mm: Use static initialization for "srcu"

The MM-notifier code currently dynamically initializes the srcu_struct
named "srcu" at subsys_initcall() time, and includes a BUG_ON() to check
this initialization in do_mmu_notifier_register().  Unfortunately, there
is no foolproof way to verify that an srcu_struct has been initialized,
given the possibility of an srcu_struct being allocated on the stack or
on the heap.  This means that creating an srcu_struct_is_initialized()
function is not a reasonable course of action.  Nor is peppering
do_mmu_notifier_register() with SRCU-specific #ifdefs an attractive
alternative.

This commit therefore uses DEFINE_STATIC_SRCU() to initialize
this srcu_struct at compile time, thus eliminating both the
subsys_initcall()-time initialization and the runtime BUG_ON().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: <linux-mm@kvack.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Cc: Vegard Nossum <vegard.nossum@oracle.com>

diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
index a7652acd2ab93c2e290f31fbe80e36d157abb609..54ca545629286223a16ef931830a3757b29da77d 100644 (file)
--- a/mm/mmu_notifier.c
+++ b/mm/mmu_notifier.c
@@ -21,7 +21,7 @@
 #include <linux/slab.h>
 
 /* global SRCU for all MMs */
-static struct srcu_struct srcu;
+DEFINE_STATIC_SRCU(srcu);
 
 /*
  * This function allows mmu_notifier::release callback to delay a call to
@@ -252,12 +252,6 @@ static int do_mmu_notifier_register(struct mmu_notifier *mn,
 
        BUG_ON(atomic_read(&mm->mm_users) <= 0);
 
-       /*
-        * Verify that mmu_notifier_init() already run and the global srcu is
-        * initialized.
-        */
-       BUG_ON(!srcu.per_cpu_ref);
-
        ret = -ENOMEM;
        mmu_notifier_mm = kmalloc(sizeof(struct mmu_notifier_mm), GFP_KERNEL);
        if (unlikely(!mmu_notifier_mm))
@@ -406,9 +400,3 @@ void mmu_notifier_unregister_no_release(struct mmu_notifier *mn,
        mmdrop(mm);
 }
 EXPORT_SYMBOL_GPL(mmu_notifier_unregister_no_release);
-
-static int __init mmu_notifier_init(void)
-{
-       return init_srcu_struct(&srcu);
-}
-subsys_initcall(mmu_notifier_init);