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Subject[RFC PATCH 07/11] PCI: hv: Prepare hv_compose_msi_msg() for the VMBus-channel-interrupt-to-vCPU reassignment functionality
Date
The current implementation of hv_compose_msi_msg() is incompatible with
the new functionality that allows changing the vCPU a VMBus channel will
interrupt: if this function always calls hv_pci_onchannelcallback() in
the polling loop, the interrupt going to a different CPU could cause
hv_pci_onchannelcallback() to be running simultaneously in a tasklet,
which will break. The current code also has a problem in that it is not
synchronized with vmbus_reset_channel_cb(): hv_compose_msi_msg() could
be accessing the ring buffer via the call of hv_pci_onchannelcallback()
well after the time that vmbus_reset_channel_cb() has finished.

Fix these issues as follows. Disable the channel tasklet before
entering the polling loop in hv_compose_msi_msg() and re-enable it when
done. This will prevent hv_pci_onchannelcallback() from running in a
tasklet on a different CPU. Moreover, poll by always calling
hv_pci_onchannelcallback(), but check the channel callback function for
NULL and invoke the callback within a sched_lock critical section. This
will prevent hv_compose_msi_msg() from accessing the ring buffer after
vmbus_reset_channel_cb() has acquired the sched_lock spinlock.

Suggested-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Andrew Murray <amurray@thegoodpenguin.co.uk>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: <linux-pci@vger.kernel.org>
---
drivers/pci/controller/pci-hyperv.c | 44 ++++++++++++++++++-----------
1 file changed, 28 insertions(+), 16 deletions(-)

diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
index 9977abff92fc5..e6020480a28b1 100644
--- a/drivers/pci/controller/pci-hyperv.c
+++ b/drivers/pci/controller/pci-hyperv.c
@@ -1350,11 +1350,11 @@ static void hv_compose_msi_msg(struct irq_data *data, struct msi_msg *msg)
{
struct irq_cfg *cfg = irqd_cfg(data);
struct hv_pcibus_device *hbus;
+ struct vmbus_channel *channel;
struct hv_pci_dev *hpdev;
struct pci_bus *pbus;
struct pci_dev *pdev;
struct cpumask *dest;
- unsigned long flags;
struct compose_comp_ctxt comp;
struct tran_int_desc *int_desc;
struct {
@@ -1372,6 +1372,7 @@ static void hv_compose_msi_msg(struct irq_data *data, struct msi_msg *msg)
dest = irq_data_get_effective_affinity_mask(data);
pbus = pdev->bus;
hbus = container_of(pbus->sysdata, struct hv_pcibus_device, sysdata);
+ channel = hbus->hdev->channel;
hpdev = get_pcichild_wslot(hbus, devfn_to_wslot(pdev->devfn));
if (!hpdev)
goto return_null_message;
@@ -1428,43 +1429,52 @@ static void hv_compose_msi_msg(struct irq_data *data, struct msi_msg *msg)
goto free_int_desc;
}

+ /*
+ * Prevents hv_pci_onchannelcallback() from running concurrently
+ * in the tasklet.
+ */
+ tasklet_disable(&channel->callback_event);
+
/*
* Since this function is called with IRQ locks held, can't
* do normal wait for completion; instead poll.
*/
while (!try_wait_for_completion(&comp.comp_pkt.host_event)) {
+ unsigned long flags;
+
/* 0xFFFF means an invalid PCI VENDOR ID. */
if (hv_pcifront_get_vendor_id(hpdev) == 0xFFFF) {
dev_err_once(&hbus->hdev->device,
"the device has gone\n");
- goto free_int_desc;
+ goto enable_tasklet;
}

/*
- * When the higher level interrupt code calls us with
- * interrupt disabled, we must poll the channel by calling
- * the channel callback directly when channel->target_cpu is
- * the current CPU. When the higher level interrupt code
- * calls us with interrupt enabled, let's add the
- * local_irq_save()/restore() to avoid race:
- * hv_pci_onchannelcallback() can also run in tasklet.
+ * Make sure that the ring buffer data structure doesn't get
+ * freed while we dereference the ring buffer pointer. Test
+ * for the channel's onchannel_callback being NULL within a
+ * sched_lock critical section. See also the inline comments
+ * in vmbus_reset_channel_cb().
*/
- local_irq_save(flags);
-
- if (hbus->hdev->channel->target_cpu == smp_processor_id())
- hv_pci_onchannelcallback(hbus);
-
- local_irq_restore(flags);
+ spin_lock_irqsave(&channel->sched_lock, flags);
+ if (unlikely(channel->onchannel_callback == NULL)) {
+ spin_unlock_irqrestore(&channel->sched_lock, flags);
+ goto enable_tasklet;
+ }
+ hv_pci_onchannelcallback(hbus);
+ spin_unlock_irqrestore(&channel->sched_lock, flags);

if (hpdev->state == hv_pcichild_ejecting) {
dev_err_once(&hbus->hdev->device,
"the device is being ejected\n");
- goto free_int_desc;
+ goto enable_tasklet;
}

udelay(100);
}

+ tasklet_enable(&channel->callback_event);
+
if (comp.comp_pkt.completion_status < 0) {
dev_err(&hbus->hdev->device,
"Request for interrupt failed: 0x%x",
@@ -1488,6 +1498,8 @@ static void hv_compose_msi_msg(struct irq_data *data, struct msi_msg *msg)
put_pcichild(hpdev);
return;

+enable_tasklet:
+ tasklet_enable(&channel->callback_event);
free_int_desc:
kfree(int_desc);
drop_reference:
--
2.24.0
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