Re: [PATCH RFC 0/5] Remove some notrace RCU APIs

From: "Paul E. McKenney" <paulmck@linux.ibm.com>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: Steven Rostedt <rostedt@goodmis.org>,
	linux-kernel@vger.kernel.org,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Ingo Molnar <mingo@redhat.com>, Jonathan Corbet <corbet@lwn.net>,
	Josh Triplett <josh@joshtriplett.org>,
	kvm-ppc@vger.kernel.org, Lai Jiangshan <jiangshanlai@gmail.com>,
	linux-doc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
	Paul Mackerras <paulus@ozlabs.org>,
	rcu@vger.kernel.org
Subject: Re: [PATCH RFC 0/5] Remove some notrace RCU APIs
Date: Sat, 25 May 2019 08:50:35 -0700	[thread overview]
Message-ID: <20190525155035.GE28207@linux.ibm.com> (raw)
In-Reply-To: <20190525141954.GA176647@google.com>

On Sat, May 25, 2019 at 10:19:54AM -0400, Joel Fernandes wrote:
> On Sat, May 25, 2019 at 07:08:26AM -0400, Steven Rostedt wrote:
> > On Sat, 25 May 2019 04:14:44 -0400
> > Joel Fernandes <joel@joelfernandes.org> wrote:
> > 
> > > > I guess the difference between the _raw_notrace and just _raw variants
> > > > is that _notrace ones do a rcu_check_sparse(). Don't we want to keep
> > > > that check?  
> > > 
> > > This is true.
> > > 
> > > Since the users of _raw_notrace are very few, is it worth keeping this API
> > > just for sparse checking? The API naming is also confusing. I was expecting
> > > _raw_notrace to do fewer checks than _raw, instead of more. Honestly, I just
> > > want to nuke _raw_notrace as done in this series and later we can introduce a
> > > sparse checking version of _raw if need-be. The other option could be to
> > > always do sparse checking for _raw however that used to be the case and got
> > > changed in http://lists.infradead.org/pipermail/linux-afs/2016-July/001016.html
> > 
> > What if we just rename _raw to _raw_nocheck, and _raw_notrace to _raw ?
> 
> That would also mean changing 160 usages of _raw to _raw_nocheck in the
> kernel :-/.
> 
> The tracing usage of _raw_notrace is only like 2 or 3 users. Can we just call
> rcu_check_sparse directly in the calling code for those and eliminate the APIs?
> 
> I wonder what Paul thinks about the matter as well.

My thought is that it is likely that a goodly number of the current uses
of _raw should really be some form of _check, with lockdep expressions
spelled out.  Not that working out what exactly those lockdep expressions
should be is necessarily a trivial undertaking.  ;-)

That aside, if we are going to change the name of an API that is
used 160 places throughout the tree, we would need to have a pretty
good justification.  Without such a justification, it will just look
like pointless churn to the various developers and maintainers on the
receiving end of the patches.

							Thanx, Paul

> thanks, Steven!
> 