int ElfMemImage::kInvalidBaseSentinel = 0;
 
 ElfMemImage::ElfMemImage(const void *base) {
   ABSL_RAW_CHECK(base != kInvalidBase, "bad pointer");
diff --git a/absl/debugging/internal/elf_mem_image.h b/absl/debugging/internal/elf_mem_image.h
index 7f3dbb971bd6..20a32a490cb2 100644
--- a/absl/debugging/internal/elf_mem_image.h
+++ b/absl/debugging/internal/elf_mem_image.h
@@ -43,9 +43,14 @@ namespace debug_internal {
 
 // An in-memory ELF image (may not exist on disk).
 class ElfMemImage {
+ private:
+  // Sentinel: there could never be an elf image at &kInvalidBaseSentinel.
+  static const int kInvalidBaseSentinel;
+
  public:
   // Sentinel: there could never be an elf image at this address.
-  static const void *const kInvalidBase;
+  static constexpr const void *const kInvalidBase =
+    static_cast<const void*>(&kInvalidBaseSentinel);
 
   // Information about a single vdso symbol.
   // All pointers are into .dynsym, .dynstr, or .text of the VDSO.
diff --git a/absl/debugging/internal/stacktrace_x86-inl.inc b/absl/debugging/internal/stacktrace_x86-inl.inc
index 6e1af017878e..9bdaa542ce27 100644
--- a/absl/debugging/internal/stacktrace_x86-inl.inc
+++ b/absl/debugging/internal/stacktrace_x86-inl.inc
@@ -114,7 +114,9 @@ static const int kMaxFrameBytes = 100000;
 // vuc is a ucontext_t *.  We use void* to avoid the use
 // of ucontext_t on non-POSIX systems.
 static uintptr_t GetFP(const void *vuc) {
-#if defined(__linux__)
+#if !defined(__linux__)
+  static_cast<void>(vuc);  // Avoid an unused argument compiler warning.
+#else
   if (vuc != nullptr) {
     auto *uc = reinterpret_cast<const ucontext_t *>(vuc);
 #if defined(__i386__)
diff --git a/absl/debugging/internal/vdso_support.cc b/absl/debugging/internal/vdso_support.cc
index 5026e1c1a9c9..815e702f5699 100644
--- a/absl/debugging/internal/vdso_support.cc
+++ b/absl/debugging/internal/vdso_support.cc
@@ -20,10 +20,15 @@
 
 #ifdef ABSL_HAVE_VDSO_SUPPORT     // defined in vdso_support.h
 
+#include <errno.h>
 #include <fcntl.h>
 #include <sys/syscall.h>
 #include <unistd.h>
 
+#if __GLIBC_PREREQ(2, 16)  // GLIBC-2.16 implements getauxval.
+#include <sys/auxv.h>
+#endif
+
 #include "absl/base/dynamic_annotations.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/base/port.h"
@@ -35,8 +40,10 @@
 namespace absl {
 namespace debug_internal {
 
