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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, Jun 06, 2018 at 03:55:01PM -0500, Kim Phillips wrote:
> 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);
> +

Locks are to protect data, not code, be careful here please.

That's the big issue with the module reference counting, it "protects"
code, not data.  If at all possible, never grab a module reference
count, as you should always be able to unload a module, unless you have
a file handle open, and if you have that, the kernel core will properly
protect you.

thanks,

greg k-h