C++ READ_ONCE函数代码示例

static unsigned int mce_chrdev_poll(struct file *file, poll_table *wait)
{
	poll_wait(file, &mce_chrdev_wait, wait);
	if (READ_ONCE(mcelog.next))
		return POLLIN | POLLRDNORM;
	if (!mce_apei_read_done && apei_check_mce())
		return POLLIN | POLLRDNORM;
	return 0;
}

static void *array_of_map_lookup_elem(struct bpf_map *map, void *key)
{
	struct bpf_map **inner_map = array_map_lookup_elem(map, key);

	if (!inner_map)
		return NULL;

	return READ_ONCE(*inner_map);
}

/*
 * A timer is active, when it is enqueued into the rbtree or the
 * callback function is running or it's in the state of being migrated
 * to another cpu.
 *
 * It is important for this function to not return a false negative.
 */
bool hrtimer_active(const struct hrtimer *timer)
{
	struct hrtimer_cpu_base *cpu_base;
	unsigned int seq;

	do {
		cpu_base = READ_ONCE(timer->base->cpu_base);
		seq = raw_read_seqcount_begin(&cpu_base->seq);

		if (timer->state != HRTIMER_STATE_INACTIVE ||
		    cpu_base->running == timer)
			return true;

	} while (read_seqcount_retry(&cpu_base->seq, seq) ||
		 cpu_base != READ_ONCE(timer->base->cpu_base));

	return false;
}

/*
 * Update host ring buffer after iterating over packets.
 */
void hv_pkt_iter_close(struct vmbus_channel *channel)
{
	struct hv_ring_buffer_info *rbi = &channel->inbound;
	u32 curr_write_sz, pending_sz, bytes_read, start_read_index;

	/*
	 * Make sure all reads are done before we update the read index since
	 * the writer may start writing to the read area once the read index
	 * is updated.
	 */
	virt_rmb();
	start_read_index = rbi->ring_buffer->read_index;
	rbi->ring_buffer->read_index = rbi->priv_read_index;

	if (!rbi->ring_buffer->feature_bits.feat_pending_send_sz)
		return;

	/*
	 * Issue a full memory barrier before making the signaling decision.
	 * Here is the reason for having this barrier:
	 * If the reading of the pend_sz (in this function)
	 * were to be reordered and read before we commit the new read
	 * index (in the calling function)  we could
	 * have a problem. If the host were to set the pending_sz after we
	 * have sampled pending_sz and go to sleep before we commit the
	 * read index, we could miss sending the interrupt. Issue a full
	 * memory barrier to address this.
	 */
	virt_mb();

	pending_sz = READ_ONCE(rbi->ring_buffer->pending_send_sz);
	if (!pending_sz)
		return;

	/*
	 * Ensure the read of write_index in hv_get_bytes_to_write()
	 * happens after the read of pending_send_sz.
	 */
	virt_rmb();
	curr_write_sz = hv_get_bytes_to_write(rbi);
	bytes_read = hv_pkt_iter_bytes_read(rbi, start_read_index);

	/*
	 * If there was space before we began iteration,
	 * then host was not blocked.
	 */

	if (curr_write_sz - bytes_read > pending_sz)
		return;

	/* If pending write will not fit, don't give false hope. */
	if (curr_write_sz <= pending_sz)
		return;

	vmbus_setevent(channel);
}

/*
 * Workqueue handler to drive one grace period and invoke any callbacks
 * that become ready as a result.  Single-CPU and !PREEMPT operation
 * means that we get away with murder on synchronization.  ;-)
 */
void srcu_drive_gp(struct work_struct *wp)
{
	int idx;
	struct rcu_head *lh;
	struct rcu_head *rhp;
	struct srcu_struct *sp;

	sp = container_of(wp, struct srcu_struct, srcu_work);
	if (sp->srcu_gp_running || !READ_ONCE(sp->srcu_cb_head))
		return; /* Already running or nothing to do. */

	/* Remove recently arrived callbacks and wait for readers. */
	WRITE_ONCE(sp->srcu_gp_running, true);
	local_irq_disable();
	lh = sp->srcu_cb_head;
	sp->srcu_cb_head = NULL;
	sp->srcu_cb_tail = &sp->srcu_cb_head;
	local_irq_enable();
	idx = sp->srcu_idx;
	WRITE_ONCE(sp->srcu_idx, !sp->srcu_idx);
	WRITE_ONCE(sp->srcu_gp_waiting, true);  /* srcu_read_unlock() wakes! */
	swait_event_exclusive(sp->srcu_wq, !READ_ONCE(sp->srcu_lock_nesting[idx]));
	WRITE_ONCE(sp->srcu_gp_waiting, false); /* srcu_read_unlock() cheap. */

