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From: Matthias Kaehlcke <mka@chromium.org>
To: Doug Anderson <dianders@chromium.org>
Cc: Lina Iyer <ilina@codeaurora.org>,
	Andy Gross <andy.gross@linaro.org>,
	David Brown <david.brown@linaro.org>,
	linux-arm-msm@vger.kernel.org,
	"open list:ARM/QUALCOMM SUPPORT" <linux-soc@vger.kernel.org>,
	Rajendra Nayak <rnayak@codeaurora.org>,
	Bjorn Andersson <bjorn.andersson@linaro.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Stephen Boyd <sboyd@kernel.org>,
	Evan Green <evgreen@chromium.org>
Subject: Re: [PATCH v7 04/10] drivers: qcom: rpmh: add RPMH helper functions
Date: Fri, 4 May 2018 13:50:05 -0700	[thread overview]
Message-ID: <20180504205005.GC19594@google.com> (raw)
In-Reply-To: <CAD=FV=VVrqHnVTATzbXYsZgj8gA9sO=Ky_HRvztfRmjLZoq0=Q@mail.gmail.com>

On Thu, May 03, 2018 at 01:26:07PM -0700, Doug Anderson wrote:
> Hi,
> 
> On Wed, May 2, 2018 at 12:37 PM, Lina Iyer <ilina@codeaurora.org> wrote:
> > +static struct rpmh_ctrlr rpmh_rsc[RPMH_MAX_CTRLR];
> > +static DEFINE_SPINLOCK(rpmh_rsc_lock);
> > +
> > +static struct rpmh_ctrlr *get_rpmh_ctrlr(const struct device *dev)
> > +{
> > +       int i;
> > +       struct rsc_drv *p, *drv = dev_get_drvdata(dev->parent);
> > +       struct rpmh_ctrlr *ctrlr = ERR_PTR(-EINVAL);
> > +       unsigned long flags;
> > +
> > +       if (!drv)
> > +               return ctrlr;
> > +
> > +       for (i = 0; i < RPMH_MAX_CTRLR; i++) {
> > +               if (rpmh_rsc[i].drv == drv) {
> > +                       ctrlr = &rpmh_rsc[i];
> > +                       return ctrlr;
> > +               }
> > +       }
> > +
> > +       spin_lock_irqsave(&rpmh_rsc_lock, flags);
> > +       list_for_each_entry(p, &rsc_drv_list, list) {
> > +               if (drv == p) {
> > +                       for (i = 0; i < RPMH_MAX_CTRLR; i++) {
> > +                               if (!rpmh_rsc[i].drv)
> > +                                       break;
> > +                       }
> > +                       if (i == RPMH_MAX_CTRLR) {
> > +                               ctrlr = ERR_PTR(-ENOMEM);
> > +                               break;
> > +                       }
> > +                       rpmh_rsc[i].drv = drv;
> > +                       ctrlr = &rpmh_rsc[i];
> > +                       break;
> > +               }
> > +       }
> > +       spin_unlock_irqrestore(&rpmh_rsc_lock, flags);
> 
> I may have missed something, but to me it appears that this whole
> "rsc_drv_list" is pretty pointless.  I wrote up a patch atop your
> series to remove it at
> <https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/1042883/>
> and it simplifies the code a whole bunch.  From that patch, my
> justification was:
> 
> > The global rsc_drv_list was (as far as I can tell) racy and not useful
> > for anything.
> >
> > I say it is racy because in general you need some sort of mutual
> > exclusion for lists.  If someone is adding to a list while someone
> > else is iterating over it then you get badness.
> >
> > I say it is not useful because the only user of it was
> > get_rpmh_ctrlr() and the only thing it did was to verify that the
> > "struct rsc_drv *" that it alrady had was in the list.  How could it
> > not be?

I agree that the list doesn't seem to be very useful.

> Note that in v7 of your series you added a spinlock around your access
> of "rsc_drv_list", but this doesn't actually remove the race.
> Specifically I'm pretty sure that the list primitives don't support
> calling list_add() while someone might be iterating over the list and
> your spinlock isn't grabbed in rpmh_rsc_probe().

Actually adding a lock was my suggestion, but to protect against
another race that is still/again present with your patch:

> if (!rpmh_rsc[i].drv) {
>     rpmh_rsc[i].drv = drv;

This could be executed concurrently with both/all instances seeing
rpmh_rsc[i].drv == NULL and then clobbering each other.

>     spin_lock_init(&rpmh_rsc[i].lock);
>     INIT_LIST_HEAD(&rpmh_rsc[i].cache);
>     return &rpmh_rsc[i];
> }

  reply	other threads:[~2018-05-04 20:50 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-02 19:37 [PATCH v7 00/10] drivers/qcom: add RPMH communication support Lina Iyer
2018-05-02 19:37 ` [PATCH v7 01/10] drivers: qcom: rpmh-rsc: add RPMH controller for QCOM SoCs Lina Iyer
2018-05-02 19:37 ` [PATCH v7 02/10] dt-bindings: introduce RPMH RSC bindings for Qualcomm SoCs Lina Iyer
2018-05-02 20:37   ` Stephen Boyd
2018-05-02 19:37 ` [PATCH v7 03/10] drivers: qcom: rpmh-rsc: log RPMH requests in FTRACE Lina Iyer
2018-05-02 19:45   ` Steven Rostedt
2018-05-02 19:37 ` [PATCH v7 04/10] drivers: qcom: rpmh: add RPMH helper functions Lina Iyer
2018-05-03 20:26   ` Doug Anderson
2018-05-04 20:50     ` Matthias Kaehlcke [this message]
2018-05-08 16:05     ` ilina
2018-05-10 22:37       ` Doug Anderson
2018-05-11 15:06         ` Lina Iyer
2018-05-11 20:14           ` Doug Anderson
2018-05-14 15:00             ` Lina Iyer
2018-05-02 19:37 ` [PATCH v7 05/10] drivers: qcom: rpmh-rsc: write sleep/wake requests to TCS Lina Iyer
2018-05-03 21:35   ` Matthias Kaehlcke
2018-05-08 16:16     ` ilina
2018-05-02 19:37 ` [PATCH v7 06/10] drivers: qcom: rpmh-rsc: allow invalidation of sleep/wake TCS Lina Iyer
2018-05-03 22:06   ` Matthias Kaehlcke
2018-05-08 16:14     ` ilina
2018-05-08 17:25       ` Matthias Kaehlcke
2018-05-02 19:37 ` [PATCH v7 07/10] drivers: qcom: rpmh: cache sleep/wake state requests Lina Iyer
2018-05-04 21:39   ` Matthias Kaehlcke
2018-05-02 19:37 ` [PATCH v7 08/10] drivers: qcom: rpmh: allow requests to be sent asynchronously Lina Iyer
2018-05-02 19:37 ` [PATCH v7 09/10] drivers: qcom: rpmh: add support for batch RPMH request Lina Iyer
2018-05-02 19:37 ` [PATCH v7 10/10] drivers: qcom: rpmh-rsc: allow active requests from wake TCS Lina Iyer

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