aio: make the lookup_ioctx() lockless

The mm->ioctx_list is currently protected by a reader-writer lock,
so we always grab that lock on the read side for doing ioctx
lookups. As the workload is extremely reader biased, turn this into
an rcu hlist so we can make lookup_ioctx() lockless. Get rid of
the rwlock and use a spinlock for providing update side exclusion.

There's usually only 1 entry on this list, so it doesn't make sense
to look into fancier data structures.

Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/arch/s390/mm/pgtable.c b/arch/s390/mm/pgtable.c
index ef3635b52fc0..0767827540b1 100644
--- a/arch/s390/mm/pgtable.c
+++ b/arch/s390/mm/pgtable.c
@@ -263,7 +263,7 @@ int s390_enable_sie(void)
         /* lets check if we are allowed to replace the mm */
         task_lock(tsk);
         if (!tsk->mm || atomic_read(&tsk->mm->mm_users) > 1 ||
-            tsk->mm != tsk->active_mm || tsk->mm->ioctx_list) {
+            tsk->mm != tsk->active_mm || !hlist_empty(&tsk->mm->ioctx_list)) {
                 task_unlock(tsk);
                 return -EINVAL;
         }
@@ -279,7 +279,7 @@ int s390_enable_sie(void)
         /* Now lets check again if something happened */
         task_lock(tsk);
         if (!tsk->mm || atomic_read(&tsk->mm->mm_users) > 1 ||
-            tsk->mm != tsk->active_mm || tsk->mm->ioctx_list) {
+            tsk->mm != tsk->active_mm || !hlist_empty(&tsk->mm->ioctx_list)) {
                 mmput(mm);
                 task_unlock(tsk);
                 return -EINVAL;