locking/lockdep: Explicitly initialize wq_barrier::done::map

With the new lockdep crossrelease feature, which checks completions usage,
a false positive is reported in the workqueue code:

> Worker A : acquired of wfc.work -> wait for cpu_hotplug_lock to be released
> Task   B : acquired of cpu_hotplug_lock -> wait for lock#3 to be released
> Task   C : acquired of lock#3 -> wait for completion of barr->done
> (Task C is in lru_add_drain_all_cpuslocked())
> Worker D : wait for wfc.work to be released -> will complete barr->done

Such a dead lock can not happen because Task C's barr->done and Worker D's
barr->done can not be the same instance.

The reason of this false positive is we initialize all wq_barrier::done
at insert_wq_barrier() via init_completion(), which makes them belong to
the same lock class, therefore, impossible circles are reported.

To fix this, explicitly initialize the lockdep map for wq_barrier::done
in insert_wq_barrier(), so that the lock class key of wq_barrier::done
is a subkey of the corresponding work_struct, as a result we won't build
a dependency between a wq_barrier with a unrelated work, and we can
differ wq barriers based on the related works, so the false positive
above is avoided.

Also define the empty lockdep_init_map_crosslock() for !CROSSRELEASE
to make the code simple and away from unnecessary #ifdefs.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Cc: Byungchul Park <byungchul.park@lge.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170817094622.12915-1-boqun.feng@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/lockdep.h

@@ -583,6 +583,7 @@ extern void crossrelease_hist_end(enum xhlock_context_t c);
 extern void lockdep_init_task(struct task_struct *task);
 extern void lockdep_free_task(struct task_struct *task);
 #else
+#define lockdep_init_map_crosslock(m, n, k, s) do {} while (0)
 /*
  * To initialize a lockdep_map statically use this macro.
  * Note that _name must not be NULL.

kernel/workqueue.c

@@ -2476,7 +2476,16 @@ static void insert_wq_barrier(struct pool_workqueue *pwq,
 	 */
 	INIT_WORK_ONSTACK(&barr->work, wq_barrier_func);
 	__set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(&barr->work));
-	init_completion(&barr->done);
+
+	/*
+	 * Explicitly init the crosslock for wq_barrier::done, make its lock
+	 * key a subkey of the corresponding work. As a result we won't
+	 * build a dependency between wq_barrier::done and unrelated work.
+	 */
+	lockdep_init_map_crosslock((struct lockdep_map *)&barr->done.map,
+				   "(complete)wq_barr::done",
+				   target->lockdep_map.key, 1);
+	__init_completion(&barr->done);
 	barr->task = current;
 
 	/*