openEuler中的Read-Copy Update机制 ​

/*
 * Definition for node within the RCU grace-period-detection hierarchy.
 */
struct rcu_node {
	raw_spinlock_t __private lock;	/* Root rcu_node's lock protects */
					/*  some rcu_state fields as well as */
					/*  following. */
	unsigned long gp_seq;	/* Track rsp->rcu_gp_seq. */
	unsigned long gp_seq_needed; /* Track rsp->rcu_gp_seq_needed. */
	unsigned long completedqs; /* All QSes done for this node. */
	unsigned long qsmask;	/* CPUs or groups that need to switch in */
				/*  order for current grace period to proceed.*/
				/*  In leaf rcu_node, each bit corresponds to */
				/*  an rcu_data structure, otherwise, each */
				/*  bit corresponds to a child rcu_node */
				/*  structure. */
	unsigned long rcu_gp_init_mask;	/* Mask of offline CPUs at GP init. */
	unsigned long qsmaskinit;
	……
				/*  Only one bit will be set in this mask. */
	int	grplo;		/* lowest-numbered CPU or group here. */
	int	grphi;		/* highest-numbered CPU or group here. */
	u8	grpnum;		/* CPU/group number for next level up. */
	u8	level;		/* root is at level 0. */
	bool	wait_blkd_tasks;/* Necessary to wait for blocked tasks to */
				/*  exit RCU read-side critical sections */
				/*  before propagating offline up the */
				/*  rcu_node tree? */
				bool	wait_blkd_tasks;/* Necessary to wait for blocked tasks to */
				/*  exit RCU read-side critical sections */
				/*  before propagating offline up the */
				/*  rcu_node tree? */
	struct rcu_node *parent;
	struct list_head blkd_tasks;
				/* Tasks blocked in RCU read-side critical */
				/*  section.  Tasks are placed at the head */
				/*  of this list and age towards the tail. */
	struct list_head *gp_tasks;
				/* Pointer to the first task blocking the */
				/*  current grace period, or NULL if there */
				/*  is no such task. */
	
				
	
	……
} ____cacheline_internodealigned_in_smp;

/* Per-CPU data for read-copy update. */
struct rcu_data {
	/* 1) quiescent-state and grace-period handling : */
	unsigned long	gp_seq;		/* Track rsp->rcu_gp_seq counter. */
	unsigned long	gp_seq_needed;	/* Track rsp->rcu_gp_seq_needed ctr. */
	unsigned long	rcu_qs_ctr_snap;/* Snapshot of rcu_qs_ctr to check */
					/*  for rcu_all_qs() invocations. */
	union rcu_noqs	cpu_no_qs;	/* No QSes yet for this CPU. */
	bool		core_needs_qs;	/* Core waits for quiesc state. */
	bool		beenonline;	/* CPU online at least once. */
	bool		gpwrap;		/* Possible ->gp_seq wrap. */
	struct rcu_node *mynode;	/* This CPU's leaf of hierarchy */
	unsigned long grpmask;		/* Mask to apply to leaf qsmask. */
	/* 2) batch handling */
	struct rcu_segcblist cblist;	/* Segmented callback list, with */
					/* different callbacks waiting for */
					/* different grace periods. */
    ……

	int cpu;
	struct rcu_state *rsp;
};

/*
 * Preemptible RCU implementation for rcu_read_unlock().
 * Decrement ->rcu_read_lock_nesting.  If the result is zero (outermost
 * rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then
 * invoke rcu_read_unlock_special() to clean up after a context switch
 * in an RCU read-side critical section and other special cases.
 */
void __rcu_read_unlock(void)
{
	struct task_struct *t = current;

	if (t->rcu_read_lock_nesting != 1) {
		--t->rcu_read_lock_nesting;
	} else {
		barrier();  /* critical section before exit code. */
		t->rcu_read_lock_nesting = INT_MIN;
		barrier();  /* assign before ->rcu_read_unlock_special load */
		if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
			rcu_read_unlock_special(t);
		barrier();  /* ->rcu_read_unlock_special load before assign */
		t->rcu_read_lock_nesting = 0;
	}
#ifdef CONFIG_PROVE_LOCKING
	{
		int rrln = READ_ONCE(t->rcu_read_lock_nesting);

