netdev - Re: [PATCH] wil6210: fix debugfs_simple

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Message-ID: <87pnwnff65.fsf@kamboji.qca.qualcomm.com>
Date:   Sat, 06 Oct 2018 15:11:30 +0300
From:   Kalle Valo <kvalo@...eaurora.org>
To:     Julia Lawall <julia.lawall@...6.fr>
Cc:     YueHaibing <yuehaibing@...wei.com>,
        Maya Erez <merez@...eaurora.org>,
        linux-wireless@...r.kernel.org, wil6210@....qualcomm.com,
        kernel-janitors@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH] wil6210: fix debugfs_simple_attr.cocci warnings

Julia Lawall <julia.lawall@...6.fr> writes:

> On Fri, 5 Oct 2018, Kalle Valo wrote:
>
>> YueHaibing <yuehaibing@...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().
>> >
>> > Generated by: scripts/coccinelle/api/debugfs/debugfs_simple_attr.cocci
>>
>> Just out of curiosity, what kind of overhead are we talking about here?
>
> The log message on the commit introducing the semantic patch says the
> following:
>
>     In order to protect against file removal races, debugfs files created via
>     debugfs_create_file() now get wrapped by a struct file_operations at their
>     opening.
>
>     If the original struct file_operations are known to be safe against removal
>     races by themselves already, the proxy creation may be bypassed by creating
>     the files through debugfs_create_file_unsafe().
>
>     In order to help debugfs users who use the common
>       DEFINE_SIMPLE_ATTRIBUTE() + debugfs_create_file()
>     idiom to transition to removal safe struct file_operations, the helper
>     macro DEFINE_DEBUGFS_ATTRIBUTE() has been introduced.
>
>     Thus, the preferred strategy is to use
>       DEFINE_DEBUGFS_ATTRIBUTE() + debugfs_create_file_unsafe()
>     now.

I admit that I didn't have time to investigate this is detail but I'm
still not understanding where is that "significant overhead" coming from
and how big of overhead are we talking about? I guess it has something
to do with full_proxy_open() vs open_proxy_open()?

Not that I'm against this patch, just curious when I see someone
claiming "significant overhead" which is not obvious for me.

-- 
Kalle Valo