linux-kernel - Re: [PATCH v4 05/14] coresight: get/put module in coresight_build/release

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Date:   Wed, 6 Jun 2018 10:46:36 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kim Phillips <kim.phillips@....com>
Cc:     Mathieu Poirier <mathieu.poirier@...aro.org>,
        Leo Yan <leo.yan@...aro.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>,
        Eric Auger <eric.auger@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Gargi Sharma <gs051095@...il.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Rik van Riel <riel@...hat.com>,
        Robin Murphy <robin.murphy@....com>,
        Russell King <linux@...linux.org.uk>,
        Thierry Reding <treding@...dia.com>,
        Todd Kjos <tkjos@...gle.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 05/14] coresight: get/put module in
 coresight_build/release_path

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

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
>