linux-kernel - Q&A from "Concurrency with tools/memory-model"

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Date:   Thu, 15 Nov 2018 14:56:30 -0800
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        mingo@...nel.org
Cc:     stern@...land.harvard.edu, parri.andrea@...il.com,
        will.deacon@....com, peterz@...radead.org, boqun.feng@...il.com,
        npiggin@...il.com, dhowells@...hat.com, j.alglave@....ac.uk,
        luc.maranget@...ia.fr, akiyks@...il.com
Subject: Q&A from "Concurrency with tools/memory-model"

Hello!

Good turnout and some good questions here in Vancouver BC, please see
below for rough notes.  ;-)

							Thanx, Paul

------------------------------------------------------------------------

"Concurrency with tools/memory-model"

Andrea Parri presenting.

Rough notes of Q&A.

o	Want atomic bit operation.

o	But smp_read_barrier_depends() not there, so how to note pairing?
	A:  Note the dependency as the other end of the pairing.

o	Speculation barriers, as in Spectre and Meltdown?  A: This would
	require adding timing, not in the immediate future.

o	What ordering does system calls provide?  A: None that we know of.
	Boqun: Userspace needs to explicitly provide the needed ordering
	when interacting with the kernel.  Some architectures do provide
	full barriers, but not to be counted on.

o	Why herd7?  A: Based on other formalizations -- note that herd7
	had a number of hardware models.  Paul: Plus the founder of the
	LKMM project is a co-author of herd, which might have had some
	effect.

o	Why not also model interrupts and NMIs?  Promela and spin have
	been used for this.  A: Cannot currently model them.  You can
	emulated them with additional threads and locks, if you wish.
	Vincent Nimal and Lihao Liang have done some academic work on
	these topics.