Re: [PATCH v4 06/20] coresight: add try_get_module() in coresight_grab_device()

From: Mathieu Poirier <mathieu.poirier@linaro.org>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: tsoni@codeaurora.org,
	Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>,
	Kim Phillips <kim.phillips@arm.com>,
	Mao Jinlong <jinlmao@codeaurora.org>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	coresight@lists.linaro.org, Randy Dunlap <rdunlap@infradead.org>,
	Mian Yousaf Kaukab <ykaukab@suse.de>,
	Russell King <linux@armlinux.org.uk>,
	Tingwei Zhang <tingwei@codeaurora.org>,
	Leo Yan <leo.yan@linaro.org>,
	linux-arm-kernel@lists.infradead.org,
	Mike Leach <mike.leach@linaro.org>
Subject: Re: [PATCH v4 06/20] coresight: add try_get_module() in coresight_grab_device()
Date: Thu, 23 Jul 2020 13:15:40 -0600	[thread overview]
Message-ID: <20200723191540.GB1323936@xps15> (raw)
In-Reply-To: <20200723181837.GA2944775@kroah.com>

On Thu, Jul 23, 2020 at 08:18:37PM +0200, Greg Kroah-Hartman wrote:
> On Thu, Jul 23, 2020 at 12:04:47PM -0600, Mathieu Poirier wrote:
> > Hi Greg,
> > 
> > On Thu, Jul 23, 2020 at 01:07:07PM +0200, Greg Kroah-Hartman wrote:
> > > On Thu, Jul 23, 2020 at 12:27:48PM +0800, Tingwei Zhang wrote:
> > > > When coresight device is in an active session, driver module of
> > > > that device should not be removed. Use try_get_module() in
> > > > coresight_grab_device() to prevent module to be unloaded.
> > > 
> > > Are you sure this works?  Why is it needed at all?  Why not just tear
> > > down the children properly when a module is removed so that you don't
> > > need this at all?
> > 
> > Using the terms parent and child is somewhat ambiguous...  This is not a
> > parent-child relationship but simply an association between devices, something
> > like port 1 on device "parent" is connected to port 2 on device "child".  The
> > parent-child nomenclature was chosen to reflect that a device appears before
> > another in a coresight path.  Otherwise there is no other relation between
> > devices, hence the choice of using try_get_module()/put_module() to prevent
> > drivers from being taken away.  I'd be happy to proceed differently but haven't
> > found better options.
> > 
> > Going back to parent/child, we could have chosen left/right, up/down or A/B, all
> > of which are just as confusion. 
> 
> Ok, thanks.
> 
> But this causes confusion for everyone as seen below:
> 
> > > > Signed-off-by: Tingwei Zhang <tingwei@codeaurora.org>
> > > > ---
> > > >  drivers/hwtracing/coresight/coresight.c | 27 +++++++++++++++++++++----
> > > >  1 file changed, 23 insertions(+), 4 deletions(-)
> > > > 
> > > > diff --git a/drivers/hwtracing/coresight/coresight.c b/drivers/hwtracing/coresight/coresight.c
> > > > index b7151c5f81b1..17bc76ea86ae 100644
> > > > --- a/drivers/hwtracing/coresight/coresight.c
> > > > +++ b/drivers/hwtracing/coresight/coresight.c
> > > > @@ -640,7 +640,7 @@ struct coresight_device *coresight_get_sink_by_id(u32 id)
> > > >   * don't appear on the trace path, they should be handled along with the
> > > >   * the master device.
> > > >   */
> > > > -static void coresight_grab_device(struct coresight_device *csdev)
> > > > +static int coresight_grab_device(struct coresight_device *csdev)
> > > >  {
> > > >  	int i;
> > > >  
> > > > @@ -648,10 +648,25 @@ static void coresight_grab_device(struct coresight_device *csdev)
> > > >  		struct coresight_device *child;
> > > >  
> > > >  		child  = csdev->pdata->conns[i].child_dev;
> > > > -		if (child && child->type == CORESIGHT_DEV_TYPE_HELPER)
> > > > +		if (child && child->type == CORESIGHT_DEV_TYPE_HELPER) {
> > > > +			if (!try_module_get(child->dev.parent->driver->owner))
> > > 
> > > Why the child's parent?  Why not the child itself?
> > 
> > The device structure of each coresight_device is not associated with a driver.
> > It is there to take advantages of device goodies such as dev.type, dev.group,
> > dev.release and dev.bus.  Coresight IP blocks are discovered on the AMBA bus and as
> > such amba_device::dev::driver holds the driver itself.  In coresight_register()
> > the association coresigth::dev::parent = amba_device::dev is made.
> > 
> > > 
> > > 
> > > > +				goto err;
> > > 
> > > What about the error given to you here?  Why throw that away?
> > > 
> > > >  			pm_runtime_get_sync(child->dev.parent);
> > > > +		}
> > > >  	}
> > > > +	if (!try_module_get(csdev->dev.parent->driver->owner))
> > > > +		goto err;
> > > 
> > > You don't reduce the child's parent's driver owner module reference
> > > here?
> > 
> > Here @parent is referencing the current device.  Now that helper devices
> > connected to any of its outgoing ports have been enabled (and a reference count 
> > to the helper device driver incremented), a reference count to the current device
> > driver can also be incremented. 
> 
> I mean the fact that your error handling does not seem to roll back the
> module reference count you got up above in the other loop.

Ah! You were talking about the error condition, while I thought you were
referring to the normal execution path.  I am in agreement with all your comments on error
handling. 

> 
> Or if it does, it's really really not obvious, and should at the very
> least, be commented as to how it's all cleaning up properly.
> 
> thanks,
> 
> greg k-h

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