lkml.org 
[lkml]   [2017]   [Oct]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: Query regarding srcu_funnel_exp_start()
From
Date
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






--
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
member of the Code Aurora Forum, hosted by The Linux Foundation

\
 
 \ /
  Last update: 2017-10-28 20:19    [W:0.987 / U:0.996 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site