[ovs-dev] [PATCHv2] fatal-signal: Fix clang error due to lock.

[William Tu]

Due to not acquiring lock, clang reports:
  lib/vlog.c:618:12: error: reading variable 'log_fd' requires holding mutex
  'log_file_mutex' [-Werror,-Wthread-safety-analysis]
  return log_fd;

The patch fixes it by creating a function in vlog.c to write
directly to log file unsafely.

Tested-at: https://travis-ci.org/github/williamtu/ovs-travis/builds/666165883
Fixes: ecd4a8fcdff2 ("fatal-signal: Log backtrace when no monitor daemon.")
Suggested-by: Ilya Maximets <i.maximets at ovn.org>
Signed-off-by: William Tu <u9012063 at gmail.com>
---
 include/openvswitch/vlog.h |  4 ++--
 lib/fatal-signal.c         |  8 ++------
 lib/vlog.c                 | 15 ++++++++++++---
 3 files changed, 16 insertions(+), 11 deletions(-)

diff --git a/include/openvswitch/vlog.h b/include/openvswitch/vlog.h
index 476bf3d6d5b4..886fce5e0fd5 100644
--- a/include/openvswitch/vlog.h
+++ b/include/openvswitch/vlog.h
@@ -143,8 +143,8 @@ void vlog_set_syslog_method(const char *method);
 /* Configure syslog target. */
 void vlog_set_syslog_target(const char *target);
 
-/* Return the log_fd. */
-int vlog_get_fd(void);
+/* Write directly to log file. */
+void vlog_direct_write_to_log_file_unsafe(const char *s);
 
 /* Initialization. */
 void vlog_init(void);
diff --git a/lib/fatal-signal.c b/lib/fatal-signal.c
index 4965c1ae82c0..51cf628d994e 100644
--- a/lib/fatal-signal.c
+++ b/lib/fatal-signal.c
@@ -197,11 +197,7 @@ send_backtrace_to_monitor(void) {
          */
         char str[] = "SIGSEGV detected, backtrace:\n";
 
-        if (vlog_get_fd() < 0) {
-            return;
-        }
-
-        ignore(write(vlog_get_fd(), str, strlen(str)));
+        vlog_direct_write_to_log_file_unsafe(str);
 
         for (int i = 0; i < dep; i++) {
             char line[64];
@@ -210,7 +206,7 @@ send_backtrace_to_monitor(void) {
                      unw_bt[i].ip,
                      unw_bt[i].func,
                      unw_bt[i].offset);
-            ignore(write(vlog_get_fd(), line, strlen(line)));
+            vlog_direct_write_to_log_file_unsafe(line);
         }
     }
 }
diff --git a/lib/vlog.c b/lib/vlog.c
index 502b33fc831e..6d17d4837e9c 100644
--- a/lib/vlog.c
+++ b/lib/vlog.c
@@ -612,10 +612,19 @@ vlog_set_syslog_target(const char *target)
     ovs_rwlock_unlock(&pattern_rwlock);
 }
 
-int
-vlog_get_fd(void)
+/*
+ * This function writes directly to log file without using async writer or
+ * taking a lock.  Caller must hold 'log_file_mutex' or be sure that it's
+ * not necessary.  Could be used in exceptional cases like dumping of backtrace
+ * on fatal signals.
+ */
+void
+vlog_direct_write_to_log_file_unsafe(const char *s)
+    OVS_NO_THREAD_SAFETY_ANALYSIS
 {
-    return log_fd;
+    if (log_fd >= 0) {
+        ignore(write(log_fd, s, strlen(s)));
+    }
 }
 
 /* Returns 'false' if 'facility' is not a valid string. If 'facility'
-- 
2.7.4