[libc] implement cached process/thread identity

Author: Yifan Zhu

libc/config/linux/aarch64/entrypoints.txt

@@ -290,6 +290,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.unistd.geteuid
     libc.src.unistd.getpid
     libc.src.unistd.getppid
+    libc.src.unistd.gettid
     libc.src.unistd.getuid
     libc.src.unistd.isatty
     libc.src.unistd.link

libc/config/linux/riscv/entrypoints.txt

@@ -295,6 +295,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.unistd.geteuid
     libc.src.unistd.getpid
     libc.src.unistd.getppid
+    libc.src.unistd.gettid
     libc.src.unistd.getuid
     libc.src.unistd.isatty
     libc.src.unistd.link

libc/config/linux/x86_64/entrypoints.txt

@@ -308,6 +308,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.unistd.geteuid
     libc.src.unistd.getpid
     libc.src.unistd.getppid
+    libc.src.unistd.gettid
     libc.src.unistd.getuid
     libc.src.unistd.isatty
     libc.src.unistd.link

libc/docs/dev/undefined_behavior.rst

@@ -89,3 +89,24 @@ The C23 standard states that if the value of the ``rnd`` argument of the
 the value of a math rounding direction macro, the direction of rounding is
 unspecified. LLVM's libc chooses to use the ``FP_INT_TONEAREST`` rounding
 direction in this case.
+
+Cached ``getpid/gettid``
+------------------------
+Since version ``2.25``, glibc removes its cache mechanism for ``getpid/gettid`` 
+(See the history section in https://man7.org/linux/man-pages/man2/getpid.2.html).
+LLVM's libc still implements the cache as it is useful for fast deadlock detection.
+The cache mechanism is also implemented in MUSL and bionic.
+
+Unwrapped ``SYS_clone/SYS_fork/SYS_vfork``
+------------------------------------------
+It is highly discouraged to use unwrapped ``SYS_clone/SYS_fork/SYS_vfork``. 
+First, calling such syscalls without provided libc wrappers ignores 
+all the ``pthread_atfork`` entries as libc can no longer detect the ``fork``. 
+Second, libc relies on the ``fork/clone`` wrappers to correctly maintain cache for
+process id and thread id, and other important process-specific states such as the list 
+of robust mutexes. Third, even if the user is to call ``exec*`` functions immediately, 
+there can still be other unexpected issues. For instance, there can be signal handlers 
+inherited from parent process triggered inside the instruction window between ``fork`` 
+and ``exec*``. As libc failed to maintain its internal states correctly, even though the
+functions used inside the signal handlers are marked as ``async-signal-safe`` (such as
+``getpid``), they will still return wrong values or lead to other even worse situations.

libc/spec/posix.td

@@ -512,6 +512,11 @@ def POSIX : StandardSpec<"POSIX"> {
           RetValSpec<PidT>,
           [ArgSpec<VoidType>]
         >,
+        FunctionSpec<
+          "gettid",
+          RetValSpec<PidT>,
+          [ArgSpec<VoidType>]
+        >,
         FunctionSpec<
           "getuid",
           RetValSpec<UidT>,
@@ -567,16 +572,6 @@ def POSIX : StandardSpec<"POSIX"> {
           RetValSpec<IntType>,
           [ArgSpec<ConstCharPtr>]
         >,
-        FunctionSpec<
-          "getpid",
-          RetValSpec<IntType>,
-          [ArgSpec<VoidType>]
-        >,
-        FunctionSpec<
-          "getppid",
-          RetValSpec<IntType>,
-          [ArgSpec<VoidType>]
-        >,
         FunctionSpec<
           "link",
           RetValSpec<IntType>,

libc/src/__support/OSUtil/CMakeLists.txt

@@ -15,3 +15,12 @@ add_object_library(
   DEPENDS
     ${target_os_util}
 )
+
+if(TARGET libc.src.__support.OSUtil.${LIBC_TARGET_OS}.pid)
+  add_object_library(
+    pid
+    ALIAS
+    DEPENDS
+      .${LIBC_TARGET_OS}.pid
+  )
+endif()

libc/src/__support/OSUtil/linux/CMakeLists.txt

@@ -22,3 +22,16 @@ add_object_library(
     libc.hdr.types.struct_flock64
     libc.hdr.types.struct_f_owner_ex
 )
+
+add_object_library(
+  pid
+  SRCS
+    pid.cpp
+  HDRS
+    ../pid.h
+  DEPENDS
+    libc.src.__support.OSUtil.osutil
+    libc.src.__support.common
+    libc.hdr.types.pid_t
+    libc.include.sys_syscall
+)

libc/src/__support/OSUtil/linux/pid.cpp

@@ -0,0 +1,20 @@
+//===------------ pid_t utilities implementation ----------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/__support/OSUtil/pid.h"
+#include "src/__support/OSUtil/syscall.h"
+#include <sys/syscall.h>
+
+namespace LIBC_NAMESPACE {
+
+pid_t ProcessIdentity::cache = -1;
+pid_t ProcessIdentity::get_uncached() {
+  return syscall_impl<pid_t>(SYS_getpid);
+}
+
+} // namespace LIBC_NAMESPACE

