Messages in this thread | | | Date | Thu, 20 Jan 2022 11:57:14 +0000 | From | Mark Rutland <> | Subject | Re: [PATCH v2 0/7] kvm: fix latent guest entry/exit bugs |
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On Wed, Jan 19, 2022 at 08:30:17PM +0100, Christian Borntraeger wrote: > > > Am 19.01.22 um 20:22 schrieb Mark Rutland: > > On Wed, Jan 19, 2022 at 07:25:20PM +0100, Christian Borntraeger wrote: > > > Am 19.01.22 um 11:58 schrieb Mark Rutland: > > > > > > > > > CCing new emails for Anup and Atish so that they are aware of this thread. > > > > Ah; whoops. I'd meant to fix the Ccs on the patches. > > > > Thanks! > > > > [...] > > > > > I just gave this a spin on s390 with debugging on and I got the following: > > > > > > [ 457.151295] ------------[ cut here ]------------ > > > [ 457.151311] WARNING: CPU: 14 PID: 0 at kernel/rcu/tree.c:613 rcu_eqs_enter.constprop.0+0xf8/0x118 > > > > Hmm, so IIUC that's: > > > > WARN_ON_ONCE(rdp->dynticks_nmi_nesting != DYNTICK_IRQ_NONIDLE); > > > > ... and we're clearly in the idle thread here. > > > > I wonder, is the s390 guest entry/exit *preemptible* ? > > Looks like debug_defconfig is indeed using preemption: > > CONFIG_PREEMPT_BUILD=y > # CONFIG_PREEMPT_NONE is not set > # CONFIG_PREEMPT_VOLUNTARY is not set > CONFIG_PREEMPT=y > CONFIG_PREEMPT_COUNT=y > CONFIG_PREEMPTION=y > CONFIG_PREEMPT_RCU=y > CONFIG_PREEMPT_NOTIFIERS=y > CONFIG_DEBUG_PREEMPT=y > CONFIG_PREEMPTIRQ_TRACEPOINTS=y > CONFIG_TRACE_PREEMPT_TOGGLE=y > CONFIG_PREEMPT_TRACER=y > # CONFIG_PREEMPTIRQ_DELAY_TEST is not set
Thanks for confirming!
Could you try with CONFIG_PROVE_RCU=y ? That can't be selected directly, but selecting PROVE_LOCKING=y will enable it.
If I'm right, with that we should get a splat out of rcu_irq_exit_check_preempt().
If so, I think we can solve this with preempt_{disable,enable}() around the guest_timing_{enter,exit}_irqoff() calls. We'll also need to add some more comments around arch_in_rcu_eqs() that arch-specific EQSs should be non-preemptible.
Thanks, Mark.
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