linux-kernel - Re: [PATCH V7 02/24] coresight: associating path with session rather than tracer

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Message-ID: <CANLsYkxSchGRuAqB=-F6UsdsgO_ke=de4V3ur1Zipk4J-80bjQ@mail.gmail.com>
Date:	Tue, 19 Jan 2016 16:54:07 -0700
From:	Mathieu Poirier <mathieu.poirier@...aro.org>
To:	Rabin Vincent <rabin@....in>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH V7 02/24] coresight: associating path with session rather
 than tracer

On 16 January 2016 at 06:54, Rabin Vincent <rabin@....in> wrote:
> On Wed, Jan 13, 2016 at 02:47:10PM -0700, Mathieu Poirier wrote:
>> On 10 January 2016 at 08:43, Rabin Vincent <rabin@....in> wrote:
>> >   - There is no reference taken for the csdev when it is assigned as
>> >     some other device's child_dev in coresight_orphan_match()
>>
>> The only time a device shouldn't be removed is when it is part of an
>> active trace session, something that shouldn't happen due to the
>> static nature of the coresight devices.
>
> # echo 1 > /sys/bus/coresight/devices/54162000.etb/enable_sink
> # echo 1 > /sys/bus/coresight/devices/5414c000.ptm/enable_source
> [   22.853637] coresight-etb10 54162000.etb: ETB enabled
> [   22.859069] coresight-funnel 54164000.funnel: FUNNEL inport 0 enabled
> [   22.865814] coresight-replicator 44000000.ocp:replicator: REPLICATOR enabled
> [   22.873260] coresight-funnel 54158000.funnel: FUNNEL inport 0 enabled
> [   22.880187] coresight-etm3x 5414c000.ptm: ETM tracing enabled
> # echo 44000000.ocp\:replicator > /sys/bus/platform/drivers/coresight-replicator/unbind
> # echo 0 > /sys/bus/coresight/devices/5414c000.ptm/enable_source
> [   37.261840] coresight-etm3x 5414c000.ptm: ETM tracing disabled
> [   37.268066] Unable to handle kernel NULL pointer dereference at virtual address 0000023c
> [   37.276611] pgd = ec60c000
> [   37.279571] [0000023c] *pgd=ac5d6831, *pte=00000000, *ppte=00000000
> [   37.286163] Internal error: Oops: 817 [#1] SMP ARM
> [   37.291198] Modules linked in:
> [   37.294403] CPU: 0 PID: 857 Comm: sh Not tainted 4.4.0-rc8-00063-g15ed5a7c725c #196

That is a problem of a totally different nature - no amount of
refcount management will prevent this splat.  As you highlighted here
the framework is not ready to deal with devices disappearing from a
path - I need to ponder on that.

Mathieu