libc/src/__support/OSUtil/pid.h

@@ -0,0 +1,32 @@
+//===------------ pid_t utilities -------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC___SUPPORT_OSUTIL_PID_H
+#define LLVM_LIBC_SRC___SUPPORT_OSUTIL_PID_H
+#include "hdr/types/pid_t.h"
+#include "src/__support/macros/attributes.h"
+#include "src/__support/macros/optimization.h"
+namespace LIBC_NAMESPACE {
+
+class ProcessIdentity {
+  static pid_t cache;
+  static pid_t get_uncached();
+
+public:
+  LIBC_INLINE static void invalidate_cache() { cache = -1; }
+  LIBC_INLINE static void refresh_cache() { cache = get_uncached(); }
+  LIBC_INLINE static pid_t get() {
+    if (LIBC_UNLIKELY(cache < 0))
+      return get_uncached();
+    return cache;
+  }
+};
+
+} // namespace LIBC_NAMESPACE
+
+#endif // LLVM_LIBC_SRC___SUPPORT_OSUTIL_PID_H

libc/src/__support/threads/CMakeLists.txt

@@ -45,6 +45,7 @@ add_header_library(
     libc.src.__support.CPP.optional
     libc.src.__support.CPP.string_view
     libc.src.__support.CPP.stringstream
+    libc.hdr.types.pid_t
 )
 
 if(TARGET libc.src.__support.threads.${LIBC_TARGET_OS}.thread)
@@ -80,3 +81,21 @@ if(TARGET libc.src.__support.threads.${LIBC_TARGET_OS}.CndVar)
     .${LIBC_TARGET_OS}.CndVar
   )
 endif()
+
+set(tid_dep)
+if (LLVM_LIBC_FULL_BUILD)
+  list(APPEND tid_dep libc.src.__support.thread)
+else()
+  list(APPEND tid_dep libc.src.__support.OSUtil.osutil)
+  list(APPEND tid_dep libc.include.sys_syscall)
+endif()
+
+add_header_library(
+  tid
+  HDRS
+    tid.h
+  DEPENDS
+    libc.src.__support.common
+    libc.hdr.types.pid_t
+    ${tid_dep}
+)

libc/src/__support/threads/linux/CMakeLists.txt

@@ -55,6 +55,7 @@ add_header_library(
     libc.src.__support.common
     libc.src.__support.OSUtil.osutil
     libc.src.__support.CPP.limits
+    libc.src.__support.threads.tid
   COMPILE_OPTIONS
     -DLIBC_COPT_RWLOCK_DEFAULT_SPIN_COUNT=${LIBC_CONF_RWLOCK_DEFAULT_SPIN_COUNT}
     ${monotonicity_flags}

libc/src/__support/threads/linux/rwlock.h

@@ -22,6 +22,7 @@
 #include "src/__support/threads/linux/futex_word.h"
 #include "src/__support/threads/linux/raw_mutex.h"
 #include "src/__support/threads/sleep.h"
+#include "src/__support/threads/tid.h"
 
 #ifndef LIBC_COPT_RWLOCK_DEFAULT_SPIN_COUNT
 #define LIBC_COPT_RWLOCK_DEFAULT_SPIN_COUNT 100
@@ -335,8 +336,6 @@ class RwLock {
   LIBC_INLINE Role get_preference() const {
     return static_cast<Role>(preference);
   }
-  // TODO: use cached thread id once implemented.
-  LIBC_INLINE static pid_t gettid() { return syscall_impl<pid_t>(SYS_gettid); }
 
   template <Role role> LIBC_INLINE LockResult try_lock(RwState &old) {
     if constexpr (role == Role::Reader) {
@@ -358,7 +357,7 @@ class RwLock {
         if (LIBC_LIKELY(old.compare_exchange_weak_with(
                 state, old.set_writer_bit(), cpp::MemoryOrder::ACQUIRE,
                 cpp::MemoryOrder::RELAXED))) {
-          writer_tid.store(gettid(), cpp::MemoryOrder::RELAXED);
+          writer_tid.store(gettid_inline(), cpp::MemoryOrder::RELAXED);
           return LockResult::Success;
         }
         // Notice that old is updated by the compare_exchange_weak_with
@@ -393,7 +392,7 @@ class RwLock {
             unsigned spin_count = LIBC_COPT_RWLOCK_DEFAULT_SPIN_COUNT) {
     // Phase 1: deadlock detection.
     // A deadlock happens if this is a RAW/WAW lock in the same thread.
-    if (writer_tid.load(cpp::MemoryOrder::RELAXED) == gettid())
+    if (writer_tid.load(cpp::MemoryOrder::RELAXED) == gettid_inline())
       return LockResult::Deadlock;
 
 #if LIBC_COPT_TIMEOUT_ENSURE_MONOTONICITY
@@ -519,7 +518,7 @@ class RwLock {
     if (old.has_active_writer()) {
       // The lock is held by a writer.
       // Check if we are the owner of the lock.
-      if (writer_tid.load(cpp::MemoryOrder::RELAXED) != gettid())
+      if (writer_tid.load(cpp::MemoryOrder::RELAXED) != gettid_inline())
         return LockResult::PermissionDenied;
       // clear writer tid.
       writer_tid.store(0, cpp::MemoryOrder::RELAXED);

libc/src/__support/threads/linux/thread.cpp

@@ -517,4 +517,6 @@ void thread_exit(ThreadReturnValue retval, ThreadStyle style) {
   __builtin_unreachable();
 }
 
