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

[Kim Phillips <kim.phillips@arm.com>]

On Wed, 6 Jun 2018 10:46:36 +0100
Suzuki K Poulose <suzuki.poulose@arm.com> wrote:

> 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@linaro.org>
> >> Cc: Leo Yan <leo.yan@linaro.org>
> >> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> >> Cc: Randy Dunlap <rdunlap@infradead.org>
> >> Cc: Suzuki K Poulose <Suzuki.Poulose@arm.com>
> >> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> >> Cc: Russell King <linux@armlinux.org.uk>
> >> Signed-off-by: Kim Phillips <kim.phillips@arm.com>
> >> ---
> >>   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.

OK, I'll add this for the next version:

diff --git a/drivers/hwtracing/coresight/coresight-core.c b/drivers/hwtracing/coresight/coresight-core.c
index f96258de1e9b..da702507a55c 100644
--- a/drivers/hwtracing/coresight/coresight-core.c
+++ b/drivers/hwtracing/coresight/coresight-core.c
@@ -1040,8 +1040,12 @@ EXPORT_SYMBOL_GPL(coresight_register);
 
 void coresight_unregister(struct coresight_device *csdev)
 {
+       mutex_lock(&coresight_mutex);
+
        /* Remove references of that device in the topology */
        coresight_remove_conns(csdev);
        device_unregister(&csdev->dev);
+
+       mutex_unlock(&coresight_mutex);
 }
 EXPORT_SYMBOL_GPL(coresight_unregister);

> 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.

Would this fix that?:

diff --git a/drivers/hwtracing/coresight/of_coresight.c b/drivers/hwtracing/coresight/of_coresight.c
index a33a92ebe74b..a43ab078c85e 100644
--- a/drivers/hwtracing/coresight/of_coresight.c
+++ b/drivers/hwtracing/coresight/of_coresight.c
@@ -181,6 +181,8 @@ of_get_coresight_platform_data(struct device *dev,
                        pdata->child_names[i] = dev_name(rdev);
                        pdata->child_ports[i] = rendpoint.id;
 
+                       put_device(rdev);
+
                        i++;
                } while (ep);
        }

Thanks,

Kim