From 0bf3924bfabd13ba21aa702344fc00b3b3263e5a Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Fri, 12 Jun 2020 15:55:00 +0200 Subject: [PATCH] x86/entry: Force rcu_irq_enter() when in idle task 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 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. Fixes: 3eeec3858488 ("x86/entry: Provide idtentry_entry/exit_cond_rcu()") Reported-by: "Paul E. McKenney" Signed-off-by: Thomas Gleixner Tested-by: "Paul E. McKenney" Reviewed-by: "Paul E. McKenney" Acked-by: Andy Lutomirski Acked-by: Frederic Weisbecker Link: https://lkml.kernel.org/r/87wo4cxubv.fsf@nanos.tec.linutronix.de --- arch/x86/entry/common.c | 35 ++++++++++++++++++++++++++++------- 1 file changed, 28 insertions(+), 7 deletions(-) diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c index f4d57782c14b..bd3f14175193 100644 --- a/arch/x86/entry/common.c +++ b/arch/x86/entry/common.c @@ -557,14 +557,34 @@ bool noinstr idtentry_enter_cond_rcu(struct pt_regs *regs) 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(struct pt_regs *regs) } /* - * 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(); -- 2.20.1