Add test about pthread_mutex_t owner tid limit.

Bug: 19216648
Change-Id: I7b12955bdcad31c13bf8ec2740ff88ba15223ec0

2 files changed, 15 insertions, 24 deletions

diff --git a/libc/bionic/pthread_mutex.cpp b/libc/bionic/pthread_mutex.cpp
index a0628b01d..83d6b541a 100644
--- a/libc/bionic/pthread_mutex.cpp
+++ b/libc/bionic/pthread_mutex.cpp
@@ -150,30 +150,9 @@
 /* Mutex owner field:
  *
  * This is only used for recursive and errorcheck mutexes. It holds the
- * tid of the owning thread. Note that this works because the Linux
- * kernel _only_ uses 16-bit values for tids.
- *
- * More specifically, it will wrap to 10000 when it reaches over 32768 for
- * application processes. You can check this by running the following inside
- * an adb shell session:
- *
-    OLDPID=$$;
-    while true; do
-    NEWPID=$(sh -c 'echo $$')
-    if [ "$NEWPID" -gt 32768 ]; then
-        echo "AARGH: new PID $NEWPID is too high!"
-        exit 1
-    fi
-    if [ "$NEWPID" -lt "$OLDPID" ]; then
-        echo "****** Wrapping from PID $OLDPID to $NEWPID. *******"
-    else
-        echo -n "$NEWPID!"
-    fi
-    OLDPID=$NEWPID
-    done
-
- * Note that you can run the same example on a desktop Linux system,
- * the wrapping will also happen at 32768, but will go back to 300 instead.
+ * tid of the owning thread. We use 16 bits to represent tid here,
+ * so the highest tid is 65535. There is a test to check /proc/sys/kernel/pid_max
+ * to make sure it will not exceed our limit.
  */
 #define  MUTEX_OWNER_SHIFT     16
 #define  MUTEX_OWNER_LEN       16
diff --git a/tests/pthread_test.cpp b/tests/pthread_test.cpp
index cb3207923..5dc60eeee 100644
--- a/tests/pthread_test.cpp
+++ b/tests/pthread_test.cpp
@@ -27,6 +27,7 @@
 #include <malloc.h>
 #include <pthread.h>
 #include <signal.h>
+#include <stdio.h>
 #include <sys/mman.h>
 #include <sys/syscall.h>
 #include <time.h>
@@ -1092,3 +1093,14 @@ TEST(pthread, pthread_mutex_lock_RECURSIVE) {
   ASSERT_EQ(EPERM, pthread_mutex_unlock(&lock));
   ASSERT_EQ(0, pthread_mutex_destroy(&lock));
 }
+
+TEST(pthread, pthread_mutex_owner_tid_limit) {
+  FILE* fp = fopen("/proc/sys/kernel/pid_max", "r");
+  ASSERT_TRUE(fp != NULL);
+  long pid_max;
+  ASSERT_EQ(1, fscanf(fp, "%ld", &pid_max));
+  fclose(fp);
+  // Current pthread_mutex uses 16 bits to represent owner tid.
+  // Change the implementation if we need to support higher value than 65535.
+  ASSERT_LE(pid_max, 65536);
+}