linux-kernel - Re: [patch 7/9] signals: Fix more rcu assumptions

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Date:	Thu, 10 Dec 2009 15:45:28 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Oleg Nesterov <oleg@...hat.com>
cc:	LKML <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	James Morris <jmorris@...ei.org>,
	David Howells <dhowells@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch 7/9] signals: Fix more rcu assumptions

On Thu, 10 Dec 2009, Oleg Nesterov wrote:

> On 12/10, Thomas Gleixner wrote:
> >
> > 1) Remove the misleading comment in __sigqueue_alloc() which claims
> >    that holding a spinlock is equivalent to rcu_read_lock().
> >
> > 2) Wrap the __send_signal() call in send_signal() into a rcu read side
> >    critical section to guarantee that the __sigqueue_alloc()
> >    requirement is met in any case.
> > ...
> >  static int send_signal(int sig, struct siginfo *info, struct task_struct *t,
> >  			int group)
> >  {
> > -	int from_ancestor_ns = 0;
> > +	int ret, from_ancestor_ns = 0;
> >
> >  #ifdef CONFIG_PID_NS
> >  	if (!is_si_special(info) && SI_FROMUSER(info) &&
> > @@ -954,7 +953,11 @@ static int send_signal(int sig, struct s
> >  		from_ancestor_ns = 1;
> >  #endif
> >
> > -	return __send_signal(sig, info, t, group, from_ancestor_ns);
> > +	rcu_read_lock();
> > +	ret = __send_signal(sig, info, t, group, from_ancestor_ns);
> > +	rcu_read_unlock();
> 
> But, without a comment it is very unobvious why do we need rcu_read_lock().
> 
> Perhaps it is better to modify __sigqueue_alloc() instead? It can take
> rcu_lock() around cred->user itself.

Indeed. Was too tired to see the obvious :)

Thanks,

	tglx
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