mm: change mm_update_next_owner() to update mm->owner with WRITE_ONCE

The RCU reader uses rcu_dereference() inside rcu_read_lock critical
sections, so the writer shall use WRITE_ONCE.  Just a cleanup, we still
rely on gcc to emit atomic writes in other places.

Link: http://lkml.kernel.org/r/20190325225636.11635-3-aarcange@redhat.com
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Jason Gunthorpe <jgg@mellanox.com>
Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: zhong jiang <zhongjiang@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/kernel/exit.c b/kernel/exit.c
index 2166c2d..8361a56 100644 (file)
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -422,7 +422,7 @@ retry:
         * freed task structure.
         */
        if (atomic_read(&mm->mm_users) <= 1) {
-               mm->owner = NULL;
+               WRITE_ONCE(mm->owner, NULL);
                return;
        }
 
@@ -462,7 +462,7 @@ retry:
         * most likely racing with swapoff (try_to_unuse()) or /proc or
         * ptrace or page migration (get_task_mm()).  Mark owner as NULL.
         */
-       mm->owner = NULL;
+       WRITE_ONCE(mm->owner, NULL);
        return;
 
 assign_new_owner:
@@ -483,7 +483,7 @@ assign_new_owner:
                put_task_struct(c);
                goto retry;
        }
-       mm->owner = c;
+       WRITE_ONCE(mm->owner, c);
        task_unlock(c);
        put_task_struct(c);
 }