From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-10.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0532CC4708D for ; Fri, 28 May 2021 08:52:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DE3D3613DA for ; Fri, 28 May 2021 08:52:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235665AbhE1Ixn (ORCPT ); Fri, 28 May 2021 04:53:43 -0400 Received: from jabberwock.ucw.cz ([46.255.230.98]:51938 "EHLO jabberwock.ucw.cz" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234651AbhE1Ixl (ORCPT ); Fri, 28 May 2021 04:53:41 -0400 Received: by jabberwock.ucw.cz (Postfix, from userid 1017) id 629EF1C0B76; Fri, 28 May 2021 10:52:06 +0200 (CEST) Date: Fri, 28 May 2021 10:52:05 +0200 From: Pavel Machek To: Manivannan Sadhasivam Cc: Bhaumik Bhatt , Pavel Machek , Greg Kroah-Hartman , linux-kernel@vger.kernel.org, stable@vger.kernel.org, Hemant Kumar , quic_jhugo@quicinc.com Subject: Re: [PATCH 5.10 002/299] bus: mhi: core: Clear configuration from channel context during reset Message-ID: <20210528085205.GB28312@amd> References: <20210510102004.821838356@linuxfoundation.org> <20210510102004.900838842@linuxfoundation.org> <20210510205650.GA17966@amd> <20210511061623.GA8651@thinkpad> <64a8ebbdc9fc7de48b25b9e2bc896d47@codeaurora.org> <20210524041947.GB8823@work> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="eAbsdosE1cNLO4uF" Content-Disposition: inline In-Reply-To: <20210524041947.GB8823@work> User-Agent: Mutt/1.5.23 (2014-03-12) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --eAbsdosE1cNLO4uF Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Hi! > > > > > commit 47705c08465931923e2f2b506986ca0bdf80380d upstream. > > > > > > > > > > When clearing up the channel context after client drivers are > > > > > done using channels, the configuration is currently not being > > > > > reset entirely. Ensure this is done to appropriately handle > > > > > issues where clients unaware of the context state end up calling > > > > > functions which expect a context. > > > >=20 > > > > > +++ b/drivers/bus/mhi/core/init.c > > > > > @@ -544,6 +544,7 @@ void mhi_deinit_chan_ctxt(struct mhi_con > > > > > + u32 tmp; > > > > > @@ -554,7 +555,19 @@ void mhi_deinit_chan_ctxt(struct mhi_con > > > > ... > > > > > + tmp =3D chan_ctxt->chcfg; > > > > > + tmp &=3D ~CHAN_CTX_CHSTATE_MASK; > > > > > + tmp |=3D (MHI_CH_STATE_DISABLED << CHAN_CTX_CHSTATE_SHIFT); > > > > > + chan_ctxt->chcfg =3D tmp; > > > > > + > > > > > + /* Update to all cores */ > > > > > + smp_wmb(); > > > > > } > > > >=20 > > > > This is really interesting code; author was careful to make sure ch= cfg > > > > is updated atomically, but C compiler is free to undo that. Plus, t= his > > > > will make all kinds of checkers angry. > > > >=20 > > > > Does the file need to use READ_ONCE and WRITE_ONCE? > > > >=20 > > >=20 > > > Thanks for looking into this. > > >=20 > > > I agree that the order could be mangled between chcfg read & write and > > > using READ_ONCE & WRITE_ONCE seems to be a good option. > > >=20 > > > Bhaumik, can you please submit a patch and tag stable? > > Hemant and I went over this patch and we noticed this particular functi= on is > > already being called with the channel mutex lock held. This would take = care > > of > > the atomicity and we also probably don't need the smp_wmb() barrier as = the > > mutex > > unlock will implicitly take care of it. > >=20 >=20 > okay >=20 > > To the point of compiler re-ordering, we would need some help to unders= tand > > the > > purpose of READ_ONCE()/WRITE_ONCE() for these dependent statements: > >=20 > > > + tmp =3D chan_ctxt->chcfg; > > > + tmp &=3D ~CHAN_CTX_CHSTATE_MASK; > > > + tmp |=3D (MHI_CH_STATE_DISABLED << CHAN_CTX_CHSTATE_SHIFT); > > > + chan_ctxt->chcfg =3D tmp; > >=20 > > Since RMW operation means that the chan_ctxt->chcfg is copied to a local > > variable (tmp) and the _same_ is being written back to chan_ctxt->chcfg= , can > > compiler reorder these dependent statements and cause a different resul= t? > >=20 >=20 > Well, I agree that there is a minimal guarantee with modern day CPUs on > not breaking the order of dependent memory accesses (like here tmp > variable is the one which gets read and written) but we want to make > sure that this won't break on future CPUs as well. So IMO using > READ_ONCE and WRITE_ONCE adds extra level of safety. Umm, if this is protected by locking, already, we really should not add READ_ONCE. Code should be clear, not having "extra safety levels". I assumed it was running unlocked due to the way it was written. Best regards, Pavel --=20 DENX Software Engineering GmbH, Managing Director: Wolfgang Denk HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany --eAbsdosE1cNLO4uF Content-Type: application/pgp-signature; name="signature.asc" Content-Description: Digital signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iEYEARECAAYFAmCwrzUACgkQMOfwapXb+vKgiQCgm7FALvKqkJTbtsA7LqWcF46c 5fgAoI3mX891I2KyvesWYo9xN1pj+nW1 =GsyN -----END PGP SIGNATURE----- --eAbsdosE1cNLO4uF--