From mboxrd@z Thu Jan  1 00:00:00 1970
From: Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: [RFC PATCH] fs: use a sequence counter instead of file_lock in
 fd_install
Date: Thu, 16 Apr 2015 19:09:32 +0100
Message-ID: <20150416180932.GW889@ZenIV.linux.org.uk>
References: <20150416121628.GA20615@mguzik>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Cc: Andrew Morton <akpm@linux-foundation.org>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Yann Droneaud <ydroneaud@opteya.com>,
	Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
To: Mateusz Guzik <mguzik@redhat.com>
Return-path: <linux-fsdevel-owner@vger.kernel.org>
Received: from zeniv.linux.org.uk ([195.92.253.2]:37689 "EHLO
	ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1753426AbbDPSJu (ORCPT
	<rfc822;linux-fsdevel@vger.kernel.org>);
	Thu, 16 Apr 2015 14:09:50 -0400
Content-Disposition: inline
In-Reply-To: <20150416121628.GA20615@mguzik>
Sender: linux-fsdevel-owner@vger.kernel.org
List-ID: <linux-fsdevel.vger.kernel.org>

On Thu, Apr 16, 2015 at 02:16:31PM +0200, Mateusz Guzik wrote:
> @@ -165,8 +165,10 @@ static int expand_fdtable(struct files_struct *files, int nr)
>  	cur_fdt = files_fdtable(files);
>  	if (nr >= cur_fdt->max_fds) {
>  		/* Continue as planned */
> +		write_seqcount_begin(&files->fdt_seqcount);
>  		copy_fdtable(new_fdt, cur_fdt);
>  		rcu_assign_pointer(files->fdt, new_fdt);
> +		write_seqcount_end(&files->fdt_seqcount);
>  		if (cur_fdt != &files->fdtab)
>  			call_rcu(&cur_fdt->rcu, free_fdtable_rcu);

Interesting.  AFAICS, your test doesn't step anywhere near that path,
does it?  So basically you never hit the retries during that...