mm: add new mmgrab() helper

Apart from adding the helper function itself, the rest of the kernel is
converted mechanically using:

  git grep -l 'atomic_inc.*mm_count' | xargs sed -i 's/atomic_inc(&\(.*\)->mm_count);/mmgrab\(\1\);/'
  git grep -l 'atomic_inc.*mm_count' | xargs sed -i 's/atomic_inc(&\(.*\)\.mm_count);/mmgrab\(\&\1\);/'

This is needed for a later patch that hooks into the helper, but might
be a worthwhile cleanup on its own.

(Michal Hocko provided most of the kerneldoc comment.)

Link: http://lkml.kernel.org/r/20161218123229.22952-1-vegard.nossum@oracle.com
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 4 insertions, 4 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index 90b09ca35c84..8a768a3672a5 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -539,7 +539,7 @@ static void exit_mm(void)
                 __set_current_state(TASK_RUNNING);
                 down_read(&mm->mmap_sem);
         }
-        atomic_inc(&mm->mm_count);
+        mmgrab(mm);
         BUG_ON(mm != current->active_mm);
         /* more a memory barrier than a real lock */
         task_lock(current);
diff --git a/kernel/futex.c b/kernel/futex.c
index cdf365036141..b687cb22301c 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -338,7 +338,7 @@ static inline bool should_fail_futex(bool fshared)
 
 static inline void futex_get_mm(union futex_key *key)
 {
-        atomic_inc(&key->private.mm->mm_count);
+        mmgrab(key->private.mm);
         /*
          * Ensure futex_get_mm() implies a full barrier such that
          * get_futex_key() implies a full barrier. This is relied upon
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e1ae6ac15eac..6ea1925ac5c0 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2847,7 +2847,7 @@ context_switch(struct rq *rq, struct task_struct *prev,
 
         if (!mm) {
                 next->active_mm = oldmm;
-                atomic_inc(&oldmm->mm_count);
+                mmgrab(oldmm);
                 enter_lazy_tlb(oldmm, next);
         } else
                 switch_mm_irqs_off(oldmm, mm, next);
@@ -6098,7 +6098,7 @@ void __init sched_init(void)
         /*
          * The boot idle thread does lazy MMU switching as well:
          */
-        atomic_inc(&init_mm.mm_count);
+        mmgrab(&init_mm);
         enter_lazy_tlb(&init_mm, current);
 
         /*