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Re: [PATCH RFC 00/77] Re-design MSI/MSI-X interrupts enablement pattern

To: Alexander Gordeev <agordeev@redhat.com>
Subject: Re: [PATCH RFC 00/77] Re-design MSI/MSI-X interrupts enablement pattern
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Date: Sun, 06 Oct 2013 17:19:46 +1100
Cc: Ben Hutchings <bhutchings@solarflare.com>, linux-kernel@vger.kernel.org, Bjorn Helgaas <bhelgaas@google.com>, Ralf Baechle <ralf@linux-mips.org>, Michael Ellerman <michael@ellerman.id.au>, Martin Schwidefsky <schwidefsky@de.ibm.com>, Ingo Molnar <mingo@redhat.com>, Tejun Heo <tj@kernel.org>, Dan Williams <dan.j.williams@intel.com>, Andy King <acking@vmware.com>, Jon Mason <jon.mason@intel.com>, Matt Porter <mporter@kernel.crashing.org>, linux-pci@vger.kernel.org, linux-mips@linux-mips.org, linuxppc-dev@lists.ozlabs.org, linux390@de.ibm.com, linux-s390@vger.kernel.org, x86@kernel.org, linux-ide@vger.kernel.org, iss_storagedev@hp.com, linux-nvme@lists.infradead.org, linux-rdma@vger.kernel.org, netdev@vger.kernel.org, e1000-devel@lists.sourceforge.net, linux-driver@qlogic.com, Solarflare linux maintainers <linux-net-drivers@solarflare.com>, "VMware, Inc." <pv-drivers@vmware.com>, linux-scsi@vger.kernel.org
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On Sun, 2013-10-06 at 08:02 +0200, Alexander Gordeev wrote:
> On Sun, Oct 06, 2013 at 08:46:26AM +1100, Benjamin Herrenschmidt wrote:
> > On Sat, 2013-10-05 at 16:20 +0200, Alexander Gordeev wrote:
> > > So my point is - drivers should first obtain a number of MSIs they *can*
> > > get, then *derive* a number of MSIs the device is fine with and only then
> > > request that number. Not terribly different from memory or any other type
> > > of resource allocation ;)
> > 
> > What if the limit is for a group of devices ? Your interface is racy in
> > that case, another driver could have eaten into the limit in between the
> > calls.
> 
> Well, the another driver has had a better karma ;) But seriously, the
> current scheme with a loop is not race-safe wrt to any other type of
> resource which might exhaust. What makes the quota so special so we
> should care about it and should not care i.e. about lack of msi_desc's?

I'm not saying the current scheme is better but I prefer the option of
passing a min,max to the request function.

> Yeah, I know the quota might hit more likely. But why it is not addressed
> right now then? Not a single function in chains...
>   rtas_msi_check_device() -> msi_quota_for_device() -> traverse_pci_devices()
>   rtas_setup_msi_irqs() -> msi_quota_for_device() -> traverse_pci_devices()
> ...is race-safe. So if it has not been bothering anyone until now then 
> no reason to start worrying now :)
>
> In fact, in the current design to address the quota race decently the
> drivers would have to protect the *loop* to prevent the quota change
> between a pci_enable_msix() returned a positive number and the the next
> call to pci_enable_msix() with that number. Is it doable?

I am not advocating for the current design, simply saying that your
proposal doesn't address this issue while Ben's does.

Cheers,
Ben.

> > Ben.
> > 
> > 
> 



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