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
