debuggerd_handler: actually wait for pseudothread to exit.

Occasionally, the pseudothread wouldn't exit in time after unlocking
the mutex to get crash_dump to proceed, resulting in spurious error
messages. Instead of using a mutex to emulate pthread_join, just
implement it correctly.

Bug: http://b/34472671
Test: debuggerd_test
Change-Id: I5c2658a84e9407ed8cc0ef2ad0fb648c388b7ad1

1 files changed, 13 insertions, 9 deletions

diff --git a/debuggerd/handler/debuggerd_handler.cpp b/debuggerd/handler/debuggerd_handler.cpp
index 6033a6b53..83db3a745 100644
--- a/debuggerd/handler/debuggerd_handler.cpp
+++ b/debuggerd/handler/debuggerd_handler.cpp
@@ -31,6 +31,7 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <inttypes.h>
+#include <linux/futex.h>
 #include <pthread.h>
 #include <sched.h>
 #include <signal.h>
@@ -46,6 +47,7 @@
 #include <sys/wait.h>
 #include <unistd.h>
 
+#include "private/bionic_futex.h"
 #include "private/libc_logging.h"
 
 // see man(2) prctl, specifically the section about PR_GET_NAME
@@ -151,7 +153,6 @@ static bool have_siginfo(int signum) {
 }
 
 struct debugger_thread_info {
-  pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
   bool crash_dump_started = false;
   pid_t crashing_tid;
   pid_t pseudothread_tid;
@@ -228,7 +229,7 @@ static int debuggerd_dispatch_pseudothread(void* arg) {
     }
   }
 
-  pthread_mutex_unlock(&thread_info->mutex);
+  syscall(__NR_exit, 0);
   return 0;
 }
 
@@ -267,23 +268,26 @@ static void debuggerd_signal_handler(int signal_number, siginfo_t* info, void*)
   }
 
   debugger_thread_info thread_info = {
+    .pseudothread_tid = -1,
     .crashing_tid = gettid(),
     .signal_number = signal_number,
     .info = info
   };
-  pthread_mutex_lock(&thread_info.mutex);
 
   // Essentially pthread_create without CLONE_FILES (see debuggerd_dispatch_pseudothread).
-  pid_t child_pid = clone(debuggerd_dispatch_pseudothread, pseudothread_stack,
-                          CLONE_THREAD | CLONE_SIGHAND | CLONE_VM | CLONE_CHILD_SETTID,
-                          &thread_info, nullptr, nullptr, &thread_info.pseudothread_tid);
+  pid_t child_pid =
+    clone(debuggerd_dispatch_pseudothread, pseudothread_stack,
+          CLONE_THREAD | CLONE_SIGHAND | CLONE_VM | CLONE_CHILD_SETTID | CLONE_CHILD_CLEARTID,
+          &thread_info, nullptr, nullptr, &thread_info.pseudothread_tid);
   if (child_pid == -1) {
     fatal("failed to spawn debuggerd dispatch thread: %s", strerror(errno));
   }
 
-  // Wait for the child to finish and unlock the mutex.
-  // This relies on bionic behavior that isn't guaranteed by the standard.
-  pthread_mutex_lock(&thread_info.mutex);
+  // Wait for the child to start...
+  __futex_wait(&thread_info.pseudothread_tid, -1, nullptr);
+
+  // and then wait for it to finish.
+  __futex_wait(&thread_info.pseudothread_tid, child_pid, nullptr);
 
   // Signals can either be fatal or nonfatal.
   // For fatal signals, crash_dump will PTRACE_CONT us with the signal we