[bug report] rcu/nocb: Add bypass callback queueing

[Dan Carpenter <dan.carpenter@oracle.com>]

Hello Paul E. McKenney,

The patch 1afc4b18724f: "rcu/nocb: Add bypass callback queueing" from
Jul 2, 2019, leads to the following static checker warning:

	kernel/rcu/tree_plugin.h:1792 rcu_nocb_try_bypass()
	warn: unsigned 'c' is never less than zero.

kernel/rcu/tree_plugin.h
  1761  static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
  1762                                  bool *was_alldone, unsigned long flags)
  1763  {
  1764          unsigned long c;
                ^^^^^^^^^^^^^^^
  1765          unsigned long cur_gp_seq;
  1766          unsigned long j = jiffies;
  1767          long ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
  1768  
  1769          if (!rcu_segcblist_is_offloaded(&rdp->cblist)) {
  1770                  *was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
  1771                  return false; /* Not offloaded, no bypassing. */
  1772          }
  1773          lockdep_assert_irqs_disabled();
  1774  
  1775          // Don't use ->nocb_bypass during early boot.
  1776          if (rcu_scheduler_active != RCU_SCHEDULER_RUNNING) {
  1777                  rcu_nocb_lock(rdp);
  1778                  WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
  1779                  *was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
  1780                  return false;
  1781          }
  1782  
  1783          // If we have advanced to a new jiffy, reset counts to allow
  1784          // moving back from ->nocb_bypass to ->cblist.
  1785          if (j == rdp->nocb_nobypass_last) {
  1786                  c = rdp->nocb_nobypass_count + 1;
  1787          } else {
  1788                  WRITE_ONCE(rdp->nocb_nobypass_last, j);
  1789                  c = rdp->nocb_nobypass_count - nocb_nobypass_lim_per_jiffy;
  1790                  if (c > nocb_nobypass_lim_per_jiffy)
  1791                          c = nocb_nobypass_lim_per_jiffy;
  1792                  else if (c < 0)
                                 ^^^^^
Impossible!

  1793                          c = 0;
  1794          }

regards,
dan carpenter