linux-kernel - Re: [PATCH] target: Use WARNON_NON_RT(!irqs

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Date:   Thu, 22 Mar 2018 10:13:19 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "Nicholas A. Bellinger" <nab@...ux-iscsi.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linux SCSI List <linux-scsi@...r.kernel.org>,
        Daniel Bristot de Oliveira <daniel@...stot.me>,
        Luis Claudio R. Gonçalves 
        <lclaudio@...hat.com>, Clark Williams <williams@...hat.com>,
        target-devel <target-devel@...r.kernel.org>
Subject: Re: [PATCH] target: Use WARNON_NON_RT(!irqs_disabled())

On Wed, 21 Mar 2018, Linus Torvalds wrote:
> [ Adding PeterZ to participants due to query about lockdep_assert() ]
> 
> On Wed, Mar 21, 2018 at 8:38 AM, Arnaldo Carvalho de Melo
> <acme@...nel.org> wrote:
> >
> >         assert_spin_locked(&cmd->t_state_lock);
> > -       WARN_ON_ONCE(!irqs_disabled());
> > +       WARN_ON_ONCE_NONRT(!irqs_disabled());
> 
> Ugh.
> 
> Can't we just replace both of those with a lockdep annotation?
> 
> Does "lockdep_assert_held()" already verify the irq contextr, or do we
> need lockdep_assert_irqs_disabled() too?
> 
> Honestly, the old-fashioned way of doing verification of state by hand
> is understandable, but it's legacy and kind of pointless when we have
> much better tools these days.
> 
> I'm perfectly willing to leave old assertions in place, but if they
> need fixing anyway, I'd damn well want to fix them *right* instead of
> starting to just add more piles of hacks on top of the old model.
> 
> Because when the details of the locking rules depend on RT vs non-RT,
> I want the checks to make sense.  And presumably lockdep is the thing
> that really knows what the status of a lock is, no?

We are working on replacing the _NONRT _RT variants with proper lockdep
mechnisms which are aware of the RT vs. non-RT magic under the hood. Just
not there yet.

Thanks,

	tglx