Re: [PATCH v2] x86: mshyperv: Use kthread for vmbus interrupts on PREEMPT_RT

[Jan Kiszka]

On 14.02.26 05:26, Saurabh Singh Sengar wrote:
> On Fri, Feb 13, 2026 at 08:20:31PM -0800, Saurabh Singh Sengar wrote:
>> On Fri, Feb 06, 2026 at 07:47:54AM +0100, Jan Kiszka wrote:
>>> From: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
>>>
>>> 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@xxxxxxxxxxx>
>>
>> First I would like to share my opinion that, although support for the
>> RT kernel is not on the near-term roadmap, we should welcome RT Linux
>> patches.
>>
>> Coming back to this patch I can reproduce the stack trace referenced
>> in the commit when running with PREEMPT_RT enabled, and I have verified
>> that this patch resolves the issue. Next, I observed the storage-related
>> stack trace mentioned in Jan’s other patch; applying the storvsc patch
>> fixed that as well.
>>
>> However, when testing without PREEMPT_RT enabled, I see a another lockdep
>> warning below (both with and without Jan’s patches). IWanted to check if
>> is it possible to address this issue as part of the same fix ?
>> Doing so would make the change more useful beyond PREEMPT_RT.
> 
> 
> Sharing the stack, missed in previous email:
> 
> [    1.612866] =============================
> [    1.616197] tun: Universal TUN/TAP device driver, 1.6
> [    1.612866] [ BUG: Invalid wait context ]
> [    1.612866] 6.19.0-next-20260212+ #8 Not tainted
> [    1.612866] -----------------------------
> [    1.612866] swapper/0/0 is trying to lock:
> [    1.612866] ffff895a03dfd3f0 (&channel->sched_lock){....}-{3:3}, at: vmbus_chan_sched+0xda/0x350
> [    1.621086] PPP generic driver version 2.4.2
> [    1.612866] other info that might help us debug this:
> [    1.612866] context-{2:2}
> [    1.612866] 1 lock held by swapper/0/0:
> [    1.612866]  #0: ffffffff9b7d4660 (rcu_read_lock){....}-{1:3}, at: vmbus_chan_sched+0x38/0x350

Current context is LD_WAIT_SPIN (2), but the sched_lock is
LD_WAIT_CONFIG (3), which is invalid in RT. So this is warning us about
what this patch is addressing. But my patch likely misses some
lockdep_hardirq_threaded() before vmbus_handler is called to annotate
that fact.

Jan

> [    1.628045] i8042: PNP: No PS/2 controller found.
> [    1.612866] stack backtrace:
> [    1.612866] CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 6.19.0-next-20260212+ #8 PREEMPT
> [    1.612866] Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS Hyper-V UEFI Release v4.1 09/25/2025
> [    1.612866] Call Trace:
> [    1.612866]  <IRQ>
> [    1.612866]  dump_stack_lvl+0x6f/0xb0
> [    1.612866]  dump_stack+0x10/0x20
> [    1.612866]  __lock_acquire+0x973/0x24e0
> [    1.612866]  ? __lock_acquire+0x449/0x24e0
> [    1.612866]  lock_acquire+0xb6/0x2c0
> [    1.612866]  ? vmbus_chan_sched+0xda/0x350
> [    1.612866]  ? vmbus_chan_sched+0x38/0x350
> [    1.612866]  _raw_spin_lock+0x2f/0x50
> [    1.612866]  ? vmbus_chan_sched+0xda/0x350
> [    1.612866]  vmbus_chan_sched+0xda/0x350
> [    1.612866]  ? sched_clock_idle_wakeup_event+0x22/0x50
> [    1.612866]  vmbus_isr+0x26/0x170
> [    1.612866]  __sysvec_hyperv_callback+0x43/0x80
> [    1.612866]  sysvec_hyperv_callback+0x85/0xb0
> [    1.612866]  </IRQ>
> [    1.612866]  <TASK>
> [    1.612866]  asm_sysvec_hyperv_callback+0x1b/0x20
> [    1.612866] RIP: 0010:pv_native_safe_halt+0xb/0x10
> [    1.612866] Code: 48 2b 05 d8 00 97 00 5d c3 cc cc cc cc 66 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 66 90 0f 00 2d 99 33 1f 00 fb f4 <e9> 40 9e 01 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 55
> [    1.612866] RSP: 0000:ffffffff9b603dd8 EFLAGS: 00000242
> [    1.612866] RAX: 0000000000040d85 RBX: 0000000000000000 RCX: 0000000000000000
> [    1.612866] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff9981172f
> [    1.612866] RBP: ffffffff9b603de0 R08: 0000000000000001 R09: 0000000000000000
> [    1.612866] R10: 0000000000000001 R11: 0000000000000001 R12: 0000000000000000
> [    1.612866] R13: 0000000000000000 R14: 0000000000000000 R15: ffffffff9b628490
> [    1.612866]  ? do_idle+0x20f/0x290
> [    1.612866]  ? default_idle+0x9/0x20
> [    1.612866]  arch_cpu_idle+0x9/0x10
> [    1.612866]  default_idle_call+0x83/0x210
> [    1.612866]  do_idle+0x20f/0x290
> [    1.612866]  cpu_startup_entry+0x29/0x30
> [    1.612866]  rest_init+0x104/0x200
> [    1.612866]  start_kernel+0xa13/0xc90
> [    1.612866]  ? sme_unmap_bootdata+0x14/0x70
> [    1.612866]  x86_64_start_reservations+0x18/0x30
> [    1.612866]  x86_64_start_kernel+0x148/0x1a0
> [    1.612866]  ? soft_restart_cpu+0x14/0x14
> [    1.612866]  common_startup_64+0x13e/0x141
> [    1.612866]  </TASK>
> 
> - Saurabh

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