linux-kernel - Re: [BUG] Deadlock due due to interactions of block, RCU, and cpu offline

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Message-ID: <a0627c12-3406-5e13-8346-649bd8e27edf@redhat.com>
Date:   Tue, 22 Aug 2017 18:12:04 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     paulmck@...ux.vnet.ibm.com, Jeffrey Hugo <jhugo@...eaurora.org>
Cc:     linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        pprakash@...eaurora.org, Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Jens Axboe <axboe@...nel.dk>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Richard Cochran <rcochran@...utronix.de>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Richard Weinberger <richard@....at>
Subject: Re: [BUG] Deadlock due due to interactions of block, RCU, and cpu
 offline

On 20/08/2017 22:56, Paul E. McKenney wrote:
>>       KVM: async_pf: avoid async pf injection when in guest mode
>>       KVM: cpuid: Fix read/write out-of-bounds vulnerability in cpuid emulation
>>       arm: KVM: Allow unaligned accesses at HYP
>>       arm64: KVM: Allow unaligned accesses at EL2
>>       arm64: KVM: Preserve RES1 bits in SCTLR_EL2
>>       KVM: arm/arm64: Handle possible NULL stage2 pud when ageing pages
>>       KVM: nVMX: Fix exception injection
>>       kvm: async_pf: fix rcu_irq_enter() with irqs enabled
>>       KVM: arm/arm64: vgic-v3: Fix nr_pre_bits bitfield extraction
>>       KVM: s390: fix ais handling vