logd: reader/writer element locks

Switch to a reader writer lock for the Element List lock.  Also setup
for a reader writer lock for the Times list, but continue to use a
mutex where rdlock() and wrlock() are the same implementation for now.

This should improve general reader performance and prevent blocking of
other reader operations or exit by a single hung logd.reader.per
thread.  For example, a full length logcat of an empty buffer (eg:
crash log buffer) will hold a lock while the iterator scans the entire
list.

Test: gTest liblog-unit-tests, logd-unit-tests, logcat-unit-tests
Bug: 37378309
Bug: 37483775
Change-Id: If5723ff4a978e17d828a75321e8f0ba91d4a09e0
diff --git a/logd/LogBuffer.h b/logd/LogBuffer.h
index 1272c20..51edd86 100644
--- a/logd/LogBuffer.h
+++ b/logd/LogBuffer.h
@@ -76,7 +76,7 @@
 
 class LogBuffer {
     LogBufferElementCollection mLogElements;
-    pthread_mutex_t mLogElementsLock;
+    pthread_rwlock_t mLogElementsLock;
 
     LogStatistics stats;
 
@@ -154,7 +154,7 @@
         return tags.tagToName(tag);
     }
 
-    // helper must be protected directly or implicitly by lock()/unlock()
+    // helper must be protected directly or implicitly by wrlock()/unlock()
     const char* pidToName(pid_t pid) {
         return stats.pidToName(pid);
     }
@@ -164,11 +164,14 @@
     const char* uidToName(uid_t uid) {
         return stats.uidToName(uid);
     }
-    void lock() {
-        pthread_mutex_lock(&mLogElementsLock);
+    void wrlock() {
+        pthread_rwlock_wrlock(&mLogElementsLock);
+    }
+    void rdlock() {
+        pthread_rwlock_rdlock(&mLogElementsLock);
     }
     void unlock() {
-        pthread_mutex_unlock(&mLogElementsLock);
+        pthread_rwlock_unlock(&mLogElementsLock);
     }
 
    private: