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

From: Tingwei Zhang <[email-masked]>
Date: Sat, 25 Jul 2020 18:05:16 +0800

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.

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
> 
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