linux-kernel - Re: [PATCH v2] interconnect: avoid memory allocation when 'icc_bw

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Message-ID: <90bfae80-f3d3-4c1e-9a5c-9f8205bf90b9@gmail.com>
Date: Thu, 19 Jun 2025 15:03:50 +0200
From: Gabor Juhos <j4g8y7@...il.com>
To: Johan Hovold <johan@...nel.org>
Cc: Georgi Djakov <djakov@...nel.org>,
 Raviteja Laggyshetty <quic_rlaggysh@...cinc.com>,
 Johan Hovold <johan+linaro@...nel.org>,
 Bryan O'Donoghue <bryan.odonoghue@...aro.org>, linux-pm@...r.kernel.org,
 linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] interconnect: avoid memory allocation when
 'icc_bw_lock' is held

2025. 06. 19. 12:07 keltezéssel, Johan Hovold írta:
> On Wed, Jun 18, 2025 at 09:58:31PM +0200, Gabor Juhos wrote:
>> The 'icc_bw_lock' mutex is introduced in commit af42269c3523
>> ("interconnect: Fix locking for runpm vs reclaim") in order
>> to decouple serialization of bw aggregation from codepaths
>> that require memory allocation.
>>
>> However commit d30f83d278a9 ("interconnect: core: Add dynamic
>> id allocation support") added a devm_kasprintf() call into a
>> path protected by the 'icc_bw_lock' which causes this lockdep
>> warning (at least on the IPQ9574 platform):
>>
>>     ======================================================
>>     WARNING: possible circular locking dependency detected
>>     6.15.0-next-20250529 #0 Not tainted
> 
>> Move the memory allocation part of the code outside of the protected
>> path to eliminate the warning, and add a note about why it is moved
>> to there. Also add memory allocation failure handling, while we are
>> at it.
>>
>> Fixes: d30f83d278a9 ("interconnect: core: Add dynamic id allocation support")
>> Signed-off-by: Gabor Juhos <j4g8y7@...il.com>
>> ---
>> Changes in v2:
>>   - move memory allocation outside of icc_lock
>>   - issue a warning and return without modifying the node name in case of
>>     memory allocation failure, and adjust the commit description
>>   - remove offered tags from Johan and Bryan
>>     Note: since I was not sure that that the added WARN_ON() is a substantial
>>     change or not, I have removed the offered tags intentionally to be on the
>>     safe side
> 
> Bah, what a mess (thanks for dropping the tags).
> 
> This dynamic id feature looks like a very ad-hoc and badly designed
> interface.
> 
> icc_node_add() should not be allocating memory in the first place as it
> is not designed to ever fail (e.g. does not return errors).
> 
> Generating the name could have been done as part of of
> icc_node_create_dyn() or yet another helper for the caller could have
> been added for that. In any case, it should be done before calling
> icc_node_add().
> 
> Perhaps the best minimal fix of the regression is to move the allocation
> into the two users of this interface. They already handle both dynamic
> and non-dynamic node allocation explicitly.

Ok, I will change the patch. Just to be clear, do you mean the
qcom_icc_rpmh_probe() and qcom_osm_l3_probe() functions, right?

> 
> Then whoever cares about this code can come up with a common interface
> for allocating the name (e.g. move it into icc_node_create_dyn() or add
> a new icc_node_init() helper or similar).
> 
>> ---
>>  drivers/interconnect/core.c | 19 +++++++++++++++----
>>  1 file changed, 15 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/interconnect/core.c b/drivers/interconnect/core.c
>> index 1a41e59c77f85a811f78986e98401625f4cadfa3..32d969c349093bc356dc66234c62484aa9b9e872 100644
>> --- a/drivers/interconnect/core.c
>> +++ b/drivers/interconnect/core.c
>> @@ -1022,6 +1022,21 @@ void icc_node_add(struct icc_node *node, struct icc_provider *provider)
>>  	if (WARN_ON(node->provider))
>>  		return;
>>  
>> +	if (node->id >= ICC_DYN_ID_START) {
>> +		char *name;
>> +
>> +		/*
>> +		 * Memory allocation must be done outside of codepaths
>> +		 * protected by icc_bw_lock.
>> +		 */
>> +		name = devm_kasprintf(provider->dev, GFP_KERNEL, "%s@%s",
>> +				      node->name, dev_name(provider->dev));
>> +		if (WARN_ON(!name))
>> +			return;
> 
> But this won't do. We'd need to return an error to the caller (even if
> this small allocation will never fail in practice).

I admit that it is ugly, but I thought that an explicit warning is better than a
hidden null pointer dereference.

Regards,
Gabor