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From: "Paul E. McKenney" <paulmck@kernel.org>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Valentin Schneider <valentin.schneider@arm.com>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, rcu@vger.kernel.org,
	linux-rt-users@vger.kernel.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Steven Rostedt <rostedt@goodmis.org>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Frederic Weisbecker <frederic@kernel.org>,
	Josh Triplett <josh@joshtriplett.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Davidlohr Bueso <dave@stgolabs.net>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Steven Price <steven.price@arm.com>,
	Ard Biesheuvel <ardb@kernel.org>,
	Boqun Feng <boqun.feng@gmail.com>, Mike Galbraith <efault@gmx.de>,
	Scott Wood <swood@redhat.com>
Subject: Re: [PATCH] rcutorture: Avoid problematic critical section nesting on RT
Date: Thu, 19 Aug 2021 11:20:35 -0700	[thread overview]
Message-ID: <20210819182035.GF4126399@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <20210819154708.3efz6jtgwtuhpeds@linutronix.de>

On Thu, Aug 19, 2021 at 05:47:08PM +0200, Sebastian Andrzej Siewior wrote:
> On 2021-08-19 17:39:29 [+0200], To Paul E. McKenney wrote:
> > up with following which I can explain:
> > 
> > diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> > index 40ef5417d9545..5c8b31b7eff03 100644
> > --- a/kernel/rcu/rcutorture.c
> > +++ b/kernel/rcu/rcutorture.c
> > @@ -1432,28 +1432,34 @@ static void rcutorture_one_extend(int *readstate, int newstate,
> >  	/* First, put new protection in place to avoid critical-section gap. */
> >  	if (statesnew & RCUTORTURE_RDR_BH)
> >  		local_bh_disable();
> > +	if (statesnew & RCUTORTURE_RDR_RBH)
> > +		rcu_read_lock_bh();
> >  	if (statesnew & RCUTORTURE_RDR_IRQ)
> >  		local_irq_disable();
> >  	if (statesnew & RCUTORTURE_RDR_PREEMPT)
> >  		preempt_disable();
> > -	if (statesnew & RCUTORTURE_RDR_RBH)
> > -		rcu_read_lock_bh();
> >  	if (statesnew & RCUTORTURE_RDR_SCHED)
> >  		rcu_read_lock_sched();
> >  	if (statesnew & RCUTORTURE_RDR_RCU)
> >  		idxnew = cur_ops->readlock() << RCUTORTURE_RDR_SHIFT;
> 
> So the ordering in the enable and disable part regarding BH is
> important. First BH, then preemption or IRQ.
> 
> > -	/* Next, remove old protection, irq first due to bh conflict. */
> > +	/*
> > +	 * Next, remove old protection, in decreasing order of strength
> > +	 * to avoid unlock paths that aren't safe in the stronger
> > +	 * context. Namely: BH can not be enabled with disabled interrupts.
> > +	 * Additionally PREEMPT_RT requires that BH is enabled in preemptible
> > +	 * context.
> > +	 */
> >  	if (statesold & RCUTORTURE_RDR_IRQ)
> >  		local_irq_enable();
> > -	if (statesold & RCUTORTURE_RDR_BH)
> > -		local_bh_enable();
> >  	if (statesold & RCUTORTURE_RDR_PREEMPT)
> >  		preempt_enable();
> > -	if (statesold & RCUTORTURE_RDR_RBH)
> > -		rcu_read_unlock_bh();
> >  	if (statesold & RCUTORTURE_RDR_SCHED)
> >  		rcu_read_unlock_sched();
> > +	if (statesold & RCUTORTURE_RDR_BH)
> > +		local_bh_enable();
> > +	if (statesold & RCUTORTURE_RDR_RBH)
> > +		rcu_read_unlock_bh();
> >  	if (statesold & RCUTORTURE_RDR_RCU) {
