rcu: Stop overflowing signed integers

The C standard does not specify the result of an operation that
overflows a signed integer, so such operations need to be
avoided.  This patch changes the type of several fields from
"long" to "unsigned long" and adjusts operations as needed.
ULONG_CMP_GE() and ULONG_CMP_LT() macros are introduced to do
the modular comparisons that are appropriate given that overflow
is an expected event.

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
Cc: dhowells@redhat.com
LKML-Reference: <1266887105-1528-17-git-send-email-paulmck@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 5 insertions, 6 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 29d88c08d875..dd0d31dffcdc 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -500,7 +500,7 @@ static void print_cpu_stall(struct rcu_state *rsp)
         trigger_all_cpu_backtrace();
 
         spin_lock_irqsave(&rnp->lock, flags);
-        if ((long)(jiffies - rsp->jiffies_stall) >= 0)
+        if (ULONG_CMP_GE(jiffies, rsp->jiffies_stall))
                 rsp->jiffies_stall =
                         jiffies + RCU_SECONDS_TILL_STALL_RECHECK;
         spin_unlock_irqrestore(&rnp->lock, flags);
@@ -1216,8 +1216,7 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
                 rsp->n_force_qs_lh++; /* Inexact, can lose counts.  Tough! */
                 return; /* Someone else is already on the job. */
         }
-        if (relaxed &&
-            (long)(rsp->jiffies_force_qs - jiffies) >= 0)
+        if (relaxed && ULONG_CMP_GE(rsp->jiffies_force_qs, jiffies))
                 goto unlock_fqs_ret; /* no emergency and done recently. */
         rsp->n_force_qs++;
         spin_lock(&rnp->lock);  /* irqs already disabled */
@@ -1295,7 +1294,7 @@ __rcu_process_callbacks(struct rcu_state *rsp, struct rcu_data *rdp)
          * If an RCU GP has gone long enough, go check for dyntick
          * idle CPUs and, if needed, send resched IPIs.
          */
-        if ((long)(ACCESS_ONCE(rsp->jiffies_force_qs) - jiffies) < 0)
+        if (ULONG_CMP_LT(ACCESS_ONCE(rsp->jiffies_force_qs), jiffies))
                 force_quiescent_state(rsp, 1);
 
         /*
@@ -1392,7 +1391,7 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
                         force_quiescent_state(rsp, 0);
                 rdp->n_force_qs_snap = rsp->n_force_qs;
                 rdp->qlen_last_fqs_check = rdp->qlen;
-        } else if ((long)(ACCESS_ONCE(rsp->jiffies_force_qs) - jiffies) < 0)
+        } else if (ULONG_CMP_LT(ACCESS_ONCE(rsp->jiffies_force_qs), jiffies))
                 force_quiescent_state(rsp, 1);
         local_irq_restore(flags);
 }
@@ -1525,7 +1524,7 @@ static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
 
         /* Has an RCU GP gone long enough to send resched IPIs &c? */
         if (rcu_gp_in_progress(rsp) &&
-            ((long)(ACCESS_ONCE(rsp->jiffies_force_qs) - jiffies) < 0)) {
+            ULONG_CMP_LT(ACCESS_ONCE(rsp->jiffies_force_qs), jiffies)) {
                 rdp->n_rp_need_fqs++;
                 return 1;
         }