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From: "Paul E. McKenney" <paulmck@linux.ibm.com>
To: Colin Ian King <colin.king@canonical.com>
Cc: Josh Triplett <josh@joshtriplett.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	rcu@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: rcu/nocb: Add bypass callback queueing, bug report
Date: Tue, 13 Aug 2019 08:23:50 -0700	[thread overview]
Message-ID: <20190813152350.GC28441@linux.ibm.com> (raw)
In-Reply-To: <e89f5af3-dcf3-986f-828f-14e10cef9915@canonical.com>

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);
 

      reply	other threads:[~2019-08-13 15:24 UTC|newest]

Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-13 12:34 rcu/nocb: Add bypass callback queueing, bug report Colin Ian King
2019-08-13 15:23 ` Paul E. McKenney [this message]

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