On 2020-07-22 18:48, Greg KH wrote:
On Wed, Jul 22, 2020 at 11:49:48AM +0100, Suzuki K Poulose wrote:
Hi Tingwei,
On 07/17/2020 06:45 AM, 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.
Is this really sufficient ? AFAIU, a device could be removed, but the module may still be alive due to the refcount on the module. This could imply that we have stale pointers in the _path_, which could lead to corruption elsewhere. Should we do a get/put_device() instead ?
Remember there are two separate things here, code and data. There are two different reference counts for them, do not confuse the two.
get/put is needed when you have a reference to the data, module stuff is when you are calling into code.
But note that you do not always need to grab a reference count to the module, as long as the module can properly tear the data down when it is asked to be removed. Look at networking drivers as a great example of that.
thanks,
greg k-h
Hi Greg,
Understand your point. I made the attempt to not hold the refcount of module but stop/clean up active session in v1 of this series. Link is as below. https://lore.kernel.org/linux-arm-kernel/20200701071427.10477-1-tingwei@code...
However, there's a lot of tricks to play to make it work especially on perf trace when trace session is dynamically created/start/stop by perf framework. It may have performance overhead as well.
With the suggestion from Mathieu, I'm wondering whether this complexity introduced really worth. Remove a coresight module driver with active trace session ongoing is a corner case. From v2, I turned to another alternative to grad a reference count to module when the operation function in that module could be called by coresight framework. That's a simpler and cleaner solution in my opinion.
Thanks, Tingwei
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