	/* Invoke the callbacks we removed above. */
	while (lh) {
		rhp = lh;
		lh = lh->next;
		local_bh_disable();
		rhp->func(rhp);
		local_bh_enable();
	}

	/*
	 * Enable rescheduling, and if there are more callbacks,
	 * reschedule ourselves.  This can race with a call_srcu()
	 * at interrupt level, but the ->srcu_gp_running checks will
	 * straighten that out.
	 */
	WRITE_ONCE(sp->srcu_gp_running, false);
	if (READ_ONCE(sp->srcu_cb_head))
		schedule_work(&sp->srcu_work);
}

/*
 * Determine number of bytes available in ring buffer after
 * the current iterator (priv_read_index) location.
 *
 * This is similar to hv_get_bytes_to_read but with private
 * read index instead.
 */
static u32 hv_pkt_iter_avail(const struct hv_ring_buffer_info *rbi)
{
	u32 priv_read_loc = rbi->priv_read_index;
	u32 write_loc = READ_ONCE(rbi->ring_buffer->write_index);

	if (write_loc >= priv_read_loc)
		return write_loc - priv_read_loc;
	else
		return (rbi->ring_datasize - priv_read_loc) + write_loc;
}

/*
 * Fill out an ACK packet.
 */
static size_t rxrpc_fill_out_ack(struct rxrpc_call *call,
				 struct rxrpc_ack_buffer *pkt,
				 rxrpc_seq_t *_hard_ack,
				 rxrpc_seq_t *_top,
				 u8 reason)
{
	rxrpc_serial_t serial;
	rxrpc_seq_t hard_ack, top, seq;
	int ix;
	u32 mtu, jmax;
	u8 *ackp = pkt->acks;

	/* Barrier against rxrpc_input_data(). */
	serial = call->ackr_serial;
	hard_ack = READ_ONCE(call->rx_hard_ack);
	top = smp_load_acquire(&call->rx_top);
	*_hard_ack = hard_ack;
	*_top = top;

	pkt->ack.bufferSpace	= htons(8);
	pkt->ack.maxSkew	= htons(call->ackr_skew);
	pkt->ack.firstPacket	= htonl(hard_ack + 1);
	pkt->ack.previousPacket	= htonl(call->ackr_prev_seq);
	pkt->ack.serial		= htonl(serial);
	pkt->ack.reason		= reason;
	pkt->ack.nAcks		= top - hard_ack;

	if (reason == RXRPC_ACK_PING)
		pkt->whdr.flags |= RXRPC_REQUEST_ACK;

	if (after(top, hard_ack)) {
		seq = hard_ack + 1;
		do {
			ix = seq & RXRPC_RXTX_BUFF_MASK;
			if (call->rxtx_buffer[ix])
				*ackp++ = RXRPC_ACK_TYPE_ACK;
			else
				*ackp++ = RXRPC_ACK_TYPE_NACK;
			seq++;
		} while (before_eq(seq, top));
	}

	mtu = call->conn->params.peer->if_mtu;
	mtu -= call->conn->params.peer->hdrsize;
	jmax = (call->nr_jumbo_bad > 3) ? 1 : rxrpc_rx_jumbo_max;
	pkt->ackinfo.rxMTU	= htonl(rxrpc_rx_mtu);
	pkt->ackinfo.maxMTU	= htonl(mtu);
	pkt->ackinfo.rwind	= htonl(call->rx_winsize);
	pkt->ackinfo.jumbo_max	= htonl(jmax);

	*ackp++ = 0;
	*ackp++ = 0;
	*ackp++ = 0;
	return top - hard_ack + 3;
}

void *thread_update(void *arg)
{
	WRITE_ONCE(y, 1);
#ifndef FORCE_FAILURE_2
	synchronize_srcu(&ss);
#endif
	might_sleep();
	__unbuffered_tpr_x = READ_ONCE(x);

	return NULL;
}

static void __init kasan_pud_populate(pgd_t *pgdp, unsigned long addr,
				      unsigned long end, int node, bool early)
{
	unsigned long next;
	pud_t *pudp = kasan_pud_offset(pgdp, addr, node, early);

	do {
		next = pud_addr_end(addr, end);
		kasan_pmd_populate(pudp, addr, next, node, early);
	} while (pudp++, addr = next, addr != end && pud_none(READ_ONCE(*pudp)));
}

void *test_xchg_lock(void *arg)
{
	int me = (long)arg;

	run_on(me);
	atomic_inc(&nthreadsrunning);
	while (READ_ONCE(goflag) == GOFLAG_INIT)
		poll(NULL, 0, 1);
	while (READ_ONCE(goflag) == GOFLAG_RUN) {
		xchg_lock(&testlock);
		if (owner != -1)
			lockerr++;
		lockacqs++;
		owner = me;
		poll(NULL, 0, 1);
		owner = -1;
		xchg_unlock(&testlock);
	}
	return NULL;
}

