Subject: Re: [PATCH v4 06/20] coresight: add try_get_module() in coresight_grab_device()

From: Tingwei Zhang <[email-masked]>
Date: Sun, 26 Jul 2020 09:32:05 +0800

Hi Mike,
On Sat, Jul 25, 2020 at 09:51:31PM +0800, Mike Leach wrote:
> Hi Tingwei,
> 
> On Sat, 25 Jul 2020 at 11:05, Tingwei Zhang <[email-masked]>
> wrote:
> >
> > Hi Mike,
> >
> >  Thu, Jul 23, 2020 at 06:55:47PM +0800, Mike Leach wrote:
> > > Hi Tingwei,
> > >
> > > On Thu, 23 Jul 2020 at 05:28, Tingwei Zhang <[email-masked]>
> > > wrote:
> > > >
> > > > When coresight device is in an active session, driver module of
> > > > that device should not be removed. Use try_get_module() in
> > > > coresight_grab_device() to prevent module to be unloaded.
> > > >
> > > > Signed-off-by: Tingwei Zhang <[email-masked]>
> > > > ---
> > > >  drivers/hwtracing/coresight/coresight.c | 27
> +++++++++++++++++++++----
> > > >  1 file changed, 23 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/drivers/hwtracing/coresight/coresight.c
> > > b/drivers/hwtracing/coresight/coresight.c
> > > > index b7151c5f81b1..17bc76ea86ae 100644
> > > > --- a/drivers/hwtracing/coresight/coresight.c
> > > > +++ b/drivers/hwtracing/coresight/coresight.c
> > > > @@ -640,7 +640,7 @@ struct coresight_device
> > > *coresight_get_sink_by_id(u32 id)
> > > >   * don't appear on the trace path, they should be handled along
> with
> > > the
> > > >   * the master device.
> > > >   */
> > > > -static void coresight_grab_device(struct coresight_device *csdev)
> > > > +static int coresight_grab_device(struct coresight_device *csdev)
> > > >  {
> > > >         int i;
> > > >
> > > > @@ -648,10 +648,25 @@ static void coresight_grab_device(struct
> > > coresight_device *csdev)
> > > >                 struct coresight_device *child;
> > > >
> > > >                 child  = csdev->pdata->conns[i].child_dev;
> > > > -               if (child && child->type ==
> CORESIGHT_DEV_TYPE_HELPER)
> > > > +               if (child && child->type ==
> CORESIGHT_DEV_TYPE_HELPER) {
> > > > +                       if
> > > (!try_module_get(child->dev.parent->driver->owner))
> > > > +                               goto err;
> > > >                         pm_runtime_get_sync(child->dev.parent);
> > > > +               }
> > > >         }
> > > > +       if (!try_module_get(csdev->dev.parent->driver->owner))
> > > > +               goto err;
> > > >         pm_runtime_get_sync(csdev->dev.parent);
> > > > +       return 0;
> > > > +err:
> > > > +       for (i--; i >= 0; i--) {
> > > > +               struct coresight_device *child;
> > > > +
> > > > +               child  = csdev->pdata->conns[i].child_dev;
> > > > +               if (child && child->type ==
> CORESIGHT_DEV_TYPE_HELPER)
> > > > +                       pm_runtime_put(child->dev.parent);
> > > > +       }
> > > > +       return -ENODEV;
> > > >  }
> > > >
> > > >  /*
> > > > @@ -663,12 +678,15 @@ static void coresight_drop_device(struct
> > > coresight_device *csdev)
> > > >         int i;
> > > >
> > > >         pm_runtime_put(csdev->dev.parent);
> > > > +       module_put(csdev->dev.parent->driver->owner);
> > > >         for (i = 0; i < csdev->pdata->nr_outport; i++) {
> > > >                 struct coresight_device *child;
> > > >
> > > >                 child  = csdev->pdata->conns[i].child_dev;
> > > > -               if (child && child->type ==
> CORESIGHT_DEV_TYPE_HELPER)
> > > > +               if (child && child->type ==
> CORESIGHT_DEV_TYPE_HELPER) {
> > > >                         pm_runtime_put(child->dev.parent);
> > > > +
> module_put(child->dev.parent->driver->owner);
> > > > +               }
> > > >         }
> > > >  }
> > > >
> > > > @@ -721,7 +739,8 @@ static int _coresight_build_path(struct
> > > coresight_device *csdev,
> > > >         if (!node)
> > > >                 return -ENOMEM;
> > > >
> > > > -       coresight_grab_device(csdev);
> > > > +       if (coresight_grab_device(csdev))
> > > > +               return -ENODEV;
> > > >         node->csdev = csdev;
> > > >         list_add(&node->link, path);
> > > >
> > > > --
> > > > The Qualcomm Innovation Center, Inc. is a member of the Code Aurora
> > > Forum,
> > > > a Linux Foundation Collaborative Project
> > > >
> > >
> > > The CTI devices are associated with other coresight components using
> > > csdev->ect_dev; These are not handled in the main linear trace path as
> > > these have a star topology interlinking all components.
> > > However, when a component has an associated ect_dev then is enabled at
> > > the same time as the other component is enabled.
> > >
> > > So a module_get/put will be needed for this association to prevent the
> > > CTI being removed while a trace session is active.
> > > I suggest in coresight.c : coresight_control_assoc_ectdev()
> > >
> >
> > In module unload process, devices of that module will be removed. In
> > this case, cti_remove() is called on all cti device when unloading
> > cti module. All cti connections is cleaned at that time.
> > csdev->ect_dev is set to NULL. coresight_control_assoc_ectdev()
> > will return since since ect_csdev is NULL.
> >
> > I think we are safe without holding module reference since cti
> > driver has done a pretty good job on clean up.
> >
> 
> The issue here is not about clean up. We need to keep all the
> programmed coresight modules loaded and operational while a coresight
> trace session is in progress. The CTI can be used to control the
> generation of trace, trigger trace related events etc.
> If the module is removed while a session is in progress then this
> programming is lost and the trace data collected may not be what is
> expected.
>

Got your point now. In my opinion, CTI is kinda different to other
coresight components since it's optional.

For other components, they have to been there when path is built or
the enablement fails. Use can either get a successfully return which
means it's good and all devices/modules are there until path is
released, or a failed return which means trace session can't be
setup.

In CTI case, there's no garrantee that CTI related to the trace
session is there even the enablement is successful. For example, 
One cti is required for ETM tracing session. If cti module is
unloaded before enable that trace session. The enablement will
still be successfully done since there's no ect_dev connected
at all.

My point is holding cti module reference in enable path won't
fix all the problem. Altenatively, user should be aware that
unload cti module leads to all cti functioniol failure.
With current design, the behavior is consistant on cti. User
can unload cti module whenever he wants but it will break
the function.

Thanks, Tingwei
 
> Regards
> 
> Mike
> 
> > Let me know if you still think we need hold the module reference.
> >
> > Thanks, Tingwei
> > > Regards
> > >
> > > Mike
> > >
> > >
> > > --
> > > Mike Leach
> > > Principal Engineer, ARM Ltd.
> > > Manchester Design Centre. UK
> > >
> > > _______________________________________________
> > > linux-arm-kernel mailing list
> > > [email-masked]
> > > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
> 
> 
> 
> -- 
> Mike Leach
> Principal Engineer, ARM Ltd.
> Manchester Design Centre. UK
> 
> _______________________________________________
> linux-arm-kernel mailing list
> [email-masked]
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

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