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Date:   Thu, 19 Oct 2023 23:49:29 +0200
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Subject: Re: [PATCH net-next v3 01/11] devlink: retain error in struct
 devlink_fmsg
Content-Language: en-US
To:     Simon Horman <horms@kernel.org>
CC:     Jiri Pirko <jiri@resnulli.us>, <netdev@vger.kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Shannon Nelson <shannon.nelson@amd.com>,
        Michael Chan <michael.chan@broadcom.com>,
        "Cai Huoqing" <cai.huoqing@linux.dev>,
        George Cherian <george.cherian@marvell.com>,
        Danielle Ratson <danieller@nvidia.com>,
        "Moshe Shemesh" <moshe@nvidia.com>,
        Saeed Mahameed <saeedm@nvidia.com>,
        Ariel Elior <aelior@marvell.com>,
        Manish Chopra <manishc@marvell.com>,
        Igor Russkikh <irusskikh@marvell.com>,
        Coiby Xu <coiby.xu@gmail.com>,
        Brett Creeley <brett.creeley@amd.com>,
        Sunil Goutham <sgoutham@marvell.com>,
        Linu Cherian <lcherian@marvell.com>,
        Geetha sowjanya <gakula@marvell.com>,
        Jerin Jacob <jerinj@marvell.com>,
        hariprasad <hkelam@marvell.com>,
        Subbaraya Sundeep <sbhatta@marvell.com>,
        Ido Schimmel <idosch@nvidia.com>,
        Petr Machata <petrm@nvidia.com>,
        Eran Ben Elisha <eranbe@nvidia.com>,
        Aya Levin <ayal@mellanox.com>,
        Leon Romanovsky <leon@kernel.org>,
        <linux-kernel@vger.kernel.org>,
        Jesse Brandeburg <jesse.brandeburg@intel.com>,
        Jiri Pirko <jiri@nvidia.com>
References: <20231018202647.44769-1-przemyslaw.kitszel@intel.com>
 <20231018202647.44769-2-przemyslaw.kitszel@intel.com>
 <20231019130037.GI2100445@kernel.org>
From:   Przemek Kitszel <przemyslaw.kitszel@intel.com>
In-Reply-To: <20231019130037.GI2100445@kernel.org>
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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@intel.com>
>> Reviewed-by: Jiri Pirko <jiri@nvidia.com>
>> Signed-off-by: Przemek Kitszel <przemyslaw.kitszel@intel.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
>>
>