		WARN_ON_ONCE(rrln < 0 && rrln > INT_MIN / 2);
	}
#endif /* #ifdef CONFIG_PROVE_LOCKING */
}
EXPORT_SYMBOL_GPL(__rcu_read_unlock);

/**
 * synchronize_rcu - wait until a grace period has elapsed.
 *
 * Control will return to the caller some time after a full grace
 * period has elapsed, in other words after all currently executing RCU
 * read-side critical sections have completed.  Note, however, that
 * upon return from synchronize_rcu(), the caller might well be executing
 * concurrently with new RCU read-side critical sections that began while
 * synchronize_rcu() was waiting.  RCU read-side critical sections are
 * delimited by rcu_read_lock() and rcu_read_unlock(), and may be nested.
 *
 * See the description of synchronize_sched() for more detailed
 * information on memory-ordering guarantees.  However, please note
 * that -only- the memory-ordering guarantees apply.  For example,
 * synchronize_rcu() is -not- guaranteed to wait on things like code
 * protected by preempt_disable(), instead, synchronize_rcu() is -only-
 * guaranteed to wait on RCU read-side critical sections, that is, sections
 * of code protected by rcu_read_lock().
 */
void synchronize_rcu(void)
{
	RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) ||
			 lock_is_held(&rcu_lock_map) ||
			 lock_is_held(&rcu_sched_lock_map),
			 "Illegal synchronize_rcu() in RCU read-side critical section");
	if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
		return;
	if (rcu_gp_is_expedited())
		synchronize_rcu_expedited();
	else
		wait_rcu_gp(call_rcu);
}
EXPORT_SYMBOL_GPL(synchronize_rcu);

/*
 * Check to see if this CPU is in a non-context-switch quiescent state
 * (user mode or idle loop for rcu, non-softirq execution for rcu_bh).
 * Also schedule RCU core processing.
 *
 * This function must be called from hardirq context.  It is normally
 * invoked from the scheduling-clock interrupt.
 */
void rcu_check_callbacks(int user)
{
	trace_rcu_utilization(TPS("Start scheduler-tick"));
	increment_cpu_stall_ticks();
	if (user || rcu_is_cpu_rrupt_from_idle()) {

		/*
		 * Get here if this CPU took its interrupt from user
		 * mode or from the idle loop, and if this is not a
		 * nested interrupt.  In this case, the CPU is in
		 * a quiescent state, so note it.
		 *
		 * No memory barrier is required here because both
		 * rcu_sched_qs() and rcu_bh_qs() reference only CPU-local
		 * variables that other CPUs neither access nor modify,
		 * at least not while the corresponding CPU is online.
		 */

		rcu_sched_qs();
		rcu_bh_qs();
		rcu_note_voluntary_context_switch(current);

	} else if (!in_softirq()) {

		/*
		 * Get here if this CPU did not take its interrupt from
		 * softirq, in other words, if it is not interrupting
		 * a rcu_bh read-side critical section.  This is an _bh
		 * critical section, so note it.
		 */

		rcu_bh_qs();
	}
	rcu_preempt_check_callbacks();
	/* The load-acquire pairs with the store-release setting to true. */
	if (smp_load_acquire(this_cpu_ptr(&rcu_dynticks.rcu_urgent_qs))) {
		/* Idle and userspace execution already are quiescent states. */
		if (!rcu_is_cpu_rrupt_from_idle() && !user) {
			set_tsk_need_resched(current);
			set_preempt_need_resched();
		}
		__this_cpu_write(rcu_dynticks.rcu_urgent_qs, false);
	}
	if (rcu_pending())
		invoke_rcu_core();

	trace_rcu_utilization(TPS("End scheduler-tick"));
}

static void invoke_rcu_core(void)
{
	if (cpu_online(smp_processor_id()))
		raise_softirq(RCU_SOFTIRQ);
}

