[patch V4 part 1 01/36] rcu: Add comments marking transitions between RCU watching and not

Thomas Gleixner
2020-05-05T14:13:51Z

From: "Paul E. McKenney" <paulmck@kernel.org>

It is not as clear as it might be just where in RCU's idle entry/exit
code RCU stops and starts watching the current CPU. This commit therefore
adds comments calling out the transitions.

Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200313024046.27622-2-paulmck@kernel.org

---
kernel/rcu/tree.c | 29 +++++++++++++++++++++++++----
1 file changed, 25 insertions(+), 4 deletions(-)

--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -225,7 +225,9 @@ void rcu_softirq_qs(void)

/*
* Record entry into an extended quiescent state. This is only to be
- * called when not already in an extended quiescent state.
+ * called when not already in an extended quiescent state, that is,
+ * RCU is watching prior to the call to this function and is no longer
+ * watching upon return.
*/
static void rcu_dynticks_eqs_enter(void)
{
@@ -238,7 +240,7 @@ static void rcu_dynticks_eqs_enter(void)
* next idle sojourn.
*/
seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks);
- /* Better be in an extended quiescent state! */
+ // RCU is no longer watching. Better be in extended quiescent state!
WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
(seq & RCU_DYNTICK_CTRL_CTR));
/* Better not have special action (TLB flush) pending! */
@@ -248,7 +250,8 @@ static void rcu_dynticks_eqs_enter(void)

/*
* Record exit from an extended quiescent state. This is only to be
- * called from an extended quiescent state.
+ * called from an extended quiescent state, that is, RCU is not watching
+ * prior to the call to this function and is watching upon return.
*/
static void rcu_dynticks_eqs_exit(void)
{
@@ -261,6 +264,7 @@ static void rcu_dynticks_eqs_exit(void)
* critical section.
*/
seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks);
+ // RCU is now watching. Better not be in an extended quiescent state!
WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
!(seq & RCU_DYNTICK_CTRL_CTR));
if (seq & RCU_DYNTICK_CTRL_MASK) {
@@ -571,6 +575,7 @@ static void rcu_eqs_enter(bool user)
WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
rdp->dynticks_nesting == 0);
if (rdp->dynticks_nesting != 1) {
+ // RCU will still be watching, so just do accounting and leave.
rdp->dynticks_nesting--;
return;
}
@@ -583,7 +588,9 @@ static void rcu_eqs_enter(bool user)
rcu_prepare_for_idle();
rcu_preempt_deferred_qs(current);
WRITE_ONCE(rdp->dynticks_nesting, 0); /* Avoid irq-access tearing. */
+ // RCU is watching here ...
rcu_dynticks_eqs_enter();
+ // ... but is no longer watching here.
rcu_dynticks_task_enter();
}

@@ -663,7 +670,9 @@ static __always_inline void rcu_nmi_exit
if (irq)
rcu_prepare_for_idle();

+ // RCU is watching here ...
rcu_dynticks_eqs_enter();
+ // ... but is no longer watching here.

if (irq)
rcu_dynticks_task_enter();
@@ -738,11 +747,14 @@ static void rcu_eqs_exit(bool user)
oldval = rdp->dynticks_nesting;
WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && oldval < 0);
if (oldval) {
+ // RCU was already watching, so just do accounting and leave.
rdp->dynticks_nesting++;
return;
}
rcu_dynticks_task_exit();
+ // RCU is not watching here ...
rcu_dynticks_eqs_exit();
+ // ... but is watching here.
rcu_cleanup_after_idle();
trace_rcu_dyntick(TPS("End"), rdp->dynticks_nesting, 1, atomic_read(&rdp->dynticks));
WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
@@ -819,7 +831,9 @@ static __always_inline void rcu_nmi_ente
if (irq)
rcu_dynticks_task_exit();

+ // RCU is not watching here ...
rcu_dynticks_eqs_exit();
+ // ... but is watching here.

if (irq)
rcu_cleanup_after_idle();
@@ -829,9 +843,16 @@ static __always_inline void rcu_nmi_ente
rdp->dynticks_nmi_nesting == DYNTICK_IRQ_NONIDLE &&
READ_ONCE(rdp->rcu_urgent_qs) &&
!READ_ONCE(rdp->rcu_forced_tick)) {
+ // We get here only if we had already exited the extended
+ // quiescent state and this was an interrupt (not an NMI).
+ // Therefore, (1) RCU is already watching and (2) The fact
+ // that we are in an interrupt handler and that the rcu_node
+ // lock is an irq-disabled lock prevents self-deadlock.
+ // So we can safely recheck under the lock.
raw_spin_lock_rcu_node(rdp->mynode);
- // Recheck under lock.
if (rdp->rcu_urgent_qs && !rdp->rcu_forced_tick) {
+ // A nohz_full CPU is in the kernel and RCU
+ // needs a quiescent state. Turn on the tick!
WRITE_ONCE(rdp->rcu_forced_tick, true);
tick_dep_set_cpu(rdp->cpu, TICK_DEP_BIT_RCU);
}