linux-kernel - Re: [PATCH v3 11/29] media: iris: implement reqbuf ioctl with vb2_queue

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Message-ID: <62c8d25f-b8d2-4364-94e8-4de33b61a52b@linaro.org>
Date: Fri, 6 Sep 2024 14:05:01 +0100
From: Bryan O'Donoghue <bryan.odonoghue@...aro.org>
To: quic_dikshita@...cinc.com, Vikash Garodia <quic_vgarodia@...cinc.com>,
 Abhinav Kumar <quic_abhinavk@...cinc.com>,
 Mauro Carvalho Chehab <mchehab@...nel.org>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Philipp Zabel <p.zabel@...gutronix.de>,
 quic_dikshita@...cinc.com, Vikash Garodia <quic_vgarodia@...cinc.com>,
 Abhinav Kumar <quic_abhinavk@...cinc.com>,
 Mauro Carvalho Chehab <mchehab@...nel.org>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Philipp Zabel <p.zabel@...gutronix.de>
Cc: linux-media@...r.kernel.org, linux-arm-msm@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 11/29] media: iris: implement reqbuf ioctl with
 vb2_queue_setup

On 06/09/2024 13:50, Bryan O'Donoghue wrote:
>> +
>> +        ret = core->hfi_ops->session_open(inst);
>> +        if (ret) {
>> +            ret = -EINVAL;
>> +            dev_err(core->dev, "session open failed\n");
>> +            goto unlock;
>> +        }
> 
> I don't understand the lifetime of the core->lock mutex here.
> 
> It has verified the state as !ISIR_CORE_ERROR and then _released_ the 
> lock so by the time you get to core->hfi_ops->session_open() you've not 
> guaranteed the state at all.
> 
> Shouldn't you continue to hold the core mutex for the duration of the 
> core->does_stuff() operation ?
> 
> i.e. the state was not !IRIS_CORE_ERROR at an indeterminate time prior 
> to the next use of core-> ...
> 
> Perhaps this is all very obvious but, I'm not immediately understanding 
> what the mutex gurantees nor for how long it does that.

You'd probably be better off

- taking the mutex at the external facing API
- validating state if you must
- doing all of your core ops
- dropping

If I'm interpreting your code right, there's alot of checking state in 
function a -> lock/check/unlock with function a then calling function b 
- which again verifies core->state and then optionally modifies say the 
linked list.

But since function b is called by function a, and function b requires 
the core->lock - you may as well have held that lock from a through b.

Moreover - what's the use case of the very granular core->stat lock 
checking ?

When is it valid for example for iris_vb2_queue_setup() to have 
core->state change state during the lifetime of iris_vb2_queue_setup() ?

iris_vb2_queue_setup() checks core->state
	-> locks - checks - release
		-> calls iris_hfi_gen1_session_open or
		         iris_hfi_gen2_session_open
			-> what is the assumed core->state @ that point?

So that's what I mean, I'm not immediately understanding why this 
granular locking scheme is in use, seems way, way, way too granular ?

---
bod