From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from pandora.armlinux.org.uk (pandora.armlinux.org.uk [78.32.30.218]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 86FF64E1BC for ; Tue, 23 Jan 2024 21:06:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=78.32.30.218 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1706043969; cv=none; b=mWFMmNW1FktGZFXfU00QkpTnMgVHzdBn4M3Gngs9dIjZREaHXgE/6VTq87HUSckYs7RixF5KBh4xeIUQ3luBmAswB+YpOGmX+09yYJHbOrEvTVTcrjzqp9eRRp6uO8E7jKant2n6Z68A4amMT+5nt5t7+PTPZ5V3RykJIZa5B2k= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1706043969; c=relaxed/simple; bh=wvda1L3RR/smUjSRPDsfcLiEuhGrOrqrle7idqMf9t8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=q1Js5je9/iBQhrDxdW8VufXCkJqDqjlh3gAkZeMqHiHCXof/s2HsPjWuWriBgu7cit0ktng5P+kmUmVn2xXZ1OS3Byn5xZWyn1M9UEjG8SA81Sn8NWtIYo31uTwKYPgzUpp4mEgG3SJp44imvjECQiBToZugVgQYW0ZlzIDnNfg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=armlinux.org.uk; spf=none smtp.mailfrom=armlinux.org.uk; dkim=pass (2048-bit key) header.d=armlinux.org.uk header.i=@armlinux.org.uk header.b=J45UVL1o; arc=none smtp.client-ip=78.32.30.218 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=armlinux.org.uk Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=armlinux.org.uk Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=armlinux.org.uk header.i=@armlinux.org.uk header.b="J45UVL1o" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=armlinux.org.uk; s=pandora-2019; h=Sender:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=oC+Xzu8aSBBLxTeOfbPxmslOua7SUo4gfiXATYXwkfg=; b=J45UVL1of3oS15/wI5Kdmn28wP 7NPwZmDwM3ad0TQT+R6Kl9P6QMirgtMipu5fPDa6WhT2/UwIxx1ne/KkfmlSKDirr4Hd8WuTmok/y FMw4Q3uBOcg8QIoVKE8OzqN9r2whlaugsZJme1hEZSobD0kFllk/tmRuVVCUd3QyKoZhjx7d9rxyt Z5ShI6oUM7qT5YIGi8U1JcpH8BQ7eJBr6osb3Hzb1YneVqu746PUAsQ9kay+HAcICWTBM9GhJ5Gn0 4x30bjcoNaX8RJ2CIwr/zI1Rt52PufU+PMTRmN1jdYXkmWxwe4NlGq0QTWfnHnBftRxNSIhd16w8y 7KsPIptw==; Received: from shell.armlinux.org.uk ([fd8f:7570:feb6:1:5054:ff:fe00:4ec]:43142) by pandora.armlinux.org.uk with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.96) (envelope-from ) id 1rSNxu-00035u-1i; Tue, 23 Jan 2024 21:05:50 +0000 Received: from linux by shell.armlinux.org.uk with local (Exim 4.94.2) (envelope-from ) id 1rSNxr-0002Fu-Qa; Tue, 23 Jan 2024 21:05:47 +0000 Date: Tue, 23 Jan 2024 21:05:47 +0000 From: "Russell King (Oracle)" To: Sean Anderson Cc: Landen.Chao@mediatek.com, UNGLinuxDriver@microchip.com, alexandre.belloni@bootlin.com, andrew@lunn.ch, angelogioacchino.delregno@collabora.com, arinc.unal@arinc9.com, claudiu.manoil@nxp.com, daniel@makrotopia.org, davem@davemloft.net, dqfext@gmail.com, edumazet@google.com, f.fainelli@gmail.com, hkallweit1@gmail.com, kuba@kernel.org, linux-arm-kernel@lists.infradead.org, linux-mediatek@lists.infradead.org, matthias.bgg@gmail.com, netdev@vger.kernel.org, olteanv@gmail.com, pabeni@redhat.com, sean.wang@mediatek.com Subject: Re: [PATCH RFC net-next 03/14] net: phylink: add support for PCS link change notifications Message-ID: References: <75773076-39a2-49dd-9eb2-15a10955a60d@seco.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: Russell King (Oracle) On Tue, Jan 23, 2024 at 03:33:57PM -0500, Sean Anderson wrote: > On 1/23/24 15:07, Russell King (Oracle) wrote: > > On Tue, Jan 23, 2024 at 02:46:15PM -0500, Sean Anderson wrote: > >> Hi Russell, > >> > >> Does there need to be any locking when calling phylink_pcs_change? I > >> noticed that you call it from threaded IRQ context in [1]. Can that race > >> with phylink_major_config? > > > > What kind of scenario are you thinking may require locking? > > Can't we at least get a spurious bounce? E.g. > > pcs_major_config() > pcs_disable(old_pcs) /* masks IRQ */ > old_pcs->phylink = NULL; > new_pcs->phylink = pl; > ... > pcs_enable(new_pcs) /* unmasks IRQ */ > ... > > pcs_handle_irq(new_pcs) /* Link up IRQ */ > phylink_pcs_change(new_pcs, true) > phylink_run_resolve(pl) > > phylink_resolve(pl) > /* Link up */ By this time, old_pcs->phylink has been set to NULL as you mentioned above. > pcs_handle_irq(old_pcs) /* Link down IRQ (pending from before pcs_disable) */ > phylink_pcs_change(old_pcs, false) > phylink_run_resolve(pl) /* Doesn't see the NULL */ So here, phylink_pcs_change(old_pcs, ...) will read old_pcs->phylink and find that it's NULL, and do nothing. > > I guess the possibility would be if pcs->phylink changes and the > > compiler reads it multiple times - READ_ONCE() should solve that. > > > > However, in terms of the mechanics, there's no race. > > > > During the initial bringup, the resolve worker isn't started until > > after phylink_major_config() has completed (it's started at > > phylink_enable_and_run_resolve().) So, if phylink_pcs_change() > > gets called while in phylink_major_config() there, it'll see > > that pl->phylink_disable_state is non-zero, and won't queue the > > work. > > > > The next one is within the worker itself - and there can only > > be one instance of the worker running in totality. So, if > > phylink_pcs_change() gets called while phylink_major_config() is > > running from this path, the only thing it'll do is re-schedule > > the resolve worker to run another iteration which is harmless > > (whether or not the PCS is still current.) > > > > The last case is phylink_ethtool_ksettings_set(). This runs under > > the state_mutex, which locks out the resolve worker (since it also > > takes that mutex). > > > > So calling phylink_pcs_change() should be pretty harmless _unless_ > > the compiler re-reads pcs->phylink multiple times inside > > phylink_pcs_change(), which I suppose with modern compilers is > > possible. Hence my suggestion above about READ_ONCE() for that. > > > > Have you encountered an OOPS because pcs->phylink has become NULL? > > Or have you spotted another issue? > > I was looking at extending this code, and I was wondering if I needed > to e.g. take RTNL first. Thanks for the quick response. Note that phylink_mac_change() gets called in irq context, so this stuff can't take any mutexes or the rtnl. It is also intended that phylink_pcs_change() is similarly callable in irq context. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!