linux-kernel - Re: [PATCH tip/core/rcu 02/19] rcu: Defer reporting RCU-preempt quiescent states when disabled

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Date:   Mon, 26 Nov 2018 11:00:27 -0800
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Ran Rozenstein <ranro@...lanox.com>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "jiangshanlai@...il.com" <jiangshanlai@...il.com>,
        "dipankar@...ibm.com" <dipankar@...ibm.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "mathieu.desnoyers@...icios.com" <mathieu.desnoyers@...icios.com>,
        "josh@...htriplett.org" <josh@...htriplett.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "fweisbec@...il.com" <fweisbec@...il.com>,
        "oleg@...hat.com" <oleg@...hat.com>,
        Maor Gottlieb <maorg@...lanox.com>,
        Tariq Toukan <tariqt@...lanox.com>,
        Eran Ben Elisha <eranbe@...lanox.com>,
        Leon Romanovsky <leonro@...lanox.com>
Subject: Re: [PATCH tip/core/rcu 02/19] rcu: Defer reporting RCU-preempt
 quiescent states when disabled

On Mon, Nov 26, 2018 at 01:55:37PM +0000, Ran Rozenstein wrote:
> > 
> > Hearing no objections, here is the updated patch.
> > 
> > 								Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > commit 970cab5d3d206029ed27274a98ea1c3d7e780e53
> > Author: Paul E. McKenney <paulmck@...ux.ibm.com>
> > Date:   Mon Oct 29 07:36:50 2018 -0700
> > 
> >     rcu: Avoid signed integer overflow in rcu_preempt_deferred_qs()
> > 
> >     Subtracting INT_MIN can be interpreted as unconditional signed integer
> >     overflow, which according to the C standard is undefined behavior.
> >     Therefore, kernel build arguments notwithstanding, it would be good to
> >     future-proof the code.  This commit therefore substitutes INT_MAX for
> >     INT_MIN in order to avoid undefined behavior.
> > 
> >     While in the neighborhood, this commit also creates some meaningful
> > names
> >     for INT_MAX and friends in order to improve readability, as suggested
> >     by Joel Fernandes.
> > 
> >     Reported-by: Ran Rozenstein <ranro@...lanox.com>
> >     Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
> > 
> >     squash! rcu: Avoid signed integer overflow in rcu_preempt_deferred_qs()
> > 
> >     While in the neighborhood, use macros to give meaningful names.
> > 
> >     Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
> 
> Hi,
> 
> What is the acceptance status of this patch?

It is queued in -rcu.  If no problems arise beforehand, I intend to submit
it as part of a pull request into -tip, which (again if no problems arise)
be pulled into mainline during the next merge window.

Oddly enough, a couple of weeks ago the C++ Standards Committee voted
in a proposal for C++20 removing undefined behavior for signed integer
overflow.  This is C++ rather than C, and C must support additional
hardware that wouldn't much like forcing twos complement for signed
integer overflow.  But still...  ;-)

							Thanx, Paul