linux-kernel - Re: [PATCH 2/2] proc: fix PAGE_SIZE limit of /proc/$PID/cmdline

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Message-ID: <CACVxJT9c38NcjqPhKXL4Tp8qra3bnGf8BOUENEph3Qg0YyXfsQ@mail.gmail.com>
Date:	Thu, 28 May 2015 14:14:41 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Jarod Wilson <jarod@...hat.com>,
	Jan Stancek <jstancek@...hat.com>
Subject: Re: [PATCH 2/2] proc: fix PAGE_SIZE limit of /proc/$PID/cmdline

On Thu, May 28, 2015 at 9:59 AM, Cyrill Gorcunov <gorcunov@...il.com> wrote:
> On Thu, May 28, 2015 at 02:12:07AM +0300, Alexey Dobriyan wrote:
>> > > >
>> > > > Could you please explain why this down/up is needed?
>> > >
>> > > Code is written this way to get constistent snapshot of data.
>> >
>> > it does not. you fetch data into local variables which is the
>> > same as simply read them locklessly in general (because later
>> > you refer to local vars).
>>
>> It is snapshot w.r.t getting both pairs not snapshot w.r.t atomicity or
>> something (unsigned long access is atomic after all). Once down_write()
>> is used in the other place, it even becomes obviously correct code!
>
> Not at all. It is correct if and only if you're operating under lock
> taken, once you fetch the pair and left the lock it simply local copies
> of values the descriptor had when lock was taken.

Yes, and?

You do not complain that signal statistics is collected
under sighand lock but printed for /proc/*/status without, do you?
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