frameworks/base: acquire lock on am only when needed

appendDropBoxProcessHeaders acquires a lock on am while accessing
ProcessRecord (even if it is null). Watchdog thread ends up invoking
this function (with a null ProcessRecord) to add the stack file to
dropbox. This function would block if and when the watchdog thread is
invoked due to unavailability of lock on am resulting in a deadlock.
This would prevent watchdog from killing system_server.

Change-Id: Ieb34b767779cb587e0c5f536b9b7ba44fb9a28d9

1 files changed, 11 insertions, 4 deletions

diff --git a/services/java/com/android/server/am/ActivityManagerService.java b/services/java/com/android/server/am/ActivityManagerService.java
index 2322ee1..92721fe 100644
--- a/services/java/com/android/server/am/ActivityManagerService.java
+++ b/services/java/com/android/server/am/ActivityManagerService.java
@@ -6730,18 +6730,25 @@ public final class ActivityManagerService extends ActivityManagerNative
      * to append various headers to the dropbox log text.
      */
     private void appendDropBoxProcessHeaders(ProcessRecord process, StringBuilder sb) {
+        // Watchdog thread ends up invoking this function (with
+        // a null ProcessRecord) to add the stack file to dropbox.
+        // Do not acquire a lock on this (am) in such cases, as it
+        // could cause a potential deadlock, if and when watchdog
+        // is invoked due to unavailability of lock on am and it
+        // would prevent watchdog from killing system_server.
+        if (process == null) {
+            sb.append("Process: system_server\n");
+            return;
+        }
         // Note: ProcessRecord 'process' is guarded by the service
         // instance.  (notably process.pkgList, which could otherwise change
         // concurrently during execution of this method)
         synchronized (this) {
-            if (process == null || process.pid == MY_PID) {
+            if (process.pid == MY_PID) {
                 sb.append("Process: system_server\n");
             } else {
                 sb.append("Process: ").append(process.processName).append("\n");
             }
-            if (process == null) {
-                return;
-            }
             int flags = process.info.flags;
             IPackageManager pm = AppGlobals.getPackageManager();
             sb.append("Flags: 0x").append(Integer.toString(flags, 16)).append("\n");