Re: [PATCH] tools/memory-model: Provide extra ordering for unlock+lock pair on the same CPU

From: Alan Stern <stern@rowland.harvard.edu>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Boqun Feng <boqun.feng@gmail.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Dan Lustig <dlustig@nvidia.com>, Will Deacon <will@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Peter Anvin <hpa@zytor.com>,
	Andrea Parri <parri.andrea@gmail.com>,
	Ingo Molnar <mingo@kernel.org>,
	Vince Weaver <vincent.weaver@maine.edu>,
	Thomas Gleixner <tglx@linutronix.de>,
	Jiri Olsa <jolsa@redhat.com>,
	Arnaldo Carvalho de Melo <acme@redhat.com>,
	Stephane Eranian <eranian@google.com>,
	palmer@dabbelt.com, paul.walmsley@sifive.com, mpe@ellerman.id.au
Subject: Re: [PATCH] tools/memory-model: Provide extra ordering for unlock+lock pair on the same CPU
Date: Thu, 30 Sep 2021 16:46:34 -0400	[thread overview]
Message-ID: <20210930204634.GB482974@rowland.harvard.edu> (raw)
In-Reply-To: <20210930181753.GH880162@paulmck-ThinkPad-P17-Gen-1>

On Thu, Sep 30, 2021 at 11:17:53AM -0700, Paul E. McKenney wrote:
> On Thu, Sep 30, 2021 at 11:20:33AM -0400, Alan Stern wrote:
> > On Thu, Sep 30, 2021 at 09:08:23PM +0800, Boqun Feng wrote:
> > > A recent discussion[1] shows that we are in favor of strengthening the
> > > ordering of unlock + lock on the same CPU: a unlock and a po-after lock
> > > should provide the so-called RCtso ordering, that is a memory access S
> > > po-before the unlock should be ordered against a memory access R
> > > po-after the lock, unless S is a store and R is a load.
> > > 
> > > The strengthening meets programmers' expection that "sequence of two
> > > locked regions to be ordered wrt each other" (from Linus), and can
> > > reduce the mental burden when using locks. Therefore add it in LKMM.
> > > 
> > > [1]: https://lore.kernel.org/lkml/20210909185937.GA12379@rowland.harvard.edu/
> > > 
> > > Co-developed-by: Alan Stern <stern@rowland.harvard.edu>
> > > Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
> > > Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> > > ---
> > > Alan,
> > > 
> > > I added the "Co-developed-by" and "Signed-off-by" tags since most of the
> > > work is done by you. Feel free to let me know if you want to change
> > > anything.
> > 
> > It looks good to me.  However, do we really want to add these litmus
> > tests to the kernel source, or would it be better to keep them with
> > the thousands of other tests in Paul's archives?
> 
> Either way works for me.  But if they are referred to from within the
> kernel, it is best to have them in the kernel source.  Which might be seen
> as a reason to minimize referring to litmus tests from the kernel.  ;-)

In this case the litmus tests are not referred to within the kernel 
source.

Alan