netdev - Re: [PATCH net-next v3 01/11] devlink: retain error in struct devlink

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Message-ID: <78a9a07b-0a94-4b38-75c7-8bc240468430@intel.com>
Date: Thu, 19 Oct 2023 23:49:29 +0200
From: Przemek Kitszel <przemyslaw.kitszel@...el.com>
To: Simon Horman <horms@...nel.org>
CC: Jiri Pirko <jiri@...nulli.us>, <netdev@...r.kernel.org>, "David S. Miller"
	<davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
	<kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Shannon Nelson
	<shannon.nelson@....com>, Michael Chan <michael.chan@...adcom.com>, "Cai
 Huoqing" <cai.huoqing@...ux.dev>, George Cherian
	<george.cherian@...vell.com>, Danielle Ratson <danieller@...dia.com>, "Moshe
 Shemesh" <moshe@...dia.com>, Saeed Mahameed <saeedm@...dia.com>, Ariel Elior
	<aelior@...vell.com>, Manish Chopra <manishc@...vell.com>, Igor Russkikh
	<irusskikh@...vell.com>, Coiby Xu <coiby.xu@...il.com>, Brett Creeley
	<brett.creeley@....com>, Sunil Goutham <sgoutham@...vell.com>, Linu Cherian
	<lcherian@...vell.com>, Geetha sowjanya <gakula@...vell.com>, Jerin Jacob
	<jerinj@...vell.com>, hariprasad <hkelam@...vell.com>, Subbaraya Sundeep
	<sbhatta@...vell.com>, Ido Schimmel <idosch@...dia.com>, Petr Machata
	<petrm@...dia.com>, Eran Ben Elisha <eranbe@...dia.com>, Aya Levin
	<ayal@...lanox.com>, Leon Romanovsky <leon@...nel.org>,
	<linux-kernel@...r.kernel.org>, Jesse Brandeburg
	<jesse.brandeburg@...el.com>, Jiri Pirko <jiri@...dia.com>
Subject: Re: [PATCH net-next v3 01/11] devlink: retain error in struct
 devlink_fmsg

On 10/19/23 15:00, Simon Horman wrote:
> On Wed, Oct 18, 2023 at 10:26:37PM +0200, Przemek Kitszel wrote:
>> Retain error value in struct devlink_fmsg, to relieve drivers from
>> checking it after each call.
>> Note that fmsg is an in-memory builder/buffer of formatted message,
>> so it's not the case that half baked message was sent somewhere.
>>
>> We could find following scheme in multiple drivers:
>>    err = devlink_fmsg_obj_nest_start(fmsg);
>>    if (err)
>>    	return err;
>>    err = devlink_fmsg_string_pair_put(fmsg, "src", src);
>>    if (err)
>>    	return err;
>>    err = devlink_fmsg_something(fmsg, foo, bar);
>>    if (err)
>> 	return err;
>>    // and so on...
>>    err = devlink_fmsg_obj_nest_end(fmsg);
>>
>> With retaining error API that translates to:
>>    devlink_fmsg_obj_nest_start(fmsg);
>>    devlink_fmsg_string_pair_put(fmsg, "src", src);
>>    devlink_fmsg_something(fmsg, foo, bar);
>>    // and so on...
>>    devlink_fmsg_obj_nest_end(fmsg);
>>
>> What means we check error just when is time to send.
>>
>> Possible error scenarios are developer error (API misuse) and memory
>> exhaustion, both cases are good candidates to choose readability
>> over fastest possible exit.
>>
>> Note that this patch keeps returning errors, to allow per-driver conversion
>> to the new API, but those are not needed at this point already.
>>
>> This commit itself is an illustration of benefits for the dev-user,
>> more of it will be in separate commits of the series.
>>
>> Reviewed-by: Jesse Brandeburg <jesse.brandeburg@...el.com>
>> Reviewed-by: Jiri Pirko <jiri@...dia.com>
>> Signed-off-by: Przemek Kitszel <przemyslaw.kitszel@...el.com>
> 
> ...
> 
>> @@ -1027,14 +934,12 @@ int devlink_fmsg_binary_pair_put(struct devlink_fmsg *fmsg, const char *name,
> 
> Hi Przemek,
> 
> The line before this hunk is:
> 
> 		err = devlink_fmsg_binary_put(fmsg, value + offset, data_size);
> 
> And, as of this patch, the implementation of
> devlink_fmsg_binary_pair_nest_start() looks like this:
> 
> int devlink_fmsg_binary_put(struct devlink_fmsg *fmsg, const void *value,
>                              u16 value_len)
> {
>          if (!fmsg->putting_binary)
>                  return -EINVAL;
> 
>          return devlink_fmsg_put_value(fmsg, value, value_len, NLA_BINARY);
> }
> 
> Which may return an error, if the if condition is met, without setting
> fmsg->err.
> 
>>   		if (err)
>>   			break;
>>   		/* Exit from loop with a break (instead of
>> -		 * return) to make sure putting_binary is turned off in
>> -		 * devlink_fmsg_binary_pair_nest_end
>> +		 * return) to make sure putting_binary is turned off
>>   		 */
>>   	}
>>   
>> -	end_err = devlink_fmsg_binary_pair_nest_end(fmsg);
>> -	if (end_err)
>> -		err = end_err;
> 
> Prior to this patch, the value of err from the loop above was preserved,
> unless devlink_fmsg_binary_pair_nest_end generated an error.
> 
>> +	err = devlink_fmsg_binary_pair_nest_end(fmsg);
> 
> But now it looks like this is only the case if fmsg->err corresponds to err
> when the loop was exited.
> 
> Or in other words, the err returned by devlink_fmsg_binary_put()
> is not propagated to the caller if !fmsg->putting_binary.
> 
> If so, is this intentional?

In scope of devlink_fmsg_binary_pair_put() all it's fine, and perhaps
that lead me into thinking that it's fine in *general*, which is not,
as devlink_fmsg_binary_put() is exported (so it's return value should
be preserved).

On the other note, it's called in two places (one of which was just
covered, and the second place is mlx5e_health_rsc_fmsg_binary(), which
really just mimics what we have here. So this is also fine in practice.

Last patch of the series fixes it for general case, so barring
pathological cherry-picks or reverts we are safe.
Do you want a v4 with that fixed?

Other thing is that this function could be changed into some internal 
helper without export.

> 
>> +	fmsg->putting_binary = false;
>>   
>>   	return err;
>>   }
>> -- 
>> 2.38.1
>>
>