linux-kernel - Re: [PATCH tip/core/rcu 03/17] signal: Explain local_irq

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Date:	Tue, 8 Jul 2014 08:50:05 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...icios.com, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
	sbw@....edu
Subject: Re: [PATCH tip/core/rcu 03/17] signal: Explain local_irq_save() call

On Tue, Jul 08, 2014 at 05:01:51PM +0800, Lai Jiangshan wrote:
> On 07/08/2014 06:38 AM, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > 
> > The explicit local_irq_save() in __lock_task_sighand() is needed to avoid
> > a potential deadlock condition, as noted in a841796f11c90d53 (signal:
> > align __lock_task_sighand() irq disabling and RCU).  However, someone
> > reading the code might be forgiven for concluding that this separate
> > local_irq_save() was completely unnecessary.  This commit therefore adds
> > a comment referencing the shiny new block comment on rcu_read_unlock().
> > 
> > Reported-by: Oleg Nesterov <oleg@...hat.com>
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Acked-by: Oleg Nesterov <oleg@...hat.com>
> > ---
> >  kernel/signal.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/kernel/signal.c b/kernel/signal.c
> > index a4077e90f19f..46161e744760 100644
> > --- a/kernel/signal.c
> > +++ b/kernel/signal.c
> > @@ -1263,6 +1263,10 @@ struct sighand_struct *__lock_task_sighand(struct task_struct *tsk,
> >  	struct sighand_struct *sighand;
> >  
> >  	for (;;) {
> > +		/*
> > +		 * Disable interrupts early to avoid deadlocks.
> > +		 * See rcu_read_unlock comment header for details.
> > +		 */
> 
> A pair of brackets are missing here: rcu_read_unlock()
> after that, please add:
> 
> Reviewed-by: Lai Jiangshan <laijs@...fujitsu.com>

Good point, added the "()" and your Reviewed-by.

> It reminds me that I should keep my effort to solve the deadlock
> problem where rcu_read_unlock() is overlapped with schedule locks.

That would be good!  I vaguely remember an earlier patch of yours that
Steven Rostedt gave feedback on, but have not been able to locate either
email.

							Thanx, Paul

> >  		local_irq_save(*flags);
> >  		rcu_read_lock();
> >  		sighand = rcu_dereference(tsk->sighand);
> 

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