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

["Bezdeka, Florian" <florian.bezdeka@siemens.com>]

On Tue, 2026-02-03 at 17:01 +0100, Jan Kiszka wrote:
> From: Jan Kiszka <jan.kiszka@siemens.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@siemens.com>

Tested-by: Florian Bezdeka <florian.bezdeka@siemens.com>


This patch survived a 24h stress test with CONFIG_PREEMPT_RT enabled and
heavy load applied to the system.

There was no lockup happening without this patch. The lockdep warning is
gone now.

Best regards,
Florian


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