Cache getpid.

In practice, with this implementation we never need to make a system call.
We get the main thread's tid (which is the same as our pid) back from
the set_tid_address system call we have to make during initialization.
A new pthread will have the same pid as its parent, and a fork child's
main (and only) thread will have a pid equal to its tid, which we get for
free from the kernel before clone returns.

The only time we'd actually have to make a getpid system call now is if
we take a signal during fork and the signal handler calls getpid. (That,
or we call getpid in the dynamic linker while it's still dealing with its
own relocations and hasn't even set up the main thread yet.)

Bug: 15387103
Change-Id: I6d4718ed0a5c912fc75b5f738c49a023dbed5189

benchmarks/unistd_benchmark.cpp

@@ -30,6 +30,17 @@ static void BM_unistd_getpid(int iters) {
 }
 BENCHMARK(BM_unistd_getpid);
 
+static void BM_unistd_getpid_syscall(int iters) {
+  StartBenchmarkTiming();
+
+  for (int i = 0; i < iters; ++i) {
+    syscall(__NR_getpid);
+  }
+
+  StopBenchmarkTiming();
+}
+BENCHMARK(BM_unistd_getpid_syscall);
+
 // Stop GCC optimizing out our pure function.
 /* Must not be static! */ pid_t (*gettid_fp)() = gettid;
 

libc/Android.mk

@@ -127,6 +127,7 @@ libc_bionic_src_files := \
     bionic/getauxval.cpp \
     bionic/getcwd.cpp \
     bionic/getpgrp.cpp \
+    bionic/getpid.cpp \
     bionic/gettid.cpp \
     bionic/inotify_init.cpp \
     bionic/lchown.cpp \

libc/SYSCALLS.TXT

@@ -97,7 +97,7 @@ ssize_t     pread64|pread(int, void*, size_t, off_t) arm64,mips64,x86_64
 ssize_t     pwrite64(int, void*, size_t, off64_t) arm,mips,x86
 ssize_t     pwrite64|pwrite(int, void*, size_t, off_t) arm64,mips64,x86_64
 int         close(int)                      all
-pid_t       getpid()    all
+pid_t       __getpid:getpid()  all
 int         munmap(void*, size_t)  all
 void*       mremap(void*, size_t, size_t, unsigned long)  all
 int         msync(const void*, size_t, int)    all

libc/arch-arm/syscalls/__getpid.S

@@ -2,7 +2,7 @@
 
 #include <private/bionic_asm.h>
 
-ENTRY(getpid)
+ENTRY(__getpid)
     mov     ip, r7
     ldr     r7, =__NR_getpid
     swi     #0
@@ -11,4 +11,4 @@ ENTRY(getpid)
     bxls    lr
     neg     r0, r0
     b       __set_errno
-END(getpid)
+END(__getpid)

libc/arch-arm64/syscalls/__getpid.S

@@ -2,7 +2,7 @@
 
 #include <private/bionic_asm.h>
 
-ENTRY(getpid)
+ENTRY(__getpid)
     mov     x8, __NR_getpid
     svc     #0
 
@@ -11,4 +11,5 @@ ENTRY(getpid)
     b.hi    __set_errno
 
     ret
-END(getpid)
+END(__getpid)
+.hidden __getpid

libc/arch-mips/syscalls/__getpid.S

@@ -2,7 +2,7 @@
 
 #include <private/bionic_asm.h>
 
-ENTRY(getpid)
+ENTRY(__getpid)
     .set noreorder
     .cpload t9
     li v0, __NR_getpid
@@ -16,4 +16,4 @@ ENTRY(getpid)
     j t9
     nop
     .set reorder
-END(getpid)
+END(__getpid)

libc/arch-mips64/syscalls/__getpid.S

@@ -2,7 +2,7 @@
 
 #include <private/bionic_asm.h>
 
-ENTRY(getpid)
+ENTRY(__getpid)
     .set push
     .set noreorder
     li v0, __NR_getpid
@@ -22,4 +22,5 @@ ENTRY(getpid)
     j t9
     move ra, t0
     .set pop
-END(getpid)
+END(__getpid)
+.hidden __getpid

