linux-kernel - Re: Query regarding srcu_funnel_exp

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Message-Id: <20171027122601.GJ3659@linux.vnet.ibm.com>
Date:   Fri, 27 Oct 2017 05:26:01 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Neeraj Upadhyay <neeraju@...eaurora.org>
Cc:     josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Query regarding srcu_funnel_exp_start()

On Fri, Oct 27, 2017 at 02:23:07PM +0530, Neeraj Upadhyay wrote:
> Hi,
> 
> One query regarding srcu_funnel_exp_start() function in
> kernel/rcu/srcutree.c.
> 
> static void srcu_funnel_exp_start(struct srcu_struct *sp, struct
> srcu_node *snp,
> 				  unsigned long s)
> {
> 	<snip>
> 	if (!ULONG_CMP_LT(sp->srcu_gp_seq_needed_exp, s))
> 		sp->srcu_gp_seq_needed_exp = s;
> 	<snip>
> }
> 
> Why is sp->srcu_gp_seq_needed_exp set to 's' if srcu_gp_seq_needed_exp is >=
> 's'. Shouldn't srcu_gp_seq_needed_exp be equal to the greater of both?

Let's suppose that it is incorrect as currently written.  Can you
construct a test case demonstrating a failure of some sort, then provide
a fix?

To start with, if it is currently incorrect, what would be the nature
of the failure?

							Thanx, Paul