From 5dd53ab6cbf9d98f2d60546835e84785a104da46 Mon Sep 17 00:00:00 2001
From: "chappedm@gmail.com"
 <chappedm@gmail.com@6b5cf1ce-ec42-a296-1ba9-69fdba395a50>
Date: Sat, 3 Nov 2012 14:52:42 +0000
Subject: [PATCH] issue-481: Replaced test mechanism for distinct address
 spaces with a more reliable mechanism

Rather than using sys_ptrace+PTRACE_PEEK_DATA to determine whether address spaces are distinct, we now use sys_waitpid+__WCLONE. See issue-481 for a more detailed rationale.


git-svn-id: http://gperftools.googlecode.com/svn/trunk@167 6b5cf1ce-ec42-a296-1ba9-69fdba395a50
---
 src/base/linuxthreads.cc | 40 ++++++++++++++++++++--------------------
 1 file changed, 20 insertions(+), 20 deletions(-)

diff --git a/src/base/linuxthreads.cc b/src/base/linuxthreads.cc
index 9d144ef..1f8d4c3 100644
--- a/src/base/linuxthreads.cc
+++ b/src/base/linuxthreads.cc
@@ -408,7 +408,7 @@ static void ListerThread(struct ListerParams *args) {
               /* Check if the marker is identical to the one we created      */
               if (sys_stat(fname, &tmp_sb) >= 0 &&
                   marker_sb.st_ino == tmp_sb.st_ino) {
-                long i, j;
+                long i;
 
                 /* Found one of our threads, make sure it is no duplicate    */
                 for (i = 0; i < num_threads; i++) {
@@ -444,28 +444,28 @@ static void ListerThread(struct ListerParams *args) {
                   sig_num_threads = num_threads;
                   goto next_entry;
                 }
-                while (sys_waitpid(pid, (int *)0, __WALL) < 0) {
+                /* Attaching to a process doesn't guarantee it'll stop before
+                 * ptrace returns; you have to wait on it.  Specifying __WCLONE
+                 * means it will only wait for clone children (i.e. threads,
+                 * not processes).
+                 */
+                while (sys_waitpid(pid, (int *)0, __WCLONE) < 0) {
                   if (errno != EINTR) {
-                    sys_ptrace_detach(pid);
-                    num_threads--;
-                    sig_num_threads = num_threads;
-                    goto next_entry;
+                    /* Assumes ECHILD                                        */
+                    if (pid == ppid) {
+                        /* The parent is not a clone                         */
+                        found_parent = true;
+                        break;
+                    } else {
+                        sys_ptrace_detach(pid);
+                        num_threads--;
+                        sig_num_threads = num_threads;
+                        goto next_entry;
+                    }
                   }
                 }
-                
-                if (sys_ptrace(PTRACE_PEEKDATA, pid, &i, &j) || i++ != j ||
-                    sys_ptrace(PTRACE_PEEKDATA, pid, &i, &j) || i   != j) {
-                  /* Address spaces are distinct, even though both
-                   * processes show the "marker". This is probably
-                   * a forked child process rather than a thread.
-                   */
-                  sys_ptrace_detach(pid);
-                  num_threads--;
-                  sig_num_threads = num_threads;
-                } else {
-                  found_parent |= pid == ppid;
-                  added_entries++;
-                }
+
+                added_entries++;
               }
             }
           }