+ABSL_CONST_INIT
 std::atomic<const void *> VDSOSupport::vdso_base_(
     debug_internal::ElfMemImage::kInvalidBase);
+
 std::atomic<VDSOSupport::GetCpuFn> VDSOSupport::getcpu_fn_(&InitAndGetCPU);
 VDSOSupport::VDSOSupport()
     // If vdso_base_ is still set to kInvalidBase, we got here
@@ -56,37 +63,44 @@ VDSOSupport::VDSOSupport()
 // Finally, even if there is a race here, it is harmless, because
 // the operation should be idempotent.
 const void *VDSOSupport::Init() {
-  if (vdso_base_.load(std::memory_order_relaxed) ==
-      debug_internal::ElfMemImage::kInvalidBase) {
-    {
-      // Valgrind zaps AT_SYSINFO_EHDR and friends from the auxv[]
-      // on stack, and so glibc works as if VDSO was not present.
-      // But going directly to kernel via /proc/self/auxv below bypasses
-      // Valgrind zapping. So we check for Valgrind separately.
-      if (RunningOnValgrind()) {
-        vdso_base_.store(nullptr, std::memory_order_relaxed);
-        getcpu_fn_.store(&GetCPUViaSyscall, std::memory_order_relaxed);
-        return nullptr;
-      }
-      int fd = open("/proc/self/auxv", O_RDONLY);
-      if (fd == -1) {
-        // Kernel too old to have a VDSO.
-        vdso_base_.store(nullptr, std::memory_order_relaxed);
-        getcpu_fn_.store(&GetCPUViaSyscall, std::memory_order_relaxed);
-        return nullptr;
-      }
-      ElfW(auxv_t) aux;
-      while (read(fd, &aux, sizeof(aux)) == sizeof(aux)) {
-        if (aux.a_type == AT_SYSINFO_EHDR) {
-          vdso_base_.store(reinterpret_cast<void *>(aux.a_un.a_val),
-                           std::memory_order_relaxed);
-          break;
-        }
+  const auto kInvalidBase = debug_internal::ElfMemImage::kInvalidBase;
+#if __GLIBC_PREREQ(2, 16)
+  if (vdso_base_.load(std::memory_order_relaxed) == kInvalidBase) {
+    errno = 0;
+    const void *const sysinfo_ehdr =
+        reinterpret_cast<const void *>(getauxval(AT_SYSINFO_EHDR));
+    if (errno == 0) {
+      vdso_base_.store(sysinfo_ehdr, std::memory_order_relaxed);
+    }
+  }
+#endif  // __GLIBC_PREREQ(2, 16)
+  if (vdso_base_.load(std::memory_order_relaxed) == kInvalidBase) {
+    // Valgrind zaps AT_SYSINFO_EHDR and friends from the auxv[]
+    // on stack, and so glibc works as if VDSO was not present.
+    // But going directly to kernel via /proc/self/auxv below bypasses
+    // Valgrind zapping. So we check for Valgrind separately.
+    if (RunningOnValgrind()) {
+      vdso_base_.store(nullptr, std::memory_order_relaxed);
+      getcpu_fn_.store(&GetCPUViaSyscall, std::memory_order_relaxed);
+      return nullptr;
+    }
+    int fd = open("/proc/self/auxv", O_RDONLY);
+    if (fd == -1) {
+      // Kernel too old to have a VDSO.
+      vdso_base_.store(nullptr, std::memory_order_relaxed);
+      getcpu_fn_.store(&GetCPUViaSyscall, std::memory_order_relaxed);
+      return nullptr;
+    }
+    ElfW(auxv_t) aux;
+    while (read(fd, &aux, sizeof(aux)) == sizeof(aux)) {
+      if (aux.a_type == AT_SYSINFO_EHDR) {
+        vdso_base_.store(reinterpret_cast<void *>(aux.a_un.a_val),
+                         std::memory_order_relaxed);
+        break;
       }
-      close(fd);
     }
-    if (vdso_base_.load(std::memory_order_relaxed) ==
-        debug_internal::ElfMemImage::kInvalidBase) {
+    close(fd);
+    if (vdso_base_.load(std::memory_order_relaxed) == kInvalidBase) {
       // Didn't find AT_SYSINFO_EHDR in auxv[].
       vdso_base_.store(nullptr, std::memory_order_relaxed);
     }
@@ -135,6 +149,7 @@ long VDSOSupport::GetCPUViaSyscall(unsigned *cpu,  // NOLINT(runtime/int)
   return syscall(SYS_getcpu, cpu, nullptr, nullptr);
 #else
   // x86_64 never implemented sys_getcpu(), except as a VDSO call.
+  static_cast<void>(cpu);  // Avoid an unused argument compiler warning.
   errno = ENOSYS;
   return -1;
 #endif