linux-kernel - Re: [PATCH] powerpc/setup_64: use DEFINE_DEBUGFS_ATTRIBUTE to define fops_rfi

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Date:   Thu, 19 Dec 2019 09:20:19 +0800
From:   Chen Zhou <chenzhou10@...wei.com>
To:     Michael Ellerman <mpe@...erman.id.au>, <benh@...nel.crashing.org>,
        <paulus@...ba.org>
CC:     <linuxppc-dev@...ts.ozlabs.org>, <linux-kernel@...r.kernel.org>,
        "Nicolai Stange" <nicstange@...il.com>,
        Julia Lawall <Julia.Lawall@...6.fr>
Subject: Re: [PATCH] powerpc/setup_64: use DEFINE_DEBUGFS_ATTRIBUTE to define
 fops_rfi_flush

Hi Michael,

On 2019/12/18 19:02, Michael Ellerman wrote:
> Chen Zhou <chenzhou10@...wei.com> writes:
>> Use DEFINE_DEBUGFS_ATTRIBUTE rather than DEFINE_SIMPLE_ATTRIBUTE for
>> debugfs files.
>>
>> Semantic patch information:
>> Rationale: DEFINE_SIMPLE_ATTRIBUTE + debugfs_create_file()
>> imposes some significant overhead as compared to
>> DEFINE_DEBUGFS_ATTRIBUTE + debugfs_create_file_unsafe().
> 
> I know you didn't write this text, but these change logs are not great.
> It doesn't really explain why you're doing it. And it is alarming that
> you're converting *to* a function with "unsafe" in the name.
> 
> The commit that added the script:
> 
>   5103068eaca2 ("debugfs, coccinelle: check for obsolete DEFINE_SIMPLE_ATTRIBUTE() usage")
> 
> Has a bit more explanation.
> 
> Maybe something like this:
> 
>   In order to protect against file removal races, debugfs files created via
>   debugfs_create_file() are wrapped by a struct file_operations at their
>   opening.
>   
>   If the original struct file_operations is known to be safe against removal
>   races already, the proxy creation may be bypassed by creating the files
>   using DEFINE_DEBUGFS_ATTRIBUTE() and debugfs_create_file_unsafe().
> 
> 
> The part that's not explained is why this file is "known to be safe
> against removal races already"?
> 
> It also seems this conversion will make the file no longer seekable,
> because DEFINE_SIMPLE_ATTRIBUTE() uses generic_file_llseek() whereas
> DEFINE_DEBUGFS_ATTRIBUTE() uses no_llseek.
> 
> That is probably fine, but should be mentioned.

Thanks for your explanation. This part indeed should be mentioned.

Chen Zhou

> 
> cheers
> 
>> Signed-off-by: Chen Zhou <chenzhou10@...wei.com>
>> ---
>>  arch/powerpc/kernel/setup_64.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/powerpc/kernel/setup_64.c b/arch/powerpc/kernel/setup_64.c
>> index 6104917..4b9fbb2 100644
>> --- a/arch/powerpc/kernel/setup_64.c
>> +++ b/arch/powerpc/kernel/setup_64.c
>> @@ -956,11 +956,11 @@ static int rfi_flush_get(void *data, u64 *val)
>>  	return 0;
>>  }
>>  
>> -DEFINE_SIMPLE_ATTRIBUTE(fops_rfi_flush, rfi_flush_get, rfi_flush_set, "%llu\n");
>> +DEFINE_DEBUGFS_ATTRIBUTE(fops_rfi_flush, rfi_flush_get, rfi_flush_set, "%llu\n");
>>  
>>  static __init int rfi_flush_debugfs_init(void)
>>  {
>> -	debugfs_create_file("rfi_flush", 0600, powerpc_debugfs_root, NULL, &fops_rfi_flush);
>> +	debugfs_create_file_unsafe("rfi_flush", 0600, powerpc_debugfs_root, NULL, &fops_rfi_flush);
>>  	return 0;
>>  }
>>  device_initcall(rfi_flush_debugfs_init);
>> -- 
>> 2.7.4
> 
> .
>