linux-kernel - Re: [PATCH v2 1/3] locking: Document the semantics of spin_is

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Message-Id: <20180403141718.GG3948@linux.vnet.ibm.com>
Date:   Tue, 3 Apr 2018 07:17:18 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     David Howells <dhowells@...hat.com>
Cc:     Andrea Parri <andrea.parri@...rulasolutions.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        Andrea Parri <parri.andrea@...il.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Will Deacon <will.deacon@....com>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>
Subject: Re: [PATCH v2 1/3] locking: Document the semantics of
 spin_is_locked()

On Tue, Apr 03, 2018 at 02:52:33PM +0100, David Howells wrote:
> Andrea Parri <andrea.parri@...rulasolutions.com> wrote:
> 
> > > It's more complicated than that.  This function is dangerous and should be
> > > used with extreme care.  In the case where CONFIG_SMP=n the value is locked
> > > one way or the other and it might be the wrong way.
> > 
> > You mean "unlocked"? (aka, return 0)
> 
> No, I mean "fixed", sorry.  We've had problems stemming from this before on UP
> systems.

Oooh...  I had forgotten about spinlocks disappearing on UP systems,
good catch!

Suggestions for a fix?  Clearly great care is required when using it
in things like WARN_ON()...

							Thanx, Paul