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

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>
Cc: stable@vger.kernel.org

fs/aio.c

@@ -793,6 +793,8 @@ void exit_aio(struct mm_struct *mm)
 
 	for (i = 0; i < table->nr; ++i) {
 		struct kioctx *ctx = table->table[i];
+		struct completion requests_done =
+			COMPLETION_INITIALIZER_ONSTACK(requests_done);
 
 		if (!ctx)
 			continue;
@@ -804,7 +806,10 @@ void exit_aio(struct mm_struct *mm)
 		 * that it needs to unmap the area, just set it to 0.
 		 */
 		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);
 	}
 
 	RCU_INIT_POINTER(mm->ioctx_table, NULL);