* re: rcu/nocb: Add bypass callback queueing, bug report
@ 2019-08-13 12:34 Colin Ian King
2019-08-13 15:23 ` Paul E. McKenney
0 siblings, 1 reply; 2+ messages in thread
From: Colin Ian King @ 2019-08-13 12:34 UTC (permalink / raw)
To: Paul E. McKenney, Josh Triplett, Steven Rostedt,
Mathieu Desnoyers, Lai Jiangshan, Joel Fernandes, rcu
Cc: linux-kernel
Hi,
Static analysis on linux-next today found an issue in the following commit:
commit 1afc4b18724f8f7b7a21fdf66cd43cc4a932812d
Author: Paul E. McKenney <paulmck@linux.ibm.com>
Date: Tue Jul 2 16:03:33 2019 -0700
rcu/nocb: Add bypass callback queueing
The coverity report is as follows:
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;
CID 85141 (#1 of 1): Unsigned compared against 0
unsigned_compare: This less-than-zero comparison of an unsigned value is
never true. c < 0UL.
1792 else if (c < 0)
1793 c = 0;
Variable c is an unsigned long so the c < 0 check is never true. I'm not
sure what the ramifications are if c is made a signed long instead, so
I'm not fixing this and reporting this issue.
Regards,
Colin
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: rcu/nocb: Add bypass callback queueing, bug report
2019-08-13 12:34 rcu/nocb: Add bypass callback queueing, bug report Colin Ian King
@ 2019-08-13 15:23 ` Paul E. McKenney
0 siblings, 0 replies; 2+ messages in thread
From: Paul E. McKenney @ 2019-08-13 15:23 UTC (permalink / raw)
To: Colin Ian King
Cc: Josh Triplett, Steven Rostedt, Mathieu Desnoyers, Lai Jiangshan,
Joel Fernandes, rcu, linux-kernel
On Tue, Aug 13, 2019 at 01:34:02PM +0100, Colin Ian King wrote:
> Hi,
>
> Static analysis on linux-next today found an issue in the following commit:
>
> commit 1afc4b18724f8f7b7a21fdf66cd43cc4a932812d
> Author: Paul E. McKenney <paulmck@linux.ibm.com>
> Date: Tue Jul 2 16:03:33 2019 -0700
>
> rcu/nocb: Add bypass callback queueing
>
>
> The coverity report is as follows:
>
> 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;
>
> CID 85141 (#1 of 1): Unsigned compared against 0
> unsigned_compare: This less-than-zero comparison of an unsigned value is
> never true. c < 0UL.
>
> 1792 else if (c < 0)
> 1793 c = 0;
>
> Variable c is an unsigned long so the c < 0 check is never true. I'm not
> sure what the ramifications are if c is made a signed long instead, so
> I'm not fixing this and reporting this issue.
Good catch!!!
How about the alleged fix shown below?
Thanx, Paul
------------------------------------------------------------------------
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 91cefa3bf943..2defc7fe74c3 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1787,10 +1787,11 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
} else {
WRITE_ONCE(rdp->nocb_nobypass_last, j);
c = rdp->nocb_nobypass_count - nocb_nobypass_lim_per_jiffy;
- if (c > nocb_nobypass_lim_per_jiffy)
- c = nocb_nobypass_lim_per_jiffy;
- else if (c < 0)
+ if (ULONG_CMP_LT(rdp->nocb_nobypass_count,
+ nocb_nobypass_lim_per_jiffy))
c = 0;
+ else if (c > nocb_nobypass_lim_per_jiffy)
+ c = nocb_nobypass_lim_per_jiffy;
}
WRITE_ONCE(rdp->nocb_nobypass_count, c);
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