/*
 * Do RCU core processing for the current CPU.
 */
static __latent_entropy void rcu_process_callbacks(struct softirq_action *unused)
{
	struct rcu_state *rsp;

	if (cpu_is_offline(smp_processor_id()))
		return;
	trace_rcu_utilization(TPS("Start RCU core"));
	for_each_rcu_flavor(rsp)
		__rcu_process_callbacks(rsp);
	trace_rcu_utilization(TPS("End RCU core"));
}

/*
 * This does the RCU core processing work for the specified rcu_state
 * and rcu_data structures.  This may be called only from the CPU to
 * whom the rdp belongs.
 */
static void
__rcu_process_callbacks(struct rcu_state *rsp)
{
	unsigned long flags;
	struct rcu_data *rdp = raw_cpu_ptr(rsp->rda);
	struct rcu_node *rnp = rdp->mynode;

	WARN_ON_ONCE(!rdp->beenonline);

	/* Update RCU state based on any recent quiescent states. */
	rcu_check_quiescent_state(rsp, rdp);

	/* No grace period and unregistered callbacks? */
	if (!rcu_gp_in_progress(rsp) &&
	    rcu_segcblist_is_enabled(&rdp->cblist)) {
		local_irq_save(flags);
		if (!rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL))
			rcu_accelerate_cbs_unlocked(rsp, rnp, rdp);
		local_irq_restore(flags);
	}

	rcu_check_gp_start_stall(rsp, rnp, rdp);

	/* If there are callbacks ready, invoke them. */
	if (rcu_segcblist_ready_cbs(&rdp->cblist))
		invoke_rcu_callbacks(rsp, rdp);

	/* Do any needed deferred wakeups of rcuo kthreads. */
	do_nocb_deferred_wakeup(rdp);
}

/*
 * Check to see if there is a new grace period of which this CPU
 * is not yet aware, and if so, set up local rcu_data state for it.
 * Otherwise, see if this CPU has just passed through its first
 * quiescent state for this grace period, and record that fact if so.
 */
static void
rcu_check_quiescent_state(struct rcu_state *rsp, struct rcu_data *rdp)
{
	/* Check for grace-period ends and beginnings. */
	note_gp_changes(rsp, rdp);

	/*
	 * Does this CPU still need to do its part for current grace period?
	 * If no, return and let the other CPUs do their part as well.
	 */
	if (!rdp->core_needs_qs)
		return;

	/*
	 * Was there a quiescent state since the beginning of the grace
	 * period? If no, then exit and wait for the next call.
	 */
	if (rdp->cpu_no_qs.b.norm)
		return;

	/*
	 * Tell RCU we are done (but rcu_report_qs_rdp() will be the
	 * judge of that).
	 */
	rcu_report_qs_rdp(rdp->cpu, rsp, rdp);
}

/*
 * Schedule RCU callback invocation.  If the specified type of RCU
 * does not support RCU priority boosting, just do a direct call,
 * otherwise wake up the per-CPU kernel kthread.  Note that because we
 * are running on the current CPU with softirqs disabled, the
 * rcu_cpu_kthread_task cannot disappear out from under us.
 */
static void invoke_rcu_callbacks(struct rcu_state *rsp, struct rcu_data *rdp)
{
	if (unlikely(!READ_ONCE(rcu_scheduler_fully_active)))
		return;
	if (likely(!rsp->boost)) {
		rcu_do_batch(rsp, rdp);
		return;
	}
	invoke_rcu_callbacks_kthread();
}

/*
 * Invoke any RCU callbacks that have made it to the end of their grace
 * period.  Thottle as specified by rdp->blimit.
 */
static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
{
	unsigned long flags;
	struct rcu_head *rhp;
	struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl);
	long bl, count;

