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From: "Paul E. McKenney" <paulmck@kernel.org>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Scott Wood <swood@redhat.com>,
	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>
Subject: Re: [PATCH v2] rcutorture: Avoid problematic critical section nesting on PREEMPT_RT
Date: Fri, 20 Aug 2021 15:10:56 -0700	[thread overview]
Message-ID: <20210820221056.GL4126399@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <20210820074236.2zli4nje7bof62rh@linutronix.de>

On Fri, Aug 20, 2021 at 09:42:36AM +0200, Sebastian Andrzej Siewior wrote:
> From: "From: Scott Wood" <swood@redhat.com>
> 
> rcutorture is generating some nesting scenarios that are not compatible on PREEMPT_RT.
> For example:
> 	preempt_disable();
> 	rcu_read_lock_bh();
> 	preempt_enable();
> 	rcu_read_unlock_bh();
> 
> The problem here is that on PREEMPT_RT the bottom halves have to be
> disabled and enabled in preemptible context.
> 
> Reorder locking: start with BH locking and continue with then with
> disabling preemption or interrupts. In the unlocking do it reverse by
> first enabling interrupts and preemption and BH at the very end.
> Ensure that on PREEMPT_RT BH locking remains unchanged if in
> non-preemptible context.
> 
> Link: https://lkml.kernel.org/r/20190911165729.11178-6-swood@redhat.com
> Link: https://lkml.kernel.org/r/20210819182035.GF4126399@paulmck-ThinkPad-P17-Gen-1
> Signed-off-by: Scott Wood <swood@redhat.com>
> [bigeasy: Drop ATOM_BH, make it only about changing BH in atomic
> context. Allow enabling RCU in IRQ-off section. Reword commit message.]
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Looks plausible.  ;-)

I have queued this for testing and further review.  If all goes well,
perhaps the v5.16 merge window.

							Thanx, Paul

> ---
> v1…v2:
>   - Drop the ATOM_BH* bits. There don't seem to be needed, Paul did not
>     ant the preempt-disable around enabling/disabling BH as it might fix
>     things that RCU should take care.
> 
>   - Allow enabling RCU with disabled interrupts on RT. Scott confirmed
>     that it was needed but might no longer be needed. Paul said that it
>     might have been required at some point. It survived multiple 6h long
>     TREE01 and TREE06 testing.
> 
>  kernel/rcu/rcutorture.c | 48 ++++++++++++++++++++++++++++++-----------
>  1 file changed, 36 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index 40ef5417d9545..d2ef535530b10 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;
>  
> -	/* 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);
>  
> @@ -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 +1512,26 @@ 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 modify BH in atomic context */
> +		if (oldmask & preempts_irq)
> +			mask &= ~bhs;
> +		if ((oldmask | mask) & preempts_irq)
> +			mask |= oldmask & bhs;
> +	}
> +
>  	return mask ?: RCUTORTURE_RDR_RCU;
>  }
>  
> -- 
> 2.33.0
> 

  reply	other threads:[~2021-08-20 22:10 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
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 [this message]
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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