Re: [PATCH v4 05/14] coresight: get/put module in coresight_build/release_path

[Suzuki K Poulose]

On 06/06/2018 09:24 AM, Greg Kroah-Hartman wrote:
> On Tue, Jun 05, 2018 at 04:07:01PM -0500, Kim Phillips wrote:
> > Increment the refcnt for driver modules in current use by calling
> > module_get in coresight_build_path and module_put in release_path.
> >
> > This prevents driver modules from being unloaded when they are in use,
> > either in sysfs or perf mode.
>
> Why does it matter?  Shouldn't you be allowed to remove any module at
> any point in time, much like a networking driver?
>
>
> > Cc: Mathieu Poirier <mathieu.poirier@xxxxxxxxxx>
> > Cc: Leo Yan <leo.yan@xxxxxxxxxx>
> > Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> > Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
> > Cc: Suzuki K Poulose <Suzuki.Poulose@xxxxxxx>
> > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > Cc: Russell King <linux@xxxxxxxxxxxxxxx>
> > Signed-off-by: Kim Phillips <kim.phillips@xxxxxxx>
> > ---
> >   drivers/hwtracing/coresight/coresight.c | 9 +++++++++
> >   1 file changed, 9 insertions(+)
> >
> > diff --git a/drivers/hwtracing/coresight/coresight.c b/drivers/hwtracing/coresight/coresight.c
> > index 338f1719641c..1c941351f1d1 100644
> > --- a/drivers/hwtracing/coresight/coresight.c
> > +++ b/drivers/hwtracing/coresight/coresight.c
> > @@ -465,6 +465,12 @@ static int _coresight_build_path(struct coresight_device *csdev,
> >   
> >   	node->csdev = csdev;
> >   	list_add(&node->link, path);
> > +
> > +	if (!try_module_get(csdev->dev.parent->driver->owner)) {
>
> What is to keep parent->driver from going away right here?  What keeps
> parent around?  This feels very fragile to me, I don't see any locking
> anywhere around this code path to try to keep things in place.

You're right. We do have coresight_mutex, which is held across the build 
path and the csdev is removed when a device is unregistered. However, I
see that we don't hold the mutex while removing the connections from
coresight_unregister(). Holding the mutex should protect us from the
csdev being removed, while we build the path.

And while we are at this, I also realised that we hold references to the
parent devices for each connection (via bus_find_device() from 
of_coresight_get_endpoint_device()), while parsing the platform data, 
which is never released.

Thanks
Suzuki

> thanks,
>
> greg k-h