PCI: hv: Disable/enable IRQs rather than BH in hv_compose_msi_msg()

Commit de0aa7b2f97d ("PCI: hv: Fix 2 hang issues in hv_compose_msi_msg()")
uses local_bh_disable()/enable(), because hv_pci_onchannelcallback() can
also run in tasklet context as the channel event callback, so bottom halves
should be disabled to prevent a race condition.

With CONFIG_PROVE_LOCKING=y in the recent mainline, or old kernels that
don't have commit f71b74bca637 ("irq/softirqs: Use lockdep to assert IRQs
are disabled/enabled"), when the upper layer IRQ code calls
hv_compose_msi_msg() with local IRQs disabled, we'll see a warning at the
beginning of __local_bh_enable_ip():

  IRQs not enabled as expected
    WARNING: CPU: 0 PID: 408 at kernel/softirq.c:162 __local_bh_enable_ip

The warning exposes an issue in de0aa7b2f97d: local_bh_enable() can
potentially call do_softirq(), which is not supposed to run when local IRQs
are disabled. Let's fix this by using local_irq_save()/restore() instead.

Note: hv_pci_onchannelcallback() is not a hot path because it's only called
when the PCI device is hot added and removed, which is infrequent.

Fixes: de0aa7b2f97d ("PCI: hv: Fix 2 hang issues in hv_compose_msi_msg()")
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Cc: stable@vger.kernel.org
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Cc: K. Y. Srinivasan <kys@microsoft.com>

drivers/pci/controller/pci-hyperv.c

@@ -1073,6 +1073,7 @@ static void hv_compose_msi_msg(struct irq_data *data, struct msi_msg *msg)
 	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 {
@@ -1164,14 +1165,15 @@ static void hv_compose_msi_msg(struct irq_data *data, struct msi_msg *msg)
 		 * 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_bh_disable()/enable() to avoid race.
+		 * local_irq_save()/restore() to avoid race:
+		 * hv_pci_onchannelcallback() can also run in tasklet.
 		 */
-		local_bh_disable();
+		local_irq_save(flags);
 
 		if (hbus->hdev->channel->target_cpu == smp_processor_id())
 			hv_pci_onchannelcallback(hbus);
 
-		local_bh_enable();
+		local_irq_restore(flags);
 
 		if (hpdev->state == hv_pcichild_ejecting) {
 			dev_err_once(&hbus->hdev->device,