linux-kernel - Re: [RFC] pci: Rework config space blocking services

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Date:	Tue, 6 Sep 2011 11:47:28 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Jan Kiszka <jan.kiszka@...mens.com>
Cc:	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Brian King <brking@...ibm.com>,
	"James E.J. Bottomley" <JBottomley@...allels.com>,
	"Hans J. Koch" <hjk@...sjkoch.de>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [RFC] pci: Rework config space blocking services

On Tue, Sep 06, 2011 at 10:27:45AM +0200, Jan Kiszka wrote:
> On 2011-09-06 10:04, Michael S. Tsirkin wrote:
> > On Tue, Sep 06, 2011 at 09:18:13AM +0200, Jan Kiszka wrote:
> >>>> @@ -401,36 +403,58 @@ int pci_vpd_truncate(struct pci_dev *dev, size_t size)
> >>>>  EXPORT_SYMBOL(pci_vpd_truncate);
> >>>>
> >>>>  /**
> >>>> - * pci_block_user_cfg_access - Block userspace PCI config reads/writes
> >>>> + * pci_block_cfg_access - Block PCI config reads/writes
> >>>
> >>> This comment seems confusing. We don't in fact block all config
> >>> reads writes. Instead we block userspace accesses and
> >>> concurrent block requests.
> >>
> >> I'm open for a better suggestion that summarize the more verbose (and
> >> hopefully clearer) explanation below.
> > 
> > I think the problem is, it doesn't block config access
> > and we call it pci_block_cfg_access.
> > 
> > Thinking about it, doesn't this behave somewhat like a lock?
> > How about
> > 
> > pci_user_cfg_access_trylock
> > pci_user_cfg_access_lock
> > pci_user_cfg_access_unlock
> > 
> > And then:
> >  * pci_user_cfg_access_lock - Lock userspace PCI config access
> 
> Except that the "userspace" here is still only half of the truth

It's the name of the lock :)

> and I
> prefer to drop it, the naming locks good to me.

OK, I think it's acceptable.

> >  *
> >  * When locked, any userspace reads or writes to config space
> >  * and concurrent lock requests will sleep, and trylock requests
> >  * will fail, until pci_user_cfg_access_unlock is called.
> > 
> > I had a brief thought of using an rwsem internally, but
> > this would make trylock fail if userspace does config read,
> > changing semantics.
> 
> Also, I bet we would make lockdep unhappy when calling
> down_write_trylock from IRQ context.
> 
> Jan

I think I did this at some point, should be fine.
But whatever.

> -- 
> Siemens AG, Corporate Technology, CT T DE IT 1
> Corporate Competence Center Embedded Linux
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