linux-kernel - Re: [PATCH] lib/vsprintf: add %pT[C012] format specifier

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Date:	Mon, 6 Jan 2014 22:34:14 +0100
From:	Pavel Machek <pavel@....cz>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	joe@...ches.com, keescook@...omium.org, akpm@...ux-foundation.org,
	geert@...ux-m68k.org, jkosina@...e.cz, viro@...iv.linux.org.uk,
	davem@...emloft.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lib/vsprintf: add %pT[C012] format specifier

On Mon 2014-01-06 23:00:56, Tetsuo Handa wrote:
> Joe Perches wrote:
> > On Sun, 2014-01-05 at 12:15 +0900, Tetsuo Handa wrote:
> > > > Since my purpose is to make reading of task_struct->comm consistent, %pT-like
> > > > extension is what I want for centralizing pointer dereferences.
> > > 
> > > If we have no objections for %pT[C012] patch,
> > 
> > I still believe emitting different output styles using
> > %pT[012] is not ideal.
> > 
> > Is this really necessary?
> 
> No problem. %pT[012] are simply optimization (reducing number of function
> arguments for saving text size) and therefore I can drop them.
> What about below patch?
> ----------
> >From f69a1db69a9ad6e14235edbb5a9b7d720530a2be Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> Date: Mon, 6 Jan 2014 22:22:00 +0900
> Subject: [PATCH] lib/vsprintf: add %pT format specifier
> 
> Since task_struct->comm can be modified by other threads while the current
> thread is reading it, it is recommended to use get_task_comm() for reading it.
> 
> However, since get_task_comm() holds task_struct->alloc_lock spinlock,
> some users cannot use get_task_comm(). Also, a lot of users are directly
> reading from task_struct->comm even if they can use get_task_comm().
> Such users might obtain inconsistent result.
> 
> This patch introduces %pT format specifier for printing task_struct->comm.
> Currently %pT does not provide consistency. I'm planning to change to use RCU
> in the future. By using RCU, the comm name read from task_struct->comm will be
> guaranteed to be consistent. But before modifying set_task_comm() to use RCU,
> we need to kill direct ->comm users who do not use get_task_comm().
> 
> An example for converting direct ->comm users is shown below. Since many debug
> printings use p == current, you can pass NULL instead of p if p == current.
> 
>   pr_info("comm=%s\n", p->comm);       => pr_info("comm=%pT\n", p);
>   pr_info("comm=%s\n", current->comm); => pr_info("comm=%pT\n", NULL);
> 
> Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>

Reviewed-by: Pavel Machek <pavel@....cz>
									Pavel
-- 
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