linux-kernel - Re: [PATCH RFC tip/core/rcu 1/3] rcu: The Bloatwatch Edition, v7

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <1255488586.7113.3094.camel@gandalf.stny.rr.com>
Date:	Tue, 13 Oct 2009 22:49:46 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	mingo@...e.hu, dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	dvhltc@...ibm.com, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	avi@...hat.com, mtosatti@...hat.com, torvalds@...ux-foundation.org
Subject: Re: [PATCH RFC tip/core/rcu 1/3] rcu: The Bloatwatch Edition, v7

On Wed, 2009-10-14 at 10:05 +0800, Lai Jiangshan wrote:
> Paul E. McKenney wrote:

> You haven't explain the tick_nohz_stop_sched_tick() in *irq_exit()*.
> (tick_nohz_stop_sched_tick() calls rcu_enter_nohz())
> 
> void irq_exit(void)
> {
> 	....
> 	rcu_irq_exit(); /* This is OK, the opposite is in irq_enter() */
> 	if (idle_cpu(smp_processor_id()) && !in_interrupt() && !need_resched())
> 		tick_nohz_stop_sched_tick(0); /* where is the opposite ??? */
> 	....
> }
> 
> This means if the interrupt -is- returning to dyntick-idle mode,
> rcu_enter_nohz() is called again.
> 
> Take this flow as example:
> 
> cpu_idle():
> while(1) {
> 
>   tick_nohz_stop_sched_tick()
>      rcu_enter_nohz()                 *****
> ------->interrupt happen
>         irq_enter()
>         irq_exit()
>            tick_nohz_stop_sched_tick()
>               rcu_enter_nohz()       *****
> <-------interrupt returns
>   tick_nohz_restart_sched_tick()
>      rcu_exit_nohz()                  *****
> 
> } /* while(1) */
> 
> 
> You can see that rcu_enter_nohz() is called twice and
> rcu_exit_nohz() is only called once in this flow.
> 
> It's because tick_nohz_stop_sched_tick()/tick_nohz_restart_sched_tick()
> are not called in pairs, so rcu_enter_nohz() and rcu_exit_nohz()
> are not called in pairs either.


But isn't the tick_nohz_stop_sched_tick have several exits?

void tick_nohz_stop_sched_tick(int inidle)
{
[..]

	if (!inidle && !ts->inidle)
		goto end;

	ts->inidle = 1;

[..]

		if (!ts->tick_stopped) {
[..]
			ts->tick_stopped = 1;
			ts->idle_jiffies = last_jiffies;
			rcu_enter_nohz();
		}
[..]


So I'm not sure calling tick_nohz_stop_sched_tick twice equals calling
rcu_enter_nohz twice.

-- Steve

> > 
> > So I do believe that rcu_enter_nohz() and rcu_exit_nohz() are in fact
> > invoked in pairs.  One strange thing about this is that the idle loop
> > first invokes rcu_enter_nohz(), then invokes rcu_exit_nohz(), while
> > an interrupt handler first invokes rcu_irq_enter() and then invokes
> > rcu_irq_exit().  So the idle loop enters dyntick-idle mode and then
> > leaves it, while an interrupt handler might leave dyntick-idle mode and
> > then re-enter it.
> > 
> > Or am I still missing something here?
> > 
> > 							Thanx, Paul
> > 
> > 
> 

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/