+pid_t Thread::get_uncached_tid() { return syscall_impl<pid_t>(SYS_gettid); }
+
 } // namespace LIBC_NAMESPACE

libc/src/__support/threads/thread.h

@@ -9,6 +9,7 @@
 #ifndef LLVM_LIBC_SRC___SUPPORT_THREADS_THREAD_H
 #define LLVM_LIBC_SRC___SUPPORT_THREADS_THREAD_H
 
+#include "hdr/types/pid_t.h"
 #include "src/__support/CPP/atomic.h"
 #include "src/__support/CPP/optional.h"
 #include "src/__support/CPP/string_view.h"
@@ -102,7 +103,7 @@ struct alignas(STACK_ALIGNMENT) ThreadAttributes {
   uintptr_t tls;                // Address to the thread TLS memory
   uintptr_t tls_size;           // The size of area pointed to by |tls|.
   unsigned char owned_stack; // Indicates if the thread owns this stack memory
-  int tid;
+  pid_t tid;
   ThreadStyle style;
   ThreadReturnValue retval;
   ThreadAtExitCallbackMgr *atexit_callback_mgr;
@@ -227,6 +228,16 @@ struct Thread {
 
   // Return the name of the thread in |name|. Return the error number of error.
   int get_name(cpp::StringStream &name) const;
+
+  static pid_t get_uncached_tid();
+
+  LIBC_INLINE void refresh_tid() { this->attrib->tid = get_uncached_tid(); }
+  LIBC_INLINE void invalidate_tid() { this->attrib->tid = -1; }
+  LIBC_INLINE pid_t get_tid() {
+    if (LIBC_UNLIKELY(this->attrib->tid < 0))
+      return get_uncached_tid();
+    return this->attrib->tid;
+  }
 };
 
 extern LIBC_THREAD_LOCAL Thread self;

libc/src/__support/threads/tid.h

@@ -0,0 +1,34 @@
+//===--- Tid wrapper --------------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC___SUPPORT_THREADS_TID_H
+#define LLVM_LIBC_SRC___SUPPORT_THREADS_TID_H
+
+// This header is for internal usage which automatically dispatches full build
+// and overlay build behaviors.
+
+#include "hdr/types/pid_t.h"
+#include "src/__support/common.h"
+#ifdef LIBC_FULL_BUILD
+#include "src/__support/threads/thread.h"
+#else
+#include "src/__support/OSUtil/syscall.h"
+#include <sys/syscall.h>
+#endif // LIBC_FULL_BUILD
+
+namespace LIBC_NAMESPACE {
+LIBC_INLINE pid_t gettid_inline() {
+#ifdef LIBC_FULL_BUILD
+  return self.get_tid();
+#else
+  return syscall_impl<pid_t>(SYS_gettid);
+#endif
+}
+} // namespace LIBC_NAMESPACE
+
+#endif // LLVM_LIBC_SRC___SUPPORT_THREADS_TID_H

libc/src/unistd/CMakeLists.txt

@@ -318,3 +318,13 @@ add_entrypoint_external(
 add_entrypoint_external(
   opterr
 )
+
+add_entrypoint_object(
+  gettid
+  SRCS
+    gettid.cpp
+  HDRS
+    gettid.h
+  DEPENDS
+    libc.src.__support.threads.tid
+)

libc/src/unistd/getpid.h

@@ -9,11 +9,11 @@
 #ifndef LLVM_LIBC_SRC_UNISTD_GETPID_H
 #define LLVM_LIBC_SRC_UNISTD_GETPID_H
 
-#include <unistd.h>
+#include "hdr/types/pid_t.h"
 
 namespace LIBC_NAMESPACE {
 
-pid_t getpid();
+pid_t getpid(void);
 
 } // namespace LIBC_NAMESPACE
 

libc/src/unistd/gettid.cpp

@@ -0,0 +1,17 @@
+//===-- Implementation file for gettid --------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/unistd/gettid.h"
+#include "src/__support/common.h"
+#include "src/__support/threads/tid.h"
+
+namespace LIBC_NAMESPACE {
+
+LLVM_LIBC_FUNCTION(pid_t, gettid, (void)) { return gettid_inline(); }
+
+} // namespace LIBC_NAMESPACE

libc/src/unistd/gettid.h

@@ -0,0 +1,21 @@
+//===-- Implementation header for gettid ------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_UNISTD_GETTID_H
+#define LLVM_LIBC_SRC_UNISTD_GETTID_H
+
+#include "hdr/types/pid_t.h"
+#include "src/__support/common.h"
+
+namespace LIBC_NAMESPACE {
+
+pid_t gettid(void);
+
+} // namespace LIBC_NAMESPACE
+
+#endif // LLVM_LIBC_SRC_UNISTD_GETTID_H