> >  		bool lockit = !statesnew && !(torture_random(trsp) & 0xffff);
> 
> The same in the unlock part so that BH is unlocked in preemptible
> context.
> Now if you need bh lock/unlock in atomic context (either with disabled
> IRQs or preemption) then I would dig out the atomic-bh part again and
> make !RT only without the preempt_disable() section around about which
> one you did complain.
> 
> > @@ -1496,6 +1502,9 @@ rcutorture_extend_mask(int oldmask, struct torture_random_state *trsp)
> >  	int mask = rcutorture_extend_mask_max();
> >  	unsigned long randmask1 = torture_random(trsp) >> 8;
> >  	unsigned long randmask2 = randmask1 >> 3;
> > +	unsigned long preempts = RCUTORTURE_RDR_PREEMPT | RCUTORTURE_RDR_SCHED;
> > +	unsigned long preempts_irq = preempts | RCUTORTURE_RDR_IRQ;
> > +	unsigned long bhs = RCUTORTURE_RDR_BH | RCUTORTURE_RDR_RBH;
> >  
> >  	WARN_ON_ONCE(mask >> RCUTORTURE_RDR_SHIFT);
> >  	/* Mostly only one bit (need preemption!), sometimes lots of bits. */
> > @@ -1503,11 +1512,37 @@ rcutorture_extend_mask(int oldmask, struct torture_random_state *trsp)
> >  		mask = mask & randmask2;
> >  	else
> >  		mask = mask & (1 << (randmask2 % RCUTORTURE_RDR_NBITS));
> > -	/* Can't enable bh w/irq disabled. */
> > -	if ((mask & RCUTORTURE_RDR_IRQ) &&
> > -	    ((!(mask & RCUTORTURE_RDR_BH) && (oldmask & RCUTORTURE_RDR_BH)) ||
> > -	     (!(mask & RCUTORTURE_RDR_RBH) && (oldmask & RCUTORTURE_RDR_RBH))))
> > -		mask |= RCUTORTURE_RDR_BH | RCUTORTURE_RDR_RBH;
> > +
> > +	/*
> > +	 * Can't enable bh w/irq disabled.
> > +	 */
> > +	if (mask & RCUTORTURE_RDR_IRQ)
> > +		mask |= oldmask & bhs;
> > +
> > +	/*
> > +	 * Ideally these sequences would be detected in debug builds
> > +	 * (regardless of RT), but until then don't stop testing
> > +	 * them on non-RT.
> > +	 */
> > +	if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> > +		/*
> > +		 * Can't release the outermost rcu lock in an irq disabled
> > +		 * section without preemption also being disabled, if irqs
> > +		 * had ever been enabled during this RCU critical section
> > +		 * (could leak a special flag and delay reporting the qs).
> > +		 */
> > +		if ((oldmask & RCUTORTURE_RDR_RCU) &&
> > +		    (mask & RCUTORTURE_RDR_IRQ) &&
> > +		    !(mask & preempts))
> > +			mask |= RCUTORTURE_RDR_RCU;
> 
> This piece above, I don't understand. I had it running for a while and
> it didn't explode. Let me try TREE01 for 30min without that piece.

This might be historical.  There was a time when interrupts being
disabled across rcu_read_unlock() meant that preemption had to have
been disabled across the entire RCU read-side critical section.

I am not seeing a purpose for it now, but I could easily be missing
something, especially given my tenuous grasp of RT.

Either way, looking forward to the next version!

							Thanx, Paul

> > +		/* Can't modify bh in atomic context */
> > +		if (oldmask & preempts_irq)
> > +			mask &= ~bhs;
> > +		if ((oldmask | mask) & preempts_irq)
> > +			mask |= oldmask & bhs;
> 
> And this is needed because we can't lock/unlock bh while atomic.
> 
> > +	}
> > +
> >  	return mask ?: RCUTORTURE_RDR_RCU;
> >  }
> >  
> 
> Sebastian

  reply	other threads:[~2021-08-19 18:20 UTC|newest]

