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

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Message-Id: <201401111028.DEG34385.FVHtLOFMQJFSOO@I-love.SAKURA.ne.jp>
Date:	Sat, 11 Jan 2014 10:28:51 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	akpm@...ux-foundation.org
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

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.

This patch and commcpy() patch ( https://lkml.org/lkml/2014/1/10/217 ) are
targeted to be merged into 3.14-rc1. This is because we can start writing
patches to use "%pT" or commcpy() only after other git trees have rebased
using 3.14-rc1. After all direct ->comm readers are replaced to use "%pT" or
commcpy(), I can start ->comm modifying functions to use RCU.
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