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

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Date:   Wed, 6 Jun 2018 10:24:22 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Kim Phillips <kim.phillips@....com>
Cc:     Mathieu Poirier <mathieu.poirier@...aro.org>,
        Leo Yan <leo.yan@...aro.org>,
        Suzuki K Poulose <Suzuki.Poulose@....com>,
        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 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.

thanks,

greg k-h