	/* If no callbacks are ready, just return. */
	if (!rcu_segcblist_ready_cbs(&rdp->cblist)) {
		trace_rcu_batch_start(rsp->name,
				      rcu_segcblist_n_lazy_cbs(&rdp->cblist),
				      rcu_segcblist_n_cbs(&rdp->cblist), 0);
		trace_rcu_batch_end(rsp->name, 0,
				    !rcu_segcblist_empty(&rdp->cblist),
				    need_resched(), is_idle_task(current),
				    rcu_is_callbacks_kthread());
		return;
	}

	/*
	 * Extract the list of ready callbacks, disabling to prevent
	 * races with call_rcu() from interrupt handlers.  Leave the
	 * callback counts, as rcu_barrier() needs to be conservative.
	 */
	local_irq_save(flags);
	WARN_ON_ONCE(cpu_is_offline(smp_processor_id()));
	bl = rdp->blimit;
	trace_rcu_batch_start(rsp->name, rcu_segcblist_n_lazy_cbs(&rdp->cblist),
			      rcu_segcblist_n_cbs(&rdp->cblist), bl);
	rcu_segcblist_extract_done_cbs(&rdp->cblist, &rcl);
	local_irq_restore(flags);

	/* Invoke callbacks. */
	rhp = rcu_cblist_dequeue(&rcl);
	for (; rhp; rhp = rcu_cblist_dequeue(&rcl)) {
		debug_rcu_head_unqueue(rhp);
		if (__rcu_reclaim(rsp->name, rhp))
			rcu_cblist_dequeued_lazy(&rcl);
		/*
		 * Stop only if limit reached and CPU has something to do.
		 * Note: The rcl structure counts down from zero.
		 */
		if (-rcl.len >= bl &&
		    (need_resched() ||
		     (!is_idle_task(current) && !rcu_is_callbacks_kthread())))
			break;
	}

	local_irq_save(flags);
	count = -rcl.len;
	trace_rcu_batch_end(rsp->name, count, !!rcl.head, need_resched(),
			    is_idle_task(current), rcu_is_callbacks_kthread());

	/* Update counts and requeue any remaining callbacks. */
	rcu_segcblist_insert_done_cbs(&rdp->cblist, &rcl);
	smp_mb(); /* List handling before counting for rcu_barrier(). */
	rcu_segcblist_insert_count(&rdp->cblist, &rcl);

	/* Reinstate batch limit if we have worked down the excess. */
	count = rcu_segcblist_n_cbs(&rdp->cblist);
	if (rdp->blimit == LONG_MAX && count <= qlowmark)
		rdp->blimit = blimit;

	/* Reset ->qlen_last_fqs_check trigger if enough CBs have drained. */
	if (count == 0 && rdp->qlen_last_fqs_check != 0) {
		rdp->qlen_last_fqs_check = 0;
		rdp->n_force_qs_snap = rsp->n_force_qs;
	} else if (count < rdp->qlen_last_fqs_check - qhimark)
		rdp->qlen_last_fqs_check = count;

	/*
	 * The following usually indicates a double call_rcu().  To track
	 * this down, try building with CONFIG_DEBUG_OBJECTS_RCU_HEAD=y.
	 */
	WARN_ON_ONCE(rcu_segcblist_empty(&rdp->cblist) != (count == 0));

	local_irq_restore(flags);

	/* Re-invoke RCU core processing if there are callbacks remaining. */
	if (rcu_segcblist_ready_cbs(&rdp->cblist))
		invoke_rcu_core();
}

static void invoke_rcu_core(void)
{
	if (cpu_online(smp_processor_id()))
		raise_softirq(RCU_SOFTIRQ);
}

/*
 * Wake up the per-CPU kthread to invoke RCU callbacks.
 */
static void invoke_rcu_callbacks_kthread(void)
{
	unsigned long flags;

	local_irq_save(flags);
	__this_cpu_write(rcu_cpu_has_work, 1);
	if (__this_cpu_read(rcu_cpu_kthread_task) != NULL &&
	    current != __this_cpu_read(rcu_cpu_kthread_task)) {
		rcu_wake_cond(__this_cpu_read(rcu_cpu_kthread_task),
			      __this_cpu_read(rcu_cpu_kthread_status));
	}
	local_irq_restore(flags);
}