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SubjectRe: [PATCH] powerpc/setup_64: use DEFINE_DEBUGFS_ATTRIBUTE to define fops_rfi_flush
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Hi Michael,

On 2019/12/18 19:02, Michael Ellerman wrote:
> Chen Zhou <chenzhou10@huawei.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@huawei.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
>
> .
>

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