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

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Message-Id: <20140110175730.102b8250.akpm@linux-foundation.org>
Date:	Fri, 10 Jan 2014 17:57:30 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:	pavel@....cz, joe@...ches.com, keescook@...omium.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 format specifier

On Sat, 11 Jan 2014 10:28:51 +0900 Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp> wrote:

> Andrew Morton wrote:
> > On Thu, 9 Jan 2014 21:52:00 +0900 Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp> wrote:
> > 
> > > 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().
> > 
> > On reflection...
> > 
> > It isn't obvious that this patch is sufficiently beneficial until we
> > have that RCU code in place.
> > 
> > So I could retain this patch in -mm until we have that all sorted out. 
> > And I'll have to avoid merging %pT users into mainline in the
> > meanwhile!
> > 
> > Am I wrong?  The patch seems fairly pointless as a standalone thing?
> > 
> 
>   Step 1: (targeted to 3.14-rc1)
>     Add "%pT" format specifier and commcpy() wrapper function.
> 
>   Step 2: (started after step 1 is reflected to other git trees)
>     Replace printk("%s", p->comm) with printk("%pT", p).
>     Replace strcpy(buf, p->comm) with commcpy(buf, p).
> 
>   Step 3: (started after step 2 is reflected to other git trees)
>     Add rcu_read_lock()/rcu_read_unlock() into commcpy().
>     Modify set_task_comm() etc. to replace ->comm using RCU.

In the absence of step 3, steps 1 and 2 are rather pointless churn.

So I think it would be better to merge (into mainline) steps 1 and 3
first and at the same time.  Then start thinking about step 2.

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