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

From: Suzuki K Poulose
Date: Thu Jun 07 2018 - 05:34:57 EST

On 06/07/2018 10:32 AM, Suzuki K Poulose wrote:
On 06/07/2018 10:13 AM, Greg Kroah-Hartman wrote:
On Thu, Jun 07, 2018 at 10:04:33AM +0100, Suzuki K Poulose wrote:
Hi Greg,

On 06/07/2018 09:34 AM, Greg Kroah-Hartman wrote:
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@xxxxxxx> 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?

The user doesn't have an explicit refcount on the individual components
in a trace session. So, when a trace session is in progress, it is as
good as having a "file" open on each component that is part of the
active trace session. So, we don't want the driver to be removed when
the component is being used in the trace collection.

Why not? What's wrong with that happening and then the trace collection
starts failing with -ENODEV or something?

Forgot to add, this will indeed hit -ENODEV, if the device driver was
removed, as we fail to build the trace path before the session.

May be I am missing something here. Can we allow the driver to be removed when one of its device is "turned ON" and we need the same
driver to "turn it OFF" when the session ends ? To make a better
comparison :

Can we unload a usb_mass_storage module when a USB disk(which uses the module driver) is mounted and is being used ? I believe, the module
will eventually get unloaded when we unmount the disk, if someone did
a unload.

We have a similar situation here. The only difference is the driver is
referenced only when one of its device is in a trace session.

Remember, removing a kernel module is something that only happens very
rarely, and is an explicit choice by someone with root permissions. If
you want to remove that module, it should be able to go, as you know
what you are doing at that point in time.

Right, but when a device is "in use" can we do that ? I thought the user
will get a module is in use or busy, error.

Don't try to "protect the user from themselves" here, they want to shoot
their foot, make it hurt if they are aiming it there :)

The module_get/put added here are only triggered when we start a trace session, where we build a path for the current session from the configured "source" to the configured "sink" and the path is destroyed
at the end of the trace session. i.e, the path is not a permanent thing.
It is constructed per session. So it is perfectly possible to remove a
device in between trace sessions.

This will be
released as soon as the session is ended. It is just like a PMU driver
where the module refcount is held to ensure the module stays until the
session is over. In this case, we have multiple components, each with
its own driver invisible to the PMU driver. Hence the coresight driver
must hold the reference.

Again, please think this through and don't add extra complexity to the
normal path, and get it right if you do it (the existing patch is not
right as I pointed out.)Â Personally, I feel the code should just be
able to be unloaded whenever they want, user beware...

Sure, will explore more to refine the code. Thanks for the trigger.