Re: [PATCH x86/entry: Force rcu_irq_enter() when in idle task

["Paul E. McKenney" <paulmck@kernel.org>]

On Fri, Jun 12, 2020 at 03:55:00PM +0200, Thomas Gleixner wrote:
> The idea of conditionally calling into rcu_irq_enter() only when RCU is
> not watching turned out to be not completely thought through.
> 
> Paul noticed occasional premature end of grace periods in RCU torture
> testing. Bisection led to the commit which made the invocation of
> rcu_irq_enter() conditional on !rcu_is_watching().
> 
> It turned out that this conditional breaks RCU assumptions about the idle
> task when the scheduler tick happens to be a nested interrupt. Nested
> interrupts can happen when the first interrupt invokes softirq processing
> on return which enables interrupts. If that nested tick interrupt does not
> invoke rcu_irq_enter() then the nest accounting in RCU claims that this is
> the first interrupt which might mark a quiescient state and end grace
> periods prematurely.

For this last sentence, how about the following?

If that nested tick interrupt does not invoke rcu_irq_enter() then the
RCU's irq-nesting checks will believe that this interrupt came directly
from idle, which will cause RCU to report a quiescent state.  Because
this interrupt instead came from a softirq handler which might have
been executing an RCU read-side critical section, this can cause the
grace period to end prematurely.

> Change the condition from !rcu_is_watching() to is_idle_task(current) which
> enforces that interrupts in the idle task unconditionally invoke
> rcu_irq_enter() independent of the RCU state.
> 
> This is also correct vs. user mode entries in NOHZ full scenarios because
> user mode entries bring RCU out of EQS and force the RCU irq nesting state
> accounting to nested. As only the first interrupt can enter from user mode
> a nested tick interrupt will enter from kernel mode and as the nesting
> state accounting is forced to nesting it will not do anything stupid even
> if rcu_irq_enter() has not been invoked.

On the testing front, just like with my busted patch yesterday, this
patch breaks the TASKS03 rcutorture scenario by preventing the Tasks
RCU grace periods from ever completing.  However, this is an unusual
configuration with NO_HZ_FULL and one CPU actually being nohz_full.
The more conventional TASKS01 and TASKS02 scenarios do just fine.

I will therefore address this issue in a follow-on patch.

> Fixes: 3eeec3858488 ("x86/entry: Provide idtentry_entry/exit_cond_rcu()")
> Reported-by: "Paul E. McKenney" <paulmck@kernel.org>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Reviewed-by: "Paul E. McKenney" <paulmck@kernel.org>
Tested-by: "Paul E. McKenney" <paulmck@kernel.org>

> ---
>  arch/x86/entry/common.c |   35 ++++++++++++++++++++++++++++-------
>  1 file changed, 28 insertions(+), 7 deletions(-)
> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -557,14 +557,34 @@ bool noinstr idtentry_enter_cond_rcu(str
>  		return false;
>  	}
>  
> -	if (!__rcu_is_watching()) {
> +	/*
> +	 * If this entry hit the idle task invoke rcu_irq_enter() whether
> +	 * RCU is watching or not.
> +	 *
> +	 * Interupts can nest when the first interrupt invokes softirq
> +	 * processing on return which enables interrupts.
> +	 *
> +	 * Scheduler ticks in the idle task can mark quiescent state and
> +	 * terminate a grace period, if and only if the timer interrupt is
> +	 * not nested into another interrupt.
> +	 *
> +	 * Checking for __rcu_is_watching() here would prevent the nesting
> +	 * interrupt to invoke rcu_irq_enter(). If that nested interrupt is
> +	 * the tick then rcu_flavor_sched_clock_irq() would wrongfully
> +	 * assume that it is the first interupt and eventually claim
> +	 * quiescient state and end grace periods prematurely.
> +	 *
> +	 * Unconditionally invoke rcu_irq_enter() so RCU state stays
> +	 * consistent.
> +	 *
> +	 * TINY_RCU does not support EQS, so let the compiler eliminate
> +	 * this part when enabled.
> +	 */
> +	if (!IS_ENABLED(CONFIG_TINY_RCU) && is_idle_task(current)) {
>  		/*
>  		 * If RCU is not watching then the same careful
>  		 * sequence vs. lockdep and tracing is required
>  		 * as in enter_from_user_mode().
> -		 *
> -		 * This only happens for IRQs that hit the idle
> -		 * loop, i.e. if idle is not using MWAIT.
>  		 */
>  		lockdep_hardirqs_off(CALLER_ADDR0);
>  		rcu_irq_enter();
> @@ -576,9 +596,10 @@ bool noinstr idtentry_enter_cond_rcu(str
>  	}
>  
>  	/*
> -	 * If RCU is watching then RCU only wants to check
> -	 * whether it needs to restart the tick in NOHZ
> -	 * mode.
> +	 * If RCU is watching then RCU only wants to check whether it needs
> +	 * to restart the tick in NOHZ mode. rcu_irq_enter_check_tick()
> +	 * already contains a warning when RCU is not watching, so no point
> +	 * in having another one here.
>  	 */
>  	instrumentation_begin();
>  	rcu_irq_enter_check_tick();