QemuMutex: support --enable-debug-mutex

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 will be 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 debugger attached to the process.

Reviewed-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180425025459.5258-4-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -456,6 +456,7 @@ replication="yes"
 vxhs=""
 libxml2=""
 docker="no"
+debug_mutex="no"
 
 # cross compilers defaults, can be overridden with --cross-cc-ARCH
 cross_cc_aarch64="aarch64-linux-gnu-gcc"
@@ -1411,6 +1412,10 @@ for opt do
   ;;
   --disable-git-update) git_update=no
   ;;
+  --enable-debug-mutex) debug_mutex=yes
+  ;;
+  --disable-debug-mutex) debug_mutex=no
+  ;;
   *)
       echo "ERROR: unknown option $opt"
       echo "Try '$0 --help' for more information"
@@ -1685,6 +1690,7 @@ 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
@@ -5951,6 +5957,7 @@ echo "seccomp support   $seccomp"
 echo "coroutine backend $coroutine"
 echo "coroutine pool    $coroutine_pool"
 echo "debug stack usage $debug_stack_usage"
+echo "mutex debugging   $debug_mutex"
 echo "crypto afalg      $crypto_afalg"
 echo "GlusterFS support $glusterfs"
 echo "gcov              $gcov_tool"
@@ -6704,6 +6711,9 @@ fi
 if test "$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

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;
 };
 

include/qemu/thread-win32.h

@@ -5,6 +5,10 @@
 
 struct QemuMutex {
     SRWLOCK lock;
+#ifdef CONFIG_DEBUG_MUTEX
+    const char *file;
+    int line;
+#endif
     bool initialized;
 };
 

util/qemu-thread-common.h

@@ -19,6 +19,10 @@
 
 static inline void qemu_mutex_post_init(QemuMutex *mutex)
 {
+#ifdef CONFIG_DEBUG_MUTEX
+    mutex->file = NULL;
+    mutex->line = 0;
+#endif
     mutex->initialized = true;
 }
 
@@ -31,12 +35,20 @@ static inline void qemu_mutex_pre_lock(QemuMutex *mutex,
 static inline void qemu_mutex_post_lock(QemuMutex *mutex,
                                         const char *file, int line)
 {
+#ifdef CONFIG_DEBUG_MUTEX
+    mutex->file = file;
+    mutex->line = line;
+#endif
     trace_qemu_mutex_locked(mutex, file, line);
 }
 
 static inline void qemu_mutex_pre_unlock(QemuMutex *mutex,
                                          const char *file, int line)
 {
+#ifdef CONFIG_DEBUG_MUTEX
+    mutex->file = NULL;
+    mutex->line = 0;
+#endif
     trace_qemu_mutex_unlock(mutex, file, line);
 }