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Message-ID: <133a95d9-8148-40ea-9acc-edfd8e3ceef4@siemens.com>
Date: Tue, 3 Feb 2026 17:01:30 +0100
From: Jan Kiszka <jan.kiszka@...mens.com>
To: "K. Y. Srinivasan" <kys@...rosoft.com>,
Haiyang Zhang <haiyangz@...rosoft.com>, Wei Liu <wei.liu@...nel.org>,
Dexuan Cui <decui@...rosoft.com>, Long Li <longli@...rosoft.com>,
Thomas Gleixner <tglx@...nel.org>, Ingo Molnar <mingo@...hat.com>,
Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
x86@...nel.org
Cc: linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
Florian Bezdeka <florian.bezdeka@...mens.com>,
RT <linux-rt-users@...r.kernel.org>, Mitchell Levy <levymitchell0@...il.com>
Subject: [PATCH] x86: mshyperv: Use kthread for vmbus interrupts on PREEMPT_RT
From: Jan Kiszka <jan.kiszka@...mens.com>
Resolves the following lockdep report when booting PREEMPT_RT on Hyper-V
with related guest support enabled:
[ 1.127941] hv_vmbus: registering driver hyperv_drm
[ 1.132518] =============================
[ 1.132519] [ BUG: Invalid wait context ]
[ 1.132521] 6.19.0-rc8+ #9 Not tainted
[ 1.132524] -----------------------------
[ 1.132525] swapper/0/0 is trying to lock:
[ 1.132526] ffff8b9381bb3c90 (&channel->sched_lock){....}-{3:3}, at: vmbus_chan_sched+0xc4/0x2b0
[ 1.132543] other info that might help us debug this:
[ 1.132544] context-{2:2}
[ 1.132545] 1 lock held by swapper/0/0:
[ 1.132547] #0: ffffffffa010c4c0 (rcu_read_lock){....}-{1:3}, at: vmbus_chan_sched+0x31/0x2b0
[ 1.132557] stack backtrace:
[ 1.132560] CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 6.19.0-rc8+ #9 PREEMPT_{RT,(lazy)}
[ 1.132565] Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS Hyper-V UEFI Release v4.1 09/25/2025
[ 1.132567] Call Trace:
[ 1.132570] <IRQ>
[ 1.132573] dump_stack_lvl+0x6e/0xa0
[ 1.132581] __lock_acquire+0xee0/0x21b0
[ 1.132592] lock_acquire+0xd5/0x2d0
[ 1.132598] ? vmbus_chan_sched+0xc4/0x2b0
[ 1.132606] ? lock_acquire+0xd5/0x2d0
[ 1.132613] ? vmbus_chan_sched+0x31/0x2b0
[ 1.132619] rt_spin_lock+0x3f/0x1f0
[ 1.132623] ? vmbus_chan_sched+0xc4/0x2b0
[ 1.132629] ? vmbus_chan_sched+0x31/0x2b0
[ 1.132634] vmbus_chan_sched+0xc4/0x2b0
[ 1.132641] vmbus_isr+0x2c/0x150
[ 1.132648] __sysvec_hyperv_callback+0x5f/0xa0
[ 1.132654] sysvec_hyperv_callback+0x88/0xb0
[ 1.132658] </IRQ>
[ 1.132659] <TASK>
[ 1.132660] asm_sysvec_hyperv_callback+0x1a/0x20
As code paths that handle vmbus IRQs use sleepy locks under PREEMPT_RT,
the complete vmbus_handler execution needs to be moved into thread
context. Open-coding this allows to skip the IPI that irq_work would
additionally bring and which we do not need, being an IRQ, never an NMI.
Signed-off-by: Jan Kiszka <jan.kiszka@...mens.com>
---
This should resolve what was once brought forward via [1]. If it
actually resolves all remaining compatibility issues of the hyperv
support with RT is not yet clear, though. So far, lockdep is happy when
using this plus [2].
[1] https://lore.kernel.org/all/20230809-b4-rt_preempt-fix-v1-0-7283bbdc8b14@gmail.com/
[2] https://lore.kernel.org/lkml/0c7fb5cd-fb21-4760-8593-e04bade84744@siemens.com/
arch/x86/kernel/cpu/mshyperv.c | 52 ++++++++++++++++++++++++++++++++--
1 file changed, 50 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index 579fb2c64cfd..1194ca452c52 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -17,6 +17,7 @@
#include <linux/irq.h>
#include <linux/kexec.h>
#include <linux/random.h>
+#include <linux/smpboot.h>
#include <asm/processor.h>
#include <asm/hypervisor.h>
#include <hyperv/hvhdk.h>
@@ -150,6 +151,43 @@ static void (*hv_stimer0_handler)(void);
static void (*hv_kexec_handler)(void);
static void (*hv_crash_handler)(struct pt_regs *regs);
+static DEFINE_PER_CPU(bool, vmbus_irq_pending);
+static DEFINE_PER_CPU(struct task_struct *, vmbus_irqd);
+
+static void vmbus_irqd_wake(void)
+{
+ struct task_struct *tsk = __this_cpu_read(vmbus_irqd);
+
+ __this_cpu_write(vmbus_irq_pending, true);
+ wake_up_process(tsk);
+}
+
+static void vmbus_irqd_setup(unsigned int cpu)
+{
+ sched_set_fifo(current);
+}
+
+static int vmbus_irqd_should_run(unsigned int cpu)
+{
+ return __this_cpu_read(vmbus_irq_pending);
+}
+
+static void run_vmbus_irqd(unsigned int cpu)
+{
+ vmbus_handler();
+ __this_cpu_write(vmbus_irq_pending, false);
+}
+
+static bool vmbus_irq_initialized;
+
+static struct smp_hotplug_thread vmbus_irq_threads = {
+ .store = &vmbus_irqd,
+ .setup = vmbus_irqd_setup,
+ .thread_should_run = vmbus_irqd_should_run,
+ .thread_fn = run_vmbus_irqd,
+ .thread_comm = "vmbus_irq/%u",
+};
+
DEFINE_IDTENTRY_SYSVEC(sysvec_hyperv_callback)
{
struct pt_regs *old_regs = set_irq_regs(regs);
@@ -158,8 +196,12 @@ DEFINE_IDTENTRY_SYSVEC(sysvec_hyperv_callback)
if (mshv_handler)
mshv_handler();
- if (vmbus_handler)
- vmbus_handler();
+ if (vmbus_handler) {
+ if (IS_ENABLED(CONFIG_PREEMPT_RT))
+ vmbus_irqd_wake();
+ else
+ vmbus_handler();
+ }
if (ms_hyperv.hints & HV_DEPRECATING_AEOI_RECOMMENDED)
apic_eoi();
@@ -174,6 +216,10 @@ void hv_setup_mshv_handler(void (*handler)(void))
void hv_setup_vmbus_handler(void (*handler)(void))
{
+ if (IS_ENABLED(CONFIG_PREEMPT_RT) && !vmbus_irq_initialized) {
+ BUG_ON(smpboot_register_percpu_thread(&vmbus_irq_threads));
+ vmbus_irq_initialized = true;
+ }
vmbus_handler = handler;
}
@@ -181,6 +227,8 @@ void hv_remove_vmbus_handler(void)
{
/* We have no way to deallocate the interrupt gate */
vmbus_handler = NULL;
+ smpboot_unregister_percpu_thread(&vmbus_irq_threads);
+ vmbus_irq_initialized = false;
}
/*
--
2.51.0
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