diff mbox series

xen/events: avoid using info_for_irq() in xen_send_IPI_one()

Message ID 20231030142732.1702-1-jgross@suse.com (mailing list archive)
State Accepted
Commit e43a64028a93f2823ca712b6335ed01b904eec69
Headers show
Series xen/events: avoid using info_for_irq() in xen_send_IPI_one() | expand

Commit Message

Jürgen Groß Oct. 30, 2023, 2:27 p.m. UTC 
xen_send_IPI_one() is being used by cpuhp_report_idle_dead() after
it calls rcu_report_dead(), meaning that any RCU usage by
xen_send_IPI_one() is a bad idea.

Unfortunately xen_send_IPI_one() is using notify_remote_via_irq()
today, which is using irq_get_chip_data() via info_for_irq(). And
irq_get_chip_data() in turn is using a maple-tree lookup requiring
RCU.

Avoid this problem by caching the ipi event channels in another
percpu variable, allowing the use notify_remote_via_evtchn() in
xen_send_IPI_one().

Fixes: 721255b9826b ("genirq: Use a maple tree for interrupt descriptor management")
Reported-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Juergen Gross <jgross@suse.com>
Tested-by: David Woodhouse <dwmw@amazon.co.uk>
---
 drivers/xen/events/events_base.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

Comments

Stefano Stabellini Oct. 31, 2023, 12:19 a.m. UTC | #1 
On Mon, 30 Oct 2023, Juergen Gross wrote:
> xen_send_IPI_one() is being used by cpuhp_report_idle_dead() after
> it calls rcu_report_dead(), meaning that any RCU usage by
> xen_send_IPI_one() is a bad idea.
> 
> Unfortunately xen_send_IPI_one() is using notify_remote_via_irq()
> today, which is using irq_get_chip_data() via info_for_irq(). And
> irq_get_chip_data() in turn is using a maple-tree lookup requiring
> RCU.
> 
> Avoid this problem by caching the ipi event channels in another
> percpu variable, allowing the use notify_remote_via_evtchn() in
> xen_send_IPI_one().
> 
> Fixes: 721255b9826b ("genirq: Use a maple tree for interrupt descriptor management")
> Reported-by: David Woodhouse <dwmw@amazon.co.uk>
> Signed-off-by: Juergen Gross <jgross@suse.com>
> Tested-by: David Woodhouse <dwmw@amazon.co.uk>

Acked-by: Stefano Stabellini <sstabellini@kernel.org>
diff mbox series

Patch

diff --git a/drivers/xen/events/events_base.c b/drivers/xen/events/events_base.c
index 1b2136fe0fa5..2cf0c2b69386 100644
--- a/drivers/xen/events/events_base.c
+++ b/drivers/xen/events/events_base.c
@@ -164,6 +164,8 @@  static DEFINE_PER_CPU(int [NR_VIRQS], virq_to_irq) = {[0 ... NR_VIRQS-1] = -1};
 
 /* IRQ <-> IPI mapping */
 static DEFINE_PER_CPU(int [XEN_NR_IPIS], ipi_to_irq) = {[0 ... XEN_NR_IPIS-1] = -1};
+/* Cache for IPI event channels - needed for hot cpu unplug (avoid RCU usage). */
+static DEFINE_PER_CPU(evtchn_port_t [XEN_NR_IPIS], ipi_to_evtchn) = {[0 ... XEN_NR_IPIS-1] = 0};
 
 /* Event channel distribution data */
 static atomic_t channels_on_cpu[NR_CPUS];
@@ -366,6 +368,7 @@  static int xen_irq_info_ipi_setup(unsigned cpu,
 	info->u.ipi = ipi;
 
 	per_cpu(ipi_to_irq, cpu)[ipi] = irq;
+	per_cpu(ipi_to_evtchn, cpu)[ipi] = evtchn;
 
 	return xen_irq_info_common_setup(info, irq, IRQT_IPI, evtchn, 0);
 }
@@ -981,6 +984,7 @@  static void __unbind_from_irq(unsigned int irq)
 			break;
 		case IRQT_IPI:
 			per_cpu(ipi_to_irq, cpu)[ipi_from_irq(irq)] = -1;
+			per_cpu(ipi_to_evtchn, cpu)[ipi_from_irq(irq)] = 0;
 			break;
 		case IRQT_EVTCHN:
 			dev = info->u.interdomain;
@@ -1631,7 +1635,7 @@  EXPORT_SYMBOL_GPL(evtchn_put);
 
 void xen_send_IPI_one(unsigned int cpu, enum ipi_vector vector)
 {
-	int irq;
+	evtchn_port_t evtchn;
 
 #ifdef CONFIG_X86
 	if (unlikely(vector == XEN_NMI_VECTOR)) {
@@ -1642,9 +1646,9 @@  void xen_send_IPI_one(unsigned int cpu, enum ipi_vector vector)
 		return;
 	}
 #endif
-	irq = per_cpu(ipi_to_irq, cpu)[vector];
-	BUG_ON(irq < 0);
-	notify_remote_via_irq(irq);
+	evtchn = per_cpu(ipi_to_evtchn, cpu)[vector];
+	BUG_ON(evtchn == 0);
+	notify_remote_via_evtchn(evtchn);
 }
 
 struct evtchn_loop_ctrl {