We have had some tracing tools for mutex but it's not easy to use them
for e.g. dead locks.  Let's provide "--enable-debug-mutex" parameter
when configure to allow QemuMutex to store the last owner that took
specific lock.  It might be extremely easy to use this tool to debug
deadlocks since we can directly know who took the lock then as long as
we can have a gdb attached to the process.

CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Fam Zheng <famz@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 configure                   | 10 ++++++++++
 include/qemu/thread-posix.h |  4 ++++
 util/qemu-thread-posix.c    |  4 ++++
 3 files changed, 18 insertions(+)

diff --git a/configure b/configure
index 0a19b033bc..9ee1064c3b 100755
--- a/configure
+++ b/configure
@@ -451,6 +451,7 @@ jemalloc="no"
 replication="yes"
 vxhs=""
 libxml2=""
+debug_mutex="no"
 
 supported_cpu="no"
 supported_os="no"
@@ -1374,6 +1375,10 @@ for opt do
   ;;
   --disable-git-update) git_update=no
   ;;
+  --enable-debug-mutex) debug_mutex=yes
+  ;;
+  --disble-debug-mutex) debug_mutex=yes
+  ;;
   *)
       echo "ERROR: unknown option $opt"
       echo "Try '$0 --help' for more information"
@@ -1631,6 +1636,7 @@ disabled with --disable-FEATURE, default is enabled if available:
   crypto-afalg    Linux AF_ALG crypto backend driver
   vhost-user      vhost-user support
   capstone        capstone disassembler support
+  debug-mutex     mutex debugging support
 
 NOTE: The object files are built at the place where configure is launched
 EOF
@@ -5874,6 +5880,7 @@ echo "avx2 optimization $avx2_opt"
 echo "replication support $replication"
 echo "VxHS block device $vxhs"
 echo "capstone          $capstone"
+echo "mutex debugging   $debug_mutex"
 
 if test "$sdl_too_old" = "yes"; then
 echo "-> Your SDL version is too old - please upgrade to have SDL support"
@@ -6602,6 +6609,9 @@ fi
 if test "$capstone" != "no" ; then
   echo "CONFIG_CAPSTONE=y" >> $config_host_mak
 fi
+if test "$debug_mutex" = "yes" ; then
+  echo "CONFIG_DEBUG_MUTEX=y" >> $config_host_mak
+fi
 
 # Hold two types of flag:
 #   CONFIG_THREAD_SETNAME_BYTHREAD  - we've got a way of setting the name on
diff --git a/include/qemu/thread-posix.h b/include/qemu/thread-posix.h
index f3f47e426f..fd27b34128 100644
--- a/include/qemu/thread-posix.h
+++ b/include/qemu/thread-posix.h
@@ -12,6 +12,10 @@ typedef QemuMutex QemuRecMutex;
 
 struct QemuMutex {
     pthread_mutex_t lock;
+#ifdef CONFIG_DEBUG_MUTEX
+    const char *file;
+    int line;
+#endif
     bool initialized;
 };
 
diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
index b789cf32e9..905a816af6 100644
--- a/util/qemu-thread-posix.c
+++ b/util/qemu-thread-posix.c
@@ -68,6 +68,10 @@ void qemu_mutex_lock_impl(QemuMutex *mutex, const char *file, const int line)
     if (err)
         error_exit(err, __func__);
 
+#ifdef CONFIG_DEBUG_MUTEX
+    mutex->file = file;
+    mutex->line = line;
+#endif
     trace_qemu_mutex_locked(mutex, file, line);
 }
 
-- 
2.14.3