linux-kernel - Re: Query regarding srcu_funnel_exp

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Message-ID: <946852c9-88cb-7363-e74d-82a1efbd3be1@codeaurora.org>
Date:   Fri, 27 Oct 2017 22:15:04 +0530
From:   Neeraj Upadhyay <neeraju@...eaurora.org>
To:     paulmck@...ux.vnet.ibm.com
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 10/27/2017 05:56 PM, Paul E. McKenney wrote:
> 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?

Will check this. Might take some time to build a test case.

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

Hi Paul,

I see below scenario, where new gp won't be expedited. Please correct
me if I am missing something here.

1. CPU0 calls synchronize_srcu_expedited()

synchronize_srcu_expedited()
   __synchronize_srcu()
     __call_srcu()
             s = rcu_seq_snap(&sp->srcu_gp_seq); // lets say srcu_gp_seq 
  = 0;
                                                 // s = 0x100
             sdp->srcu_gp_seq_needed = s // 0x100
             needgp = true
             sdp->srcu_gp_seq_needed_exp = s // 0x100
         srcu_funnel_gp_start()
                 sp->srcu_gp_seq_needed_exp = s;
             srcu_gp_start(sp);
                 rcu_seq_start(&sp->srcu_gp_seq);

2. CPU1 calls normal synchronize_srcu()

synchronize_srcu()
     __synchronize_srcu(sp, true)
         __call_srcu()
                 s = rcu_seq_snap(&sp->srcu_gp_seq); // srcu_gp_seq = 1
                                                     // s= 0x200
                 sdp->srcu_gp_seq_needed = s; // 0x200
             srcu_funnel_gp_start()
                 smp_store_release(&sp->srcu_gp_seq_needed, s); // 0x200

3. CPU3 calls synchronize_srcu_expedited()

synchronize_srcu_expedited()
   __synchronize_srcu()
     __call_srcu()
             s = rcu_seq_snap(&sp->srcu_gp_seq); // srcu_gp_seq = 1
                                                 // s = 0x200
             sdp->srcu_gp_seq_needed_exp = s // 0x200
         srcu_funnel_exp_start(sp, sdp->mynode, s);
             // sp->srcu_gp_seq_needed_exp = 0x100
             // s = 0x200 ; sp->srcu_gp_seq_needed_exp is not updated
             if (!ULONG_CMP_LT(sp->srcu_gp_seq_needed_exp, s))
                 sp->srcu_gp_seq_needed_exp = s;


Thanks
Neeraj






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