linux-kernel - Re: [PATCH] coresight: Defer probe when the child dev is not probed

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Date:   Wed, 2 Mar 2022 10:29:06 +0800
From:   Jinlong Mao <quic_jinlmao@...cinc.com>
To:     Suzuki K Poulose <suzuki.poulose@....com>,
        Mike Leach <mike.leach@...aro.org>
CC:     Mathieu Poirier <mathieu.poirier@...aro.org>,
        Leo Yan <leo.yan@...aro.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Mao Jinlong <jinlmao@....qualcomm.com>,
        <coresight@...ts.linaro.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-arm-msm@...r.kernel.org>,
        "Tingwei Zhang" <quic_tingweiz@...cinc.com>,
        Yuanfang Zhang <quic_yuanfang@...cinc.com>,
        Tao Zhang <quic_taozha@...cinc.com>,
        Hao Zhang <quic_hazha@...cinc.com>
Subject: Re: [PATCH] coresight: Defer probe when the child dev is not probed


On 3/1/2022 11:03 PM, Suzuki K Poulose wrote:
> Hi
>
> On 01/03/2022 13:30, Jinlong Mao wrote:
>> Hi Mike,
>>
>> On 3/1/2022 9:15 PM, Mike Leach wrote:
>>> Hi,
>>>
>>> On Tue, 1 Mar 2022 at 11:42, Jinlong Mao <quic_jinlmao@...cinc.com> 
>>> wrote:
>>>> On 2/28/2022 10:51 PM, Suzuki K Poulose wrote:
>>>>
>
> ...
>
>>>>
>>>> Hi Suzuki,
>>>>
>>>> This issue happens when race condition happens.
>>>> The condition is that the device and its child_device's probe 
>>>> happens at the same time.
>>>>
>>>> For example: device0 and its child device device1.
>>>> Both of them are calling coresight_register function. device0 is 
>>>> calling coresight_fixup_device_conns.
>>>> device1 is waiting for device0 to release the coresight_mutex. 
>>>> Because device1's csdev node is allocated,
>>>> coresight_make_links will be called for device0. Then in 
>>>> coresight_add_sysfs_link, has_conns_grp is true
>>>> for device0, but has_conns_grp is false for device1 as 
>>>> has_conns_grp is set to true in coresight_create_conns_sysfs_group .
>>>> The probe of device0 will fail for at this condition.
>>>>
>>>>
>>>> struct coresight_device *coresight_register(struct coresight_desc 
>>>> *desc)
>>>> {
>>>>     .........
>>>>      mutex_lock(&coresight_mutex);
>>>>
>>>>      ret = coresight_create_conns_sysfs_group(csdev);
>>>>      if (!ret)
>>>>          ret = coresight_fixup_device_conns(csdev);
>>>>      if (!ret)
>>>>          ret = coresight_fixup_orphan_conns(csdev);
>>>>      if (!ret && cti_assoc_ops && cti_assoc_ops->add)
>>>>          cti_assoc_ops->add(csdev);
>>>>
>>>>      mutex_unlock(&coresight_mutex);
>>>>
>>>> .........
>>>>
>>>> }
>>>>
>>>> static int coresight_fixup_device_conns(struct coresight_device 
>>>> *csdev)
>>>> {
>>>>     ..........
>>>>          conn->child_dev =
>>>> coresight_find_csdev_by_fwnode(conn->child_fwnode);
>>> The issue appears to be a constraint hidden in the lower layers of 
>>> the code.
>>> Would a better solution not be to alter the code here:
>>>
>>> if (conn->child_dev && conn->child_dev->has_conns_grp) {
>>>     ...
>>> } else {
>>>        csdev->orphan = true;
>>> }
>>>
>>> which would mean that the connection attempt would drop through to
>>> label the connection as an orphan, to be cleaned up by the child
>>> itself when it runs coresight_fixup_orphan_conns()
>>>
>
> Tnanks Mike, I think that is a good solution. Alternatively, we
> could make sure that device_register() and the fixup following
> that are atomic.
>
> i.e.
>
>     mutex_lock()
>
>     device_register()
>     fixup_connections()
>     create_sysfs()
>
>     mutex_unlock();
>
> The fix may be a bit invasive than Mike's proposal, but it makes
> sure we don't end up with half baked device on the coresight-bus.
>
> Suzuki

Thanks Mike & Suzuki.

I will combine your proposals and make the changes.

I will get back to you after the test.


Thanks
Jinlong Mao