linux-kernel - Re: [PATCH 01/16] x86/PCI: Replace spin_is

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Message-ID: <20181004065011.GC896@E570.localdomain>
Date:   Wed, 3 Oct 2018 23:50:11 -0700
From:   Lance Roy <ldr709@...il.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-pci@...r.kernel.org
Subject: Re: [PATCH 01/16] x86/PCI: Replace spin_is_locked() with lockdep

On Wed, Oct 03, 2018 at 11:00:51AM -0500, Bjorn Helgaas wrote:
> On Tue, Oct 02, 2018 at 10:38:47PM -0700, Lance Roy wrote:
> > lockdep_assert_held() is better suited to checking locking requirements,
> > since it won't get confused when someone else holds the lock. This is
> > also a step towards possibly removing spin_is_locked().
> >
> > Signed-off-by: Lance Roy <ldr709@...il.com>
> > Cc: Bjorn Helgaas <bhelgaas@...gle.com>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Ingo Molnar <mingo@...hat.com>
> > Cc: Borislav Petkov <bp@...en8.de>
> > Cc: "H. Peter Anvin" <hpa@...or.com>
> > Cc: <x86@...nel.org>
> > Cc: <linux-pci@...r.kernel.org>
>
> I assume you plan to merge the whole series together.  I don't object
> to that, but I don't know enough to be able to formally ack this.
>
> It would be useful to include a tiny bit more detail in the changelog.
> The spin_is_locked() documentation doesn't mention anything about
> differences with respect to the lock being held by self vs by someone
> else, so I can't tell where the confusion arises.
The difference is that spin_is_locked() will return true when someone else holds
the lock, while lockdep_assert_held() asserts that the current thread holds the
lock. How about the following for an new changelog?

lockdep_assert_held() is better suited to checking locking requirements,
since it only checks if the current thread holds the lock regardless of
whether someone else does. This is also a step towards possibly removing
spin_is_locked().

Thanks,
Lance

> Bjorn
>
> > ---
> >  arch/x86/pci/i386.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/pci/i386.c b/arch/x86/pci/i386.c
> > index ed4ac215305d..24bb58a007de 100644
> > --- a/arch/x86/pci/i386.c
> > +++ b/arch/x86/pci/i386.c
> > @@ -59,7 +59,7 @@ static struct pcibios_fwaddrmap *pcibios_fwaddrmap_lookup(struct pci_dev *dev)
> >  {
> >  	struct pcibios_fwaddrmap *map;
> >
> > -	WARN_ON_SMP(!spin_is_locked(&pcibios_fwaddrmap_lock));
> > +	lockdep_assert_held(&pcibios_fwaddrmap_lock);
> >
> >  	list_for_each_entry(map, &pcibios_fwaddrmappings, list)
> >  		if (map->dev == dev)
> > --
> > 2.19.0
> >