entry: Add arch_in_rcu_eqs()

JIRA: https://issues.redhat.com/browse/RHEL-113440

commit ee4a2e0
Author: Mark Rutland <mark.rutland@arm.com>
Date:   Tue Jul 8 19:27:41 2025 +1000

    entry: Add arch_in_rcu_eqs()

    All architectures have an interruptible RCU extended quiescent state
    (EQS) as part of their idle sequences, where interrupts can occur
    without RCU watching. Entry code must account for this and wake RCU as
    necessary; the common entry code deals with this in irqentry_enter() by
    treating any interrupt from an idle thread as potentially having
    occurred within an EQS and waking RCU for the duration of the interrupt
    via rcu_irq_enter() .. rcu_irq_exit().

    Some architectures may have other interruptible EQSs which require
    similar treatment. For example, on s390 it is necessary to enable
    interrupts around guest entry in the middle of a period where core KVM
    code has entered an EQS.

    So that architectures can wake RCU in these cases, this patch adds a
    new arch_in_rcu_eqs() hook to the common entry code which is checked in
    addition to the existing is_idle_thread() check, with RCU woken if
    either returns true. A default implementation is provided which always
    returns false, which suffices for most architectures.

    As no architectures currently implement arch_in_rcu_eqs(), there should
    be no functional change as a result of this patch alone. A subsequent
    patch will add an s390 implementation to fix a latent bug with missing
    RCU wakeups.

    [ajd@linux.ibm.com: rebase, fix commit message]

    Signed-off-by: Mark Rutland <mark.rutland@arm.com>
    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
    Cc: Heiko Carstens <hca@linux.ibm.com>
    Cc: Paolo Bonzini <pbonzini@redhat.com>
    Cc: Paul E. McKenney <paulmck@kernel.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Sven Schnelle <svens@linux.ibm.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
    Cc: Vasily Gorbik <gor@linux.ibm.com>
    Cc: Alexander Gordeev <agordeev@linux.ibm.com>
    Cc: Janosch Frank <frankja@linux.ibm.com>
    Reviewed-by: Christian Borntraeger <borntraeger@linux.ibm.com>
    Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
    Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
    Link: https://lore.kernel.org/r/20250708092742.104309-2-ajd@linux.ibm.com
    Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
    Message-ID: <20250708092742.104309-2-ajd@linux.ibm.com>

Signed-off-by: Thomas Huth <thuth@redhat.com>

Author: huth

include/linux/entry-common.h

@@ -85,6 +85,22 @@ static __always_inline void arch_enter_from_user_mode(struct pt_regs *regs);
 static __always_inline void arch_enter_from_user_mode(struct pt_regs *regs) {}
 #endif
 
+/**
+ * arch_in_rcu_eqs - Architecture specific check for RCU extended quiescent
+ * states.
+ *
+ * Returns: true if the CPU is potentially in an RCU EQS, false otherwise.
+ *
+ * Architectures only need to define this if threads other than the idle thread
+ * may have an interruptible EQS. This does not need to handle idle threads. It
+ * is safe to over-estimate at the cost of redundant RCU management work.
+ *
+ * Invoked from irqentry_enter()
+ */
+#ifndef arch_in_rcu_eqs
+static __always_inline bool arch_in_rcu_eqs(void) { return false; }
+#endif
+
 /**
  * enter_from_user_mode - Establish state when coming from user mode
  *

kernel/entry/common.c

@@ -267,7 +267,8 @@ noinstr irqentry_state_t irqentry_enter(struct pt_regs *regs)
 	 * 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 (!IS_ENABLED(CONFIG_TINY_RCU) &&
+	    (is_idle_task(current) || arch_in_rcu_eqs())) {
 		/*
 		 * If RCU is not watching then the same careful
 		 * sequence vs. lockdep and tracing is required