linux-kernel - RE: [PATCH] pci-hyperv: use kmalloc to allocate hypercall params buffer

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Message-ID: <BN3PR03MB2227BB61FB49F42EBA2323EBCE820@BN3PR03MB2227.namprd03.prod.outlook.com>
Date:   Tue, 6 Dec 2016 00:37:08 +0000
From:   Long Li <longli@...rosoft.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
CC:     KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>
Subject: RE: [PATCH] pci-hyperv: use kmalloc to allocate hypercall params
 buffer



> -----Original Message-----
> From: Stephen Hemminger [mailto:stephen@...workplumber.org]
> Sent: Monday, December 5, 2016 8:53 AM
> To: Long Li <longli@...rosoft.com>
> Cc: KY Srinivasan <kys@...rosoft.com>; Haiyang Zhang
> <haiyangz@...rosoft.com>; Bjorn Helgaas <bhelgaas@...gle.com>;
> devel@...uxdriverproject.org; linux-kernel@...r.kernel.org; linux-
> pci@...r.kernel.org
> Subject: Re: [PATCH] pci-hyperv: use kmalloc to allocate hypercall params
> buffer
> 
> On Tue,  8 Nov 2016 14:04:38 -0800
> Long Li <longli@...hange.microsoft.com> wrote:
> 
> > +	spin_lock_irqsave(&hbus->retarget_msi_interrupt_lock, flags);
> > +
> > +	params = &hbus->retarget_msi_interrupt_params;
> > +	memset(params, 0, sizeof(*params));
> > +	params->partition_id = HV_PARTITION_ID_SELF;
> > +	params->source = 1; /* MSI(-X) */
> > +	params->address = msi_desc->msg.address_lo;
> > +	params->data = msi_desc->msg.data;
> > +	params->device_id = (hbus->hdev->dev_instance.b[5] << 24) |
> >  			   (hbus->hdev->dev_instance.b[4] << 16) |
> >  			   (hbus->hdev->dev_instance.b[7] << 8) |
> >  			   (hbus->hdev->dev_instance.b[6] & 0xf8) |
> >  			   PCI_FUNC(pdev->devfn);
> > -	params.vector = cfg->vector;
> > +	params->vector = cfg->vector;
> >
> >  	for_each_cpu_and(cpu, dest, cpu_online_mask)
> > -		params.vp_mask |= (1ULL <<
> vmbus_cpu_number_to_vp_number(cpu));
> > +		params->vp_mask |= (1ULL <<
> vmbus_cpu_number_to_vp_number(cpu));
> > +
> > +	hv_do_hypercall(HVCALL_RETARGET_INTERRUPT, params, NULL);
> >
> > -	hv_do_hypercall(HVCALL_RETARGET_INTERRUPT, &params, NULL);
> > +	spin_unlock_irqrestore(&hbus->retarget_msi_interrupt_lock, flags);
> 
> It looks like the additional locking here is being overly paranoid.
> The caller is already holding the irq descriptor lock. Look at fixup_irqs.

You are right. On my test machine, there are two possible places calling hv_irq_unmask(): request _irq() and handle_edge_irq(). They both have desc->lock held when calling .irq_unmask on the chip. A review of the IRQ code shows that desc->lock is always held while calling chip->irq_unmask().

Since the lock doesn't do any harm and it is not on performance code path, we can remove the lock in the upcoming patches.