Thread overview: 52+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-11 20:13 [PATCH v3 0/4] rcu, arm64: PREEMPT_RT fixlets Valentin Schneider
2021-08-11 20:13 ` [PATCH v3 1/4] rcutorture: Don't disable softirqs with preemption disabled when PREEMPT_RT Valentin Schneider
2021-08-12 16:47   ` Paul E. McKenney
2021-08-17 12:13   ` Sebastian Andrzej Siewior
2021-08-17 13:17     ` Sebastian Andrzej Siewior
2021-08-17 14:40       ` [PATCH] rcutorture: Avoid problematic critical section nesting on RT Sebastian Andrzej Siewior
2021-08-18 22:46         ` Paul E. McKenney
2021-08-19 15:35           ` Sebastian Andrzej Siewior
2021-08-19 15:39           ` Sebastian Andrzej Siewior
2021-08-19 15:47             ` Sebastian Andrzej Siewior
2021-08-19 18:20               ` Paul E. McKenney [this message]
2021-08-19 18:45                 ` Sebastian Andrzej Siewior
2021-08-20  4:11                 ` Scott Wood
2021-08-20  7:11                   ` Sebastian Andrzej Siewior
2021-08-20  7:42                   ` [PATCH v2] rcutorture: Avoid problematic critical section nesting on PREEMPT_RT Sebastian Andrzej Siewior
2021-08-20 22:10                     ` Paul E. McKenney
2021-08-20  3:23         ` [PATCH] rcutorture: Avoid problematic critical section nesting on RT Scott Wood
2021-08-20  6:54           ` Sebastian Andrzej Siewior
2021-08-11 20:13 ` [PATCH v3 2/4] sched: Introduce migratable() Valentin Schneider
2021-08-17 14:43   ` Sebastian Andrzej Siewior
2021-08-22 17:31     ` Valentin Schneider
2021-08-17 17:09   ` Sebastian Andrzej Siewior
2021-08-17 19:30     ` Phil Auld
2021-08-22 18:14     ` Valentin Schneider
2022-01-26 16:56       ` Sebastian Andrzej Siewior
2022-01-26 18:10         ` Valentin Schneider
2022-01-27 10:07           ` Sebastian Andrzej Siewior
2022-01-27 18:23             ` Valentin Schneider
2022-01-27 19:27         ` Valentin Schneider
2022-02-04  9:24           ` Sebastian Andrzej Siewior
2021-08-11 20:13 ` [PATCH v3 3/4] rcu/nocb: Protect NOCB state via local_lock() under PREEMPT_RT Valentin Schneider
2021-08-13  0:20   ` Paul E. McKenney
2021-08-13 18:48     ` Valentin Schneider
2021-08-24 13:00     ` Frederic Weisbecker
2021-08-17 15:36   ` Sebastian Andrzej Siewior
2021-08-22 18:15     ` Valentin Schneider
2021-09-21 14:05   ` Thomas Gleixner
2021-09-21 21:12     ` rcu/tree: Protect rcu_rdp_is_offloaded() invocations on RT Thomas Gleixner
2021-09-21 23:36       ` Frederic Weisbecker
2021-09-22  2:18         ` Paul E. McKenney
2021-09-22 11:31           ` Frederic Weisbecker
2021-09-21 23:45       ` Frederic Weisbecker
2021-09-22  6:32         ` Sebastian Andrzej Siewior
2021-09-22 11:10           ` Frederic Weisbecker
2021-09-22 11:27             ` Sebastian Andrzej Siewior
2021-09-22 11:38               ` Frederic Weisbecker
2021-09-22 13:02                 ` Sebastian Andrzej Siewior
2021-09-23 10:02                   ` Frederic Weisbecker
2021-09-30  9:00       ` Valentin Schneider
2021-09-30 10:53         ` Frederic Weisbecker
2021-09-30 13:22           ` Valentin Schneider
2021-08-11 20:13 ` [PATCH v3 4/4] arm64: mm: Make arch_faults_on_old_pte() check for migratability Valentin Schneider

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