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

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Date:	Sat, 11 Jan 2014 01:50:45 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:	akpm@...ux-foundation.org, 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, Jan 11, 2014 at 12:09:46PM +0900, Tetsuo Handa wrote:
> Andrew Morton wrote:
> > 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.
> 
> Unfortunately we can't.
> Step 2 depends on step 1 for avoiding compile time errors.
> Step 3 depends on step 2 for avoiding run time errors.
> 
>   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", current->comm) with printk("%pT", NULL).
>     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.
> 
> If step 3 is merged into mainline before step 2 complete, those who are not
> using "%pT" or commcpy() might crash due to reading RCU protected ->comm
> without rcu_read_lock()/rcu_read_unlock().
> 
> 
> Let me confirm, Paul.
> 
>   I'm trying to change task_struct->comm to use RCU.
>   At step 3, I'm planning to do
> 
>   static inline void *commcpy(void *buf, const struct task_struct *tsk)
>   {
>   	rcu_read_lock();
>   	memcpy(buf, tsk->comm, TASK_COMM_LEN);
>   	rcu_read_unlock();
>   	return buf;
>   }
> 
>   and let set_task_comm() wait for readers using synchronize_rcu() or
>   call_rcu().
> 
>   Given that commcpy() can be called from any context, are synchronize_rcu()
>   and call_rcu() sufficient for waiting for commcpy() users?

Yep, that should work.

							Thanx, Paul

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