From 3dfdfaff2d49aa7895ced8a54e7d61755eac6830 Mon Sep 17 00:00:00 2001 From: Frederic Weisbecker Date: Tue, 29 Apr 2025 12:08:40 +0200 Subject: [PATCH] rcu: Robustify rcu_is_cpu_rrupt_from_idle() RCU relies on the context tracking nesting counter in order to determine if it is running in extended quiescent state. However the context tracking nesting counter is not completely synchronized with the actual context tracking state: * The nesting counter is set to 1 or incremented further _after_ the actual state is set to RCU watching. * The nesting counter is set to 0 or decremented further _before_ the actual state is set to RCU not watching. Therefore it is safe to assume that if ct_nesting() > 0, RCU is watching. But if ct_nesting() <= 0, RCU is not watching except for tiny windows. This hasn't been a problem so far because rcu_is_cpu_rrupt_from_idle() has only been called from interrupts. However the code is confusing and abuses the role of the context tracking nesting counter while there are more accurate indicators available. Clarify and robustify accordingly. Signed-off-by: Frederic Weisbecker Signed-off-by: Joel Fernandes Signed-off-by: Neeraj Upadhyay (AMD) --- kernel/rcu/tree.c | 27 +++++++++++++++++---------- 1 file changed, 17 insertions(+), 10 deletions(-) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 14d4499c6fc31..f83bbb4088958 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -377,7 +377,7 @@ EXPORT_SYMBOL_GPL(rcu_momentary_eqs); */ static int rcu_is_cpu_rrupt_from_idle(void) { - long nesting; + long nmi_nesting = ct_nmi_nesting(); /* * Usually called from the tick; but also used from smp_function_call() @@ -389,21 +389,28 @@ static int rcu_is_cpu_rrupt_from_idle(void) /* Check for counter underflows */ RCU_LOCKDEP_WARN(ct_nesting() < 0, "RCU nesting counter underflow!"); - RCU_LOCKDEP_WARN(ct_nmi_nesting() <= 0, - "RCU nmi_nesting counter underflow/zero!"); - /* Are we at first interrupt nesting level? */ - nesting = ct_nmi_nesting(); - if (nesting > 1) + /* Non-idle interrupt or nested idle interrupt */ + if (nmi_nesting > 1) return false; /* - * If we're not in an interrupt, we must be in the idle task! + * Non nested idle interrupt (interrupting section where RCU + * wasn't watching). */ - WARN_ON_ONCE(!nesting && !is_idle_task(current)); + if (nmi_nesting == 1) + return true; + + /* Not in an interrupt */ + if (!nmi_nesting) { + RCU_LOCKDEP_WARN(!in_task() || !is_idle_task(current), + "RCU nmi_nesting counter not in idle task!"); + return !rcu_is_watching_curr_cpu(); + } - /* Does CPU appear to be idle from an RCU standpoint? */ - return ct_nesting() == 0; + RCU_LOCKDEP_WARN(1, "RCU nmi_nesting counter underflow/zero!"); + + return false; } #define DEFAULT_RCU_BLIMIT (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) ? 1000 : 10) -- 2.47.2