linux-kernel - Re: [PATCH 1/2] srcu: Prevent sdp->srcu_gp_seq_needed

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Message-Id: <20180209123159.GZ3617@linux.vnet.ibm.com>
Date:   Fri, 9 Feb 2018 04:31:59 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Ildar Ismagilov <devix84@...il.com>
Cc:     Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] srcu: Prevent sdp->srcu_gp_seq_needed_exp counter
 wrap

On Tue, Feb 06, 2018 at 01:44:10PM +0300, Ildar Ismagilov wrote:
> The results of rcutorture tests in attachment. I only tested: SRCU-N,
> SRCU-P, SRCU-t and SRCU-u configs.

That is a good set for an SRUC change, thank you!

							Thanx, Paul

> 2018-02-03 12:16 GMT+03:00 Paul E. McKenney <paulmck@...ux.vnet.ibm.com>:
> > On Wed, Jan 31, 2018 at 10:39:53PM +0300, Ildar Ismagilov wrote:
> >> Signed-off-by: Ildar Ismagilov <devix84@...il.com>
> >
> > I did apply this for testing and review, thank you and good eyes!
> >
> > However, I had to:
> >
> > 1.      hand-edit it to make it work against the rcu/dev
> >         branch of my -rcu tree:
> >         git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git
> >
> > 2.      fill out the commit logs.
> >
> > Please see below for the updated patch, and please let me know if I
> > messed something up.
> >
> > In the future, could you please apply your patches against the rcu/dev
> > branch of -rcu and provide more complete commit logs?  The commit log
> > should describe the problem and how that problem is fixed.
> >
> > Also, are you able to test these using rcutorture?  If not, let's get you
> > set up for that, as I am prone to testing backlogs just now.  Plus I am
> > writing this while 34,000 feet over Western Australia.  So it would mean
> > fewer delays for you if you could test your own patches.  ;-)
> >
> >                                                         Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > commit 753abb606ba61012c8838c73e2c888aea20efe13
> > Author: Ildar Ismagilov <devix84@...il.com>
> > Date:   Wed Jan 31 22:39:53 2018 +0300
> >
> >     srcu: Prevent sdp->srcu_gp_seq_needed_exp counter wrap
> >
> >     SRCU checks each srcu_data structure's grace-period number for counter
> >     wrap four times per cycle by default.  This frequency guarantees that
> >     normal comparisons will detect potential wrap.  However, the expedited
> >     grace-period number is not checked.  The consquences are not too horrible
> >     (a failure to expedite a grace period when requested), but it would be
> >     good to avoid such things.  This commit therefore adds this check to
> >     the expedited grace-period number.
> >
> >     Signed-off-by: Ildar Ismagilov <devix84@...il.com>
> >     Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> >
> > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > index 2878387d4189..ad3e1aa5e6ea 100644
> > --- a/kernel/rcu/srcutree.c
> > +++ b/kernel/rcu/srcutree.c
> > @@ -579,6 +579,9 @@ static void srcu_gp_end(struct srcu_struct *sp)
> >                                 if (ULONG_CMP_GE(gpseq,
> >                                                  sdp->srcu_gp_seq_needed + 100))
> >                                         sdp->srcu_gp_seq_needed = gpseq;
> > +                               if (ULONG_CMP_GE(gpseq,
> > +                                                sdp->srcu_gp_seq_needed_exp + 100))
> > +                                       sdp->srcu_gp_seq_needed_exp = gpseq;
> >                                 spin_unlock_irqrestore_rcu_node(sdp, flags);
> >                         }
> >         }
> >
> 
> 
> 
> -- 
> Best regards
> Ildar