Commit ac3caf82 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu: Add comments marking transitions between RCU watching and not 



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: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
parent 52b1fc3f

kernel/rcu/tree.c

+25 −4
Original line number Diff line number Diff line
@@ -238,7 +238,9 @@ 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)

{
@@ -251,7 +253,7 @@ 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! */
@@ -261,7 +263,8 @@ 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)

{
@@ -274,6 +277,7 @@ 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) {
@@ -584,6 +588,7 @@ 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;

	}
@@ -596,7 +601,9 @@ 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();

}
@@ -676,7 +683,9 @@ static __always_inline void rcu_nmi_exit_common(bool irq) 
	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();
@@ -751,11 +760,14 @@ 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));
@@ -832,7 +844,9 @@ static __always_inline void rcu_nmi_enter_common(bool irq) 
		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();
@@ -842,9 +856,16 @@ static __always_inline void rcu_nmi_enter_common(bool irq) 
		   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);

		}