mm/mmu_notifier.c: remove mmu_notifier_synchronize()

Contrary to its name, mmu_notifier_synchronize() does not synchronize the
notifier's SRCU instance, but rather waits for RCU callbacks to finish.
i.e.  it invokes rcu_barrier().  The RCU documentation is quite clear on
this matter, explicitly calling out that rcu_barrier() does not imply
synchronize_rcu().

As there are no callers of mmu_notifier_synchronize() and it's unclear
whether any user of mmu_notifier_call_srcu() will ever want to barrier on
their callbacks, simply remove the function.

Link: http://lkml.kernel.org/r/20181106134705.14197-1-sean.j.christopherson@intel.comSigned-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mmu_notifier.h

@@ -420,7 +420,6 @@ static inline void mmu_notifier_mm_destroy(struct mm_struct *mm)
 
 extern void mmu_notifier_call_srcu(struct rcu_head *rcu,
 				   void (*func)(struct rcu_head *rcu));
-extern void mmu_notifier_synchronize(void);
 
 #else /* CONFIG_MMU_NOTIFIER */
 

mm/mmu_notifier.c

@@ -35,13 +35,6 @@ void mmu_notifier_call_srcu(struct rcu_head *rcu,
 }
 EXPORT_SYMBOL_GPL(mmu_notifier_call_srcu);
 
-void mmu_notifier_synchronize(void)
-{
-	/* Wait for any running method to finish. */
-	srcu_barrier(&srcu);
-}
-EXPORT_SYMBOL_GPL(mmu_notifier_synchronize);
-
 /*
  * This function can't run concurrently against mmu_notifier_register
  * because mm->mm_users > 0 during mmu_notifier_register and exit_mmap