/*
 * Copy the current shadow region into a new pgdir.
 */
void __init kasan_copy_shadow(pgd_t *pgdir)
{
	pgd_t *pgdp, *pgdp_new, *pgdp_end;

	pgdp = pgd_offset_k(KASAN_SHADOW_START);
	pgdp_end = pgd_offset_k(KASAN_SHADOW_END);
	pgdp_new = pgd_offset_raw(pgdir, KASAN_SHADOW_START);
	do {
		set_pgd(pgdp_new, READ_ONCE(*pgdp));
	} while (pgdp++, pgdp_new++, pgdp != pgdp_end);
}

/*
 * task->mm can be NULL if the task is the exited group leader.  So to
 * determine whether the task is using a particular mm, we examine all the
 * task's threads: if one of those is using this mm then this task was also
 * using it.
 */
static bool process_shares_mm(struct task_struct *p, struct mm_struct *mm)
{
	struct task_struct *t;

	for_each_thread(p, t) {
		struct mm_struct *t_mm = READ_ONCE(t->mm);
		if (t_mm)
			return t_mm == mm;
	}
	return false;
}

static void hv_signal_on_write(u32 old_write, struct vmbus_channel *channel,
			       bool kick_q)
{
	struct hv_ring_buffer_info *rbi = &channel->outbound;

	virt_mb();
	if (READ_ONCE(rbi->ring_buffer->interrupt_mask))
		return;

	/* check interrupt_mask before read_index */
	virt_rmb();
	/*
	 * This is the only case we need to signal when the
	 * ring transitions from being empty to non-empty.
	 */
	if (old_write == READ_ONCE(rbi->ring_buffer->read_index))
		vmbus_setevent(channel);

	return;
}

/* Called with IRQs disabled. */
__visible inline void prepare_exit_to_usermode(struct pt_regs *regs)
{
	struct thread_info *ti = current_thread_info();
	u32 cached_flags;

	addr_limit_user_check();

	lockdep_assert_irqs_disabled();
	lockdep_sys_exit();

	cached_flags = READ_ONCE(ti->flags);

	if (unlikely(cached_flags & EXIT_TO_USERMODE_LOOP_FLAGS))
		exit_to_usermode_loop(regs, cached_flags);

	/* Reload ti->flags; we may have rescheduled above. */
	cached_flags = READ_ONCE(ti->flags);

	fpregs_assert_state_consistent();
	if (unlikely(cached_flags & _TIF_NEED_FPU_LOAD))
		switch_fpu_return();

#ifdef CONFIG_COMPAT
	/*
	 * Compat syscalls set TS_COMPAT.  Make sure we clear it before
	 * returning to user mode.  We need to clear it *after* signal
	 * handling, because syscall restart has a fixup for compat
	 * syscalls.  The fixup is exercised by the ptrace_syscall_32
	 * selftest.
	 *
	 * We also need to clear TS_REGS_POKED_I386: the 32-bit tracer
	 * special case only applies after poking regs and before the
	 * very next return to user mode.
	 */
	ti->status &= ~(TS_COMPAT|TS_I386_REGS_POKED);
#endif

	user_enter_irqoff();

	mds_user_clear_cpu_buffers();
}

void quarantine_reduce(void)
{
	size_t new_quarantine_size, percpu_quarantines;
	unsigned long flags;
	struct qlist_head to_free = QLIST_INIT;
	size_t size_to_free = 0;
	struct qlist_node *last;

	if (likely(READ_ONCE(global_quarantine.bytes) <=
		   READ_ONCE(quarantine_size)))
		return;

	spin_lock_irqsave(&quarantine_lock, flags);

	/*
	 * Update quarantine size in case of hotplug. Allocate a fraction of
	 * the installed memory to quarantine minus per-cpu queue limits.
	 */
	new_quarantine_size = (READ_ONCE(totalram_pages) << PAGE_SHIFT) /
		QUARANTINE_FRACTION;
	percpu_quarantines = QUARANTINE_PERCPU_SIZE * num_online_cpus();
	new_quarantine_size = (new_quarantine_size < percpu_quarantines) ?
		0 : new_quarantine_size - percpu_quarantines;
	WRITE_ONCE(quarantine_size, new_quarantine_size);

	last = global_quarantine.head;
	while (last) {
		struct kmem_cache *cache = qlink_to_cache(last);

		size_to_free += cache->size;
		if (!last->next || size_to_free >
		    global_quarantine.bytes - QUARANTINE_LOW_SIZE)
			break;
		last = last->next;
	}
	qlist_move(&global_quarantine, last, &to_free, size_to_free);

	spin_unlock_irqrestore(&quarantine_lock, flags);

	qlist_free_all(&to_free, NULL);
}