mm/mmap_lock: remove dead code for !CONFIG_TRACING configurations

make W=1 generates the following warning in mmap_lock.c for allnoconfig

  mm/mmap_lock.c:213:6: warning: no previous prototype for `__mmap_lock_do_trace_start_locking' [-Wmissing-prototypes]
   void __mmap_lock_do_trace_start_locking(struct mm_struct *mm, bool write)
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  mm/mmap_lock.c:219:6: warning: no previous prototype for `__mmap_lock_do_trace_acquire_returned' [-Wmissing-prototypes]
   void __mmap_lock_do_trace_acquire_returned(struct mm_struct *mm, bool write,
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  mm/mmap_lock.c:226:6: warning: no previous prototype for `__mmap_lock_do_trace_released' [-Wmissing-prototypes]
   void __mmap_lock_do_trace_released(struct mm_struct *mm, bool write)

On !CONFIG_TRACING configurations, the code is dead so put it behind an
#ifdef.

[cuibixuan@huawei.com: fix warning when CONFIG_TRACING is not defined]
  Link: https://lkml.kernel.org/r/20210531033426.74031-1-cuibixuan@huawei.com

Link: https://lkml.kernel.org/r/20210520084809.8576-13-mgorman@techsingularity.netSigned-off-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Bixuan Cui <cuibixuan@huawei.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mmap_lock.c

@@ -153,6 +153,37 @@ static inline void put_memcg_path_buf(void)
 	rcu_read_unlock();
 }
 
+#define TRACE_MMAP_LOCK_EVENT(type, mm, ...)                                   \
+	do {                                                                   \
+		const char *memcg_path;                                        \
+		preempt_disable();                                             \
+		memcg_path = get_mm_memcg_path(mm);                            \
+		trace_mmap_lock_##type(mm,                                     \
+				       memcg_path != NULL ? memcg_path : "",   \
+				       ##__VA_ARGS__);                         \
+		if (likely(memcg_path != NULL))                                \
+			put_memcg_path_buf();                                  \
+		preempt_enable();                                              \
+	} while (0)
+
+#else /* !CONFIG_MEMCG */
+
+int trace_mmap_lock_reg(void)
+{
+	return 0;
+}
+
+void trace_mmap_lock_unreg(void)
+{
+}
+
+#define TRACE_MMAP_LOCK_EVENT(type, mm, ...)                                   \
+	trace_mmap_lock_##type(mm, "", ##__VA_ARGS__)
+
+#endif /* CONFIG_MEMCG */
+
+#ifdef CONFIG_TRACING
+#ifdef CONFIG_MEMCG
 /*
  * Write the given mm_struct's memcg path to a percpu buffer, and return a
  * pointer to it. If the path cannot be determined, or no buffer was available
@@ -187,33 +218,6 @@ static const char *get_mm_memcg_path(struct mm_struct *mm)
 	return buf;
 }
 
-#define TRACE_MMAP_LOCK_EVENT(type, mm, ...)                                   \
-	do {                                                                   \
-		const char *memcg_path;                                        \
-		local_lock(&memcg_paths.lock);				       \
-		memcg_path = get_mm_memcg_path(mm);                            \
-		trace_mmap_lock_##type(mm,                                     \
-				       memcg_path != NULL ? memcg_path : "",   \
-				       ##__VA_ARGS__);                         \
-		if (likely(memcg_path != NULL))                                \
-			put_memcg_path_buf();                                  \
-		local_unlock(&memcg_paths.lock);			       \
-	} while (0)
-
-#else /* !CONFIG_MEMCG */
-
-int trace_mmap_lock_reg(void)
-{
-	return 0;
-}
-
-void trace_mmap_lock_unreg(void)
-{
-}
-
-#define TRACE_MMAP_LOCK_EVENT(type, mm, ...)                                   \
-	trace_mmap_lock_##type(mm, "", ##__VA_ARGS__)
-
 #endif /* CONFIG_MEMCG */
 
 /*
@@ -239,3 +243,4 @@ void __mmap_lock_do_trace_released(struct mm_struct *mm, bool write)
 	TRACE_MMAP_LOCK_EVENT(released, mm, write);
 }
 EXPORT_SYMBOL(__mmap_lock_do_trace_released);
+#endif /* CONFIG_TRACING */