linux-kernel - Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire

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Date:   Thu, 12 Jul 2018 10:34:44 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     Daniel Lustig <dlustig@...dia.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>,
        Alan Stern <stern@...land.harvard.edu>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        LKMM Maintainers -- Akira Yokosawa <akiyks@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Nicholas Piggin <npiggin@...il.com>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and
 remove it for ordinary release/acquire

> All the discussion here[1] for example is about having ordering and
> doing an smp_cond_load_acquire() on a variable which is sometimes
> protected by a CPU's rq->lock and sometimes not?  Isn't that one of the
> key use cases for this whole discussion?

Not a "pure" one:

  http://lkml.kernel.org/r/1530629639-27767-1-git-send-email-andrea.parri@amarulasolutions.com

we also need "W->R ordering" in schedule()! so there better be an
smp_mb__after_spinlock() or a barrier providing similar ordering.

Nick was suggesting a "weaker version" of this barrier back in:

  362a61ad61199e ("fix SMP data race in pagetable setup vs walking")

c.f., the comment in mm/memory.c:__pte_alloc(), but that does not
math our pattern (UNLOCK+LOCK), AFAICT.

  Andrea


> 
> [1] https://lkml.org/lkml/2015/10/6/805
> 
> Dan