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: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 156ac8d0418befabc18852e89126acf4e06bf9d3..0bbcbf398169458f38d6bdae89e52141a7af297b 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -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);
                }