libutils: fix deadlock in elapsedRealtimeNano

If n > 2 threads all lock the mutex awaiting the clock method to
be set, the 1st thread will first set the method and unlock.  The
2nd thread will then take the mutex, but never unlock because the
clock_method has already been updated by the 1st thread.  This causes
a deadlock for threads 3-n.

Solution is to ensure the calling thread always unlocks the mutex if
it has previously locked it.

Log:

"main" prio=5 tid=1 Native
  | group="main" sCount=1 dsCount=0 obj=0x733313b0 self=0xb4cf6500
  | sysTid=12786 nice=0 cgrp=default sched=0/0 handle=0xb6f86b44
  | state=S schedstat=( 41990410 32985836 80 ) utm=3 stm=1 core=2 HZ=100
  | stack=0xbe5fb000-0xbe5fd000 stackSize=8MB
  | held mutexes=
  native: #00 pc 00017638  /system/lib/libc.so (syscall+28)
  native: #01 pc 0003ffa5  /system/lib/libc.so (_ZL33__pthread_mutex_lock_with_timeoutP24pthread_mutex_internal_tPK8timespeci+504)
  native: #02 pc 000400a9  /system/lib/libc.so (pthread_mutex_lock+26)
  native: #03 pc 0000fa01  /system/lib/libutils.so (_ZN7android19elapsedRealtimeNanoEv+16)
  native: #04 pc 0000fb1f  /system/lib/libutils.so (_ZN7android15elapsedRealtimeEv+2)
  native: #05 pc 00214d1d  /data/dalvik-cache/arm/system@framework@boot.oat (Java_android_os_SystemClock_elapsedRealtime__+72)

"Thread-6372" prio=5 tid=9 Native
  | group="main" sCount=1 dsCount=0 obj=0x32c05120 self=0xacb58100
  | sysTid=12829 nice=10 cgrp=bg_non_interactive sched=0/0 handle=0xb38c3930
  | state=S schedstat=( 869427 8219115 17 ) utm=0 stm=0 core=2 HZ=100
  | stack=0xb37c1000-0xb37c3000 stackSize=1038KB
  | held mutexes=
  native: #00 pc 00017638  /system/lib/libc.so (syscall+28)
  native: #01 pc 0003ffa5  /system/lib/libc.so (_ZL33__pthread_mutex_lock_with_timeoutP24pthread_mutex_internal_tPK8timespeci+504)
  native: #02 pc 000400a9  /system/lib/libc.so (pthread_mutex_lock+26)
  native: #03 pc 0000fa01  /system/lib/libutils.so (_ZN7android19elapsedRealtimeNanoEv+16)
  native: #04 pc 0000fb1f  /system/lib/libutils.so (_ZN7android15elapsedRealtimeEv+2)
  native: #05 pc 00214d1d  /data/dalvik-cache/arm/system@framework@boot.oat (Java_android_os_SystemClock_elapsedRealtime__+72)

HAM-1470
Change-Id: I41874d2b0ea034a35a74da030398231089c15cde

1 files changed, 8 insertions, 0 deletions

diff --git a/libutils/SystemClock.cpp b/libutils/SystemClock.cpp
index c3b5d7402..28ea8176e 100644
--- a/libutils/SystemClock.cpp
+++ b/libutils/SystemClock.cpp
@@ -121,9 +121,11 @@ int64_t elapsedRealtimeNano()
 #endif
 
     static int s_fd = -1;
+    bool need_unlock = false;
 
     if (clock_method < 0) {
         pthread_mutex_lock(&clock_lock);
+        need_unlock = true;
     }
 
     if (clock_method < 0 || clock_method == METHOD_IOCTL) {
@@ -143,6 +145,8 @@ int64_t elapsedRealtimeNano()
                 checkTimeStamps(timestamp, &prevTimestamp, &prevMethod, METHOD_IOCTL);
                 if (clock_method < 0) {
                     clock_method = METHOD_IOCTL;
+                }
+                if (need_unlock) {
                     pthread_mutex_unlock(&clock_lock);
                 }
                 return timestamp;
@@ -159,6 +163,8 @@ int64_t elapsedRealtimeNano()
                             METHOD_CLOCK_GETTIME);
             if (clock_method < 0) {
                 clock_method = METHOD_CLOCK_GETTIME;
+            }
+            if (need_unlock) {
                 pthread_mutex_unlock(&clock_lock);
             }
             return timestamp;
@@ -173,6 +179,8 @@ int64_t elapsedRealtimeNano()
                     METHOD_SYSTEMTIME);
     if (clock_method < 0) {
         clock_method = METHOD_SYSTEMTIME;
+    }
+    if (need_unlock) {
         pthread_mutex_unlock(&clock_lock);
     }
     return timestamp;