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Date: Fri, 20 Mar 2020 18:59:58 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: LKML <linux-kernel@...r.kernel.org>
Cc: x86@...nel.org, Paul McKenney <paulmck@...nel.org>,
Josh Poimboeuf <jpoimboe@...hat.com>,
"Joel Fernandes (Google)" <joel@...lfernandes.org>,
"Steven Rostedt (VMware)" <rostedt@...dmis.org>,
Masami Hiramatsu <mhiramat@...nel.org>,
Alexei Starovoitov <ast@...nel.org>,
Frederic Weisbecker <frederic@...nel.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Brian Gerst <brgerst@...il.com>,
Juergen Gross <jgross@...e.com>,
Alexandre Chartre <alexandre.chartre@...cle.com>,
Peter Zijlstra <peterz@...radead.org>,
Tom Lendacky <thomas.lendacky@....com>,
Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org
Subject: [patch V3 02/23] rcu: Add comments marking transitions between RCU
watching and not
From: "Paul E. McKenney" <paulmck@...nel.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@...utronix.de>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Thomas Gleixner <tglx@...utronix.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
@@ -224,7 +224,9 @@ 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)
{
@@ -237,7 +239,7 @@ 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! */
@@ -247,7 +249,8 @@ 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)
{
@@ -260,6 +263,7 @@ 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) {
@@ -562,6 +566,7 @@ 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;
}
@@ -574,7 +579,9 @@ 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();
}
@@ -654,7 +661,9 @@ 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();
@@ -729,11 +738,14 @@ 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));
@@ -810,7 +822,9 @@ 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();
@@ -819,9 +833,16 @@ static __always_inline void rcu_nmi_ente
} else if (irq && tick_nohz_full_cpu(rdp->cpu) &&
rdp->dynticks_nmi_nesting == DYNTICK_IRQ_NONIDLE &&
READ_ONCE(rdp->rcu_urgent_qs) && !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!
rdp->rcu_forced_tick = true;
tick_dep_set_cpu(rdp->cpu, TICK_DEP_BIT_RCU);
}
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