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: SlimRoms#1 pc 0003ffa5 /system/lib/libc.so (_ZL33__pthread_mutex_lock_with_timeoutP24pthread_mutex_internal_tPK8timespeci+504) native: SlimRoms#2 pc 000400a9 /system/lib/libc.so (pthread_mutex_lock+26) native: SlimRoms#3 pc 0000fa01 /system/lib/libutils.so (_ZN7android19elapsedRealtimeNanoEv+16) native: SlimRoms#4 pc 0000fb1f /system/lib/libutils.so (_ZN7android15elapsedRealtimeEv+2) native: #05 pc 00214d1d /data/dalvik-cache/arm/system@[email protected] (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: SlimRoms#1 pc 0003ffa5 /system/lib/libc.so (_ZL33__pthread_mutex_lock_with_timeoutP24pthread_mutex_internal_tPK8timespeci+504) native: SlimRoms#2 pc 000400a9 /system/lib/libc.so (pthread_mutex_lock+26) native: SlimRoms#3 pc 0000fa01 /system/lib/libutils.so (_ZN7android19elapsedRealtimeNanoEv+16) native: SlimRoms#4 pc 0000fb1f /system/lib/libutils.so (_ZN7android15elapsedRealtimeEv+2) native: #05 pc 00214d1d /data/dalvik-cache/arm/system@[email protected] (Java_android_os_SystemClock_elapsedRealtime__+72) HAM-1470 Change-Id: I41874d2b0ea034a35a74da030398231089c15cde
spezi77 · Aug 30, 2016 · c00328f · c00328f
1 parent 4a978c6
commit c00328f
Showing 1 changed file with 8 additions and 0 deletions.
diff --git 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;