libc/arch-x86/syscalls/__getpid.S

@@ -2,7 +2,7 @@
 
 #include <private/bionic_asm.h>
 
-ENTRY(getpid)
+ENTRY(__getpid)
     movl    $__NR_getpid, %eax
     int     $0x80
     cmpl    $-MAX_ERRNO, %eax
@@ -13,4 +13,4 @@ ENTRY(getpid)
     addl    $4, %esp
 1:
     ret
-END(getpid)
+END(__getpid)

libc/arch-x86_64/syscalls/__getpid.S

@@ -2,7 +2,7 @@
 
 #include <private/bionic_asm.h>
 
-ENTRY(getpid)
+ENTRY(__getpid)
     movl    $__NR_getpid, %eax
     syscall
     cmpq    $-MAX_ERRNO, %rax
@@ -12,4 +12,5 @@ ENTRY(getpid)
     call    __set_errno
 1:
     ret
-END(getpid)
+END(__getpid)
+.hidden __getpid

libc/bionic/fork.cpp

@@ -31,19 +31,26 @@
 
 #include "pthread_internal.h"
 
+#define FORK_FLAGS (CLONE_CHILD_SETTID | CLONE_CHILD_CLEARTID | SIGCHLD)
+
 int fork() {
   __bionic_atfork_run_prepare();
 
   pthread_internal_t* self = __get_thread();
-  int flags = CLONE_CHILD_SETTID | CLONE_CHILD_CLEARTID | SIGCHLD;
+
+  // Remember the parent pid and invalidate the cached value while we fork.
+  pid_t parent_pid = self->invalidate_cached_pid();
+
 #if defined(__x86_64__) // sys_clone's last two arguments are flipped on x86-64.
-  int result = syscall(__NR_clone, flags, NULL, NULL, &(self->tid), NULL);
+  int result = syscall(__NR_clone, FORK_FLAGS, NULL, NULL, &(self->tid), NULL);
 #else
-  int result = syscall(__NR_clone, flags, NULL, NULL, NULL, &(self->tid));
+  int result = syscall(__NR_clone, FORK_FLAGS, NULL, NULL, NULL, &(self->tid));
 #endif
   if (result == 0) {
+    self->set_cached_pid(gettid());
     __bionic_atfork_run_child();
   } else {
+    self->set_cached_pid(parent_pid);
     __bionic_atfork_run_parent();
   }
   return result;

libc/bionic/getpid.cpp

@@ -0,0 +1,47 @@
+/*
+ * Copyright (C) 2014 The Android Open Source Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *  * Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *  * Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <unistd.h>
+
+#include "pthread_internal.h"
+
+extern "C" pid_t __getpid();
+
+pid_t getpid() {
+  pthread_internal_t* self = __get_thread();
+
+  // Do we have a valid cached pid?
+  pid_t cached_pid;
+  if (__predict_true(self->get_cached_pid(&cached_pid))) {
+    return cached_pid;
+  }
+
+  // We're still in the dynamic linker or we're in the middle of forking, so ask the kernel.
+  // We don't know whether it's safe to update the cached value, so don't try.
+  return __getpid();
+}

libc/bionic/libc_init_common.cpp

@@ -86,21 +86,24 @@ static size_t get_main_thread_stack_size() {
 void __libc_init_tls(KernelArgumentBlock& args) {
   __libc_auxv = args.auxv;
 
-  uintptr_t stack_top = (__get_sp() & ~(PAGE_SIZE - 1)) + PAGE_SIZE;
-  size_t stack_size = get_main_thread_stack_size();
-  uintptr_t stack_bottom = stack_top - stack_size;
-
   static void* tls[BIONIC_TLS_SLOTS];
   static pthread_internal_t main_thread;
   main_thread.tls = tls;
 
   // Tell the kernel to clear our tid field when we exit, so we're like any other pthread.
+  // As a side-effect, this tells us our pid (which is the same as the main thread's tid).
   main_thread.tid = __set_tid_address(&main_thread.tid);
+  main_thread.set_cached_pid(main_thread.tid);
 
