aio: block exit_aio() until all context requests are completed

commit 6098b45b upstream.

It seems that exit_aio() also needs to wait for all iocbs to complete (like
io_destroy), but we missed the wait step in current implemention, so fix
it in the same way as we did in io_destroy.
Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/aio.c

@@ -797,6 +797,9 @@ void exit_aio(struct mm_struct *mm)
 	unsigned i = 0;
 
 	while (1) {
+		struct completion requests_done =
+			COMPLETION_INITIALIZER_ONSTACK(requests_done);
+
 		rcu_read_lock();
 		table = rcu_dereference(mm->ioctx_table);
 
@@ -824,7 +827,10 @@ void exit_aio(struct mm_struct *mm)
 		 */
 		ctx->mmap_size = 0;
 
-		kill_ioctx(mm, ctx, NULL);
+		kill_ioctx(mm, ctx, &requests_done);
+
+		/* Wait until all IO for the context are done. */
+		wait_for_completion(&requests_done);
 	}
 }