[PATCH v4 06/20] coresight: add try_get_module() in coresight_grab_device()
Mike Leach
mike.leach at linaro.org
Mon Jul 27 13:04:21 EDT 2020
Hi Tingwei,
On Sun, 26 Jul 2020 at 02:32, Tingwei Zhang <tingweiz at codeaurora.org> wrote:
>
> Hi Mike,
> On Sat, Jul 25, 2020 at 09:51:31PM +0800, Mike Leach wrote:
> > Hi Tingwei,
> >
> > On Sat, 25 Jul 2020 at 11:05, Tingwei Zhang <tingweiz at codeaurora.org>
> > wrote:
> > >
> > > Hi Mike,
> > >
> > > Thu, Jul 23, 2020 at 06:55:47PM +0800, Mike Leach wrote:
> > > > Hi Tingwei,
> > > >
> > > > On Thu, 23 Jul 2020 at 05:28, Tingwei Zhang <tingwei at codeaurora.org>
> > > > 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.
> > > > >
> > > > > Signed-off-by: Tingwei Zhang <tingwei at 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))
> > > > > + goto err;
> > > > > pm_runtime_get_sync(child->dev.parent);
> > > > > + }
> > > > > }
> > > > > + if (!try_module_get(csdev->dev.parent->driver->owner))
> > > > > + goto err;
> > > > > pm_runtime_get_sync(csdev->dev.parent);
> > > > > + return 0;
> > > > > +err:
> > > > > + for (i--; i >= 0; i--) {
> > > > > + struct coresight_device *child;
> > > > > +
> > > > > + child = csdev->pdata->conns[i].child_dev;
> > > > > + if (child && child->type ==
> > CORESIGHT_DEV_TYPE_HELPER)
> > > > > + pm_runtime_put(child->dev.parent);
> > > > > + }
> > > > > + return -ENODEV;
> > > > > }
> > > > >
> > > > > /*
> > > > > @@ -663,12 +678,15 @@ static void coresight_drop_device(struct
> > > > coresight_device *csdev)
> > > > > int i;
> > > > >
> > > > > pm_runtime_put(csdev->dev.parent);
> > > > > + module_put(csdev->dev.parent->driver->owner);
> > > > > for (i = 0; i < csdev->pdata->nr_outport; i++) {
> > > > > 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) {
> > > > > pm_runtime_put(child->dev.parent);
> > > > > +
> > module_put(child->dev.parent->driver->owner);
> > > > > + }
> > > > > }
> > > > > }
> > > > >
> > > > > @@ -721,7 +739,8 @@ static int _coresight_build_path(struct
> > > > coresight_device *csdev,
> > > > > if (!node)
> > > > > return -ENOMEM;
> > > > >
> > > > > - coresight_grab_device(csdev);
> > > > > + if (coresight_grab_device(csdev))
> > > > > + return -ENODEV;
> > > > > node->csdev = csdev;
> > > > > list_add(&node->link, path);
> > > > >
> > > > > --
> > > > > The Qualcomm Innovation Center, Inc. is a member of the Code Aurora
> > > > Forum,
> > > > > a Linux Foundation Collaborative Project
> > > > >
> > > >
> > > > The CTI devices are associated with other coresight components using
> > > > csdev->ect_dev; These are not handled in the main linear trace path as
> > > > these have a star topology interlinking all components.
> > > > However, when a component has an associated ect_dev then is enabled at
> > > > the same time as the other component is enabled.
> > > >
> > > > So a module_get/put will be needed for this association to prevent the
> > > > CTI being removed while a trace session is active.
> > > > I suggest in coresight.c : coresight_control_assoc_ectdev()
> > > >
> > >
> > > In module unload process, devices of that module will be removed. In
> > > this case, cti_remove() is called on all cti device when unloading
> > > cti module. All cti connections is cleaned at that time.
> > > csdev->ect_dev is set to NULL. coresight_control_assoc_ectdev()
> > > will return since since ect_csdev is NULL.
> > >
> > > I think we are safe without holding module reference since cti
> > > driver has done a pretty good job on clean up.
> > >
> >
> > The issue here is not about clean up. We need to keep all the
> > programmed coresight modules loaded and operational while a coresight
> > trace session is in progress. The CTI can be used to control the
> > generation of trace, trigger trace related events etc.
> > If the module is removed while a session is in progress then this
> > programming is lost and the trace data collected may not be what is
> > expected.
> >
>
> Got your point now. In my opinion, CTI is kinda different to other
> coresight components since it's optional.
>
> For other components, they have to been there when path is built or
> the enablement fails. Use can either get a successfully return which
> means it's good and all devices/modules are there until path is
> released, or a failed return which means trace session can't be
> setup.
>
The module get/put do more than ensure that the system can be enabled.
They ensure that no components in the coresight path can be unloaded
until all paths in use are released. This ensures that clients do not
get the trace system pulled out from underneath - corrupting trace
data, and possibly affecting the client program.
> In CTI case, there's no garrantee that CTI related to the trace
> session is there even the enablement is successful.
And there is no guarantee that it is not programmed to perform a key
task in controlling the generation of trace.
The entire programmed coresight system must be protected when in use.
That includes CTI devices. Otherwise the integrity of the captured
trace could be compromised, and indeed there may be no trace captured
if this is relying on CTI triggers.
>For example,
> One cti is required for ETM tracing session. If cti module is
> unloaded before enable that trace session. The enablement will
> still be successfully done since there's no ect_dev connected
> at all.
>
> My point is holding cti module reference in enable path won't
> fix all the problem. Altenatively, user should be aware that
> unload cti module leads to all cti functioniol failure.
> With current design, the behavior is consistant on cti. User
> can unload cti module whenever he wants but it will break
> the function.
Precisely - and this is what should never be allowed - why can the CTI
be used to break the trace system but not other components?
Moving forwards we will see users who simply use pre-programmed
settings from client programs such as perf. It may be that some users
do not have the credentials to add and remove modules - this could be
an admin function where larger systems with multiple sessions are in
use.
Regards
Mike
>
> Thanks, Tingwei
>
> > Regards
> >
> > Mike
> >
> > > Let me know if you still think we need hold the module reference.
> > >
> > > Thanks, Tingwei
> > > > Regards
> > > >
> > > > Mike
> > > >
> > > >
> > > > --
> > > > Mike Leach
> > > > Principal Engineer, ARM Ltd.
> > > > Manchester Design Centre. UK
> > > >
> > > > _______________________________________________
> > > > linux-arm-kernel mailing list
> > > > linux-arm-kernel at lists.infradead.org
> > > > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
> >
> >
> >
> > --
> > Mike Leach
> > Principal Engineer, ARM Ltd.
> > Manchester Design Centre. UK
> >
> > _______________________________________________
> > linux-arm-kernel mailing list
> > linux-arm-kernel at lists.infradead.org
> > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
--
Mike Leach
Principal Engineer, ARM Ltd.
Manchester Design Centre. UK
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