linux-kernel - Re: [PATCH v2 2/6] PCI/MSI: Factor out pci_get_msi

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Message-ID: <20131218182635.GE15119@google.com>
Date:	Wed, 18 Dec 2013 11:26:35 -0700
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Mark Lord <kernel@...rt.ca>
Cc:	Alexander Gordeev <agordeev@...hat.com>,
	Michael Ellerman <michael@...erman.id.au>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Tejun Heo <tj@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-ide@...r.kernel.org" <linux-ide@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>, Joerg Roedel <joro@...tes.org>,
	Jan Beulich <JBeulich@...e.com>, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 2/6] PCI/MSI: Factor out pci_get_msi_cap() interface

On Thu, Sep 26, 2013 at 08:32:53AM -0400, Mark Lord wrote:
> On 13-09-18 05:48 AM, Alexander Gordeev wrote:
> >
> > The last pattern makes most of sense to me and could be updated with a more
> > clear sequence - a call to (bit modified) pci_msix_table_size() followed
> > by a call to pci_enable_msix(). I think this pattern can effectively
> > supersede the currently recommended "loop" practice.
> 
> The loop is still necessary, because there's a race between those two calls,
> so that pci_enable_msix() can still fail due to lack of MSIX slots.

Hi Mark,

Can you elaborate on this race?  My understanding is that
pci_msix_table_size() depends only on the "Table Size" field in the MSI-X
Message Control register.

So if there's a concurrency problem here, it would have to be something
like "pci_enable_msix() may not be able to configure the requested number
of vectors because it has to allocate from a shared pool."

If that's the case, pci_msix_table_size() wouldn't be involved at all, and
the only question is how to coordinate between several drivers that each
call pci_enable_msix().  I think that would have to be resolved in some
arch hook used by the PCI core.

Maybe this is already taken care of; I just want to make sure we don't
overlook an issue here.

Bjorn
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