-  // We already have a stack, and we don't want to free it up on exit (because things like
-  // environment variables with global scope live on it).
+  // Work out the extent of the main thread's stack.
+  uintptr_t stack_top = (__get_sp() & ~(PAGE_SIZE - 1)) + PAGE_SIZE;
+  size_t stack_size = get_main_thread_stack_size();
+  void* stack_bottom = reinterpret_cast<void*>(stack_top - stack_size);
+
+  // We don't want to free the main thread's stack even when the main thread exits
+  // because things like environment variables with global scope live on it.
   pthread_attr_init(&main_thread.attr);
-  pthread_attr_setstack(&main_thread.attr, (void*) stack_bottom, stack_size);
+  pthread_attr_setstack(&main_thread.attr, stack_bottom, stack_size);
   main_thread.attr.flags = PTHREAD_ATTR_FLAG_USER_ALLOCATED_STACK | PTHREAD_ATTR_FLAG_MAIN_THREAD;
 
   __init_thread(&main_thread, false);

libc/bionic/pthread_create.cpp

@@ -30,6 +30,7 @@
 
 #include <errno.h>
 #include <sys/mman.h>
+#include <unistd.h>
 
 #include "pthread_internal.h"
 
@@ -220,6 +221,8 @@ int pthread_create(pthread_t* thread_out, pthread_attr_t const* attr,
   thread->start_routine = start_routine;
   thread->start_routine_arg = arg;
 
+  thread->set_cached_pid(getpid());
+
   int flags = CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGHAND | CLONE_THREAD | CLONE_SYSVSEM |
       CLONE_SETTLS | CLONE_PARENT_SETTID | CLONE_CHILD_CLEARTID;
   void* tls = thread->tls;

libc/bionic/pthread_internal.h

@@ -36,6 +36,26 @@ struct pthread_internal_t {
 
   pid_t tid;
 
+ private:
+  pid_t cached_pid_;
+
+ public:
+  pid_t invalidate_cached_pid() {
+    pid_t old_value;
+    get_cached_pid(&old_value);
+    set_cached_pid(0);
+    return old_value;
+  }
+
+  void set_cached_pid(pid_t value) {
+    cached_pid_ = value;
+  }
+
+  bool get_cached_pid(pid_t* cached_pid) {
+    *cached_pid = cached_pid_;
+    return (*cached_pid != 0);
+  }
+
   void** tls;
 
   pthread_attr_t attr;

tests/unistd_test.cpp

@@ -22,6 +22,7 @@
 #include <fcntl.h>
 #include <stdint.h>
 #include <unistd.h>
+#include <sys/syscall.h>
 #include <sys/types.h>
 #include <sys/wait.h>
 
@@ -379,3 +380,47 @@ TEST(unistd, fdatasync) {
 TEST(unistd, fsync) {
   TestFsyncFunction(fsync);
 }
+
+TEST(unistd, getpid_caching_and_fork) {
+  pid_t parent_pid = getpid();
+  ASSERT_EQ(syscall(__NR_getpid), parent_pid);
+
+  pid_t fork_result = fork();
+  ASSERT_NE(fork_result, -1);
+  if (fork_result == 0) {
+    // We're the child.
+    ASSERT_EQ(syscall(__NR_getpid), getpid());
+    ASSERT_EQ(parent_pid, getppid());
+    _exit(123);
+  } else {
+    // We're the parent.
+    ASSERT_EQ(parent_pid, getpid());
+
+    int status;
+    ASSERT_EQ(fork_result, waitpid(fork_result, &status, 0));
+    ASSERT_TRUE(WIFEXITED(status));
+    ASSERT_EQ(123, WEXITSTATUS(status));
+  }
+}
+
+static void GetPidCachingHelperHelper() {
+  ASSERT_EQ(syscall(__NR_getpid), getpid());
+}
+
+static void* GetPidCachingHelper(void*) {
+  GetPidCachingHelperHelper(); // Can't assert in a non-void function.
+  return NULL;
+}
+
+TEST(unistd, getpid_caching_and_pthread_create) {
+  pid_t parent_pid = getpid();
+
+  pthread_t t;
+  ASSERT_EQ(0, pthread_create(&t, NULL, GetPidCachingHelper, NULL));
+
+  ASSERT_EQ(parent_pid, getpid());
+
+  void* result;
+  ASSERT_EQ(0, pthread_join(t, &result));
+  ASSERT_EQ(NULL, result);
+}