linux-kernel - Re: [PATCH] XArray tests: allocation has to be GFP_ATOMIC under rcu_read

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Date:   Sat, 2 Feb 2019 15:19:44 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Li RongQing <lirongqing@...du.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] XArray tests: allocation has to be GFP_ATOMIC under
 rcu_read_lock

On Tue, Jan 29, 2019 at 07:08:42PM +0800, Li RongQing wrote:
> -	XA_BUG_ON(xa, xa_store_index(xa, 1, GFP_KERNEL) != NULL);
> +	XA_BUG_ON(xa, xa_store_index(xa, 1, GFP_ATOMIC) != NULL);

Did you try running this change in userspace too?  I don't think it'll
work.

I think the right change would be:

+++ b/lib/test_xarray.c
@@ -107,8 +107,11 @@ static noinline void check_xas_retry(struct xarray *xa)
        XA_BUG_ON(xa, xas.xa_node != XAS_RESTART);
        XA_BUG_ON(xa, xas_next_entry(&xas, ULONG_MAX) != xa_mk_value(0));
        XA_BUG_ON(xa, xas.xa_node != NULL);
+       rcu_read_unlock();
 
        XA_BUG_ON(xa, xa_store_index(xa, 1, GFP_KERNEL) != NULL);
+
+       rcu_read_lock();
        XA_BUG_ON(xa, !xa_is_internal(xas_reload(&xas)));
        xas.xa_node = XAS_RESTART;
        XA_BUG_ON(xa, xas_next_entry(&xas, ULONG_MAX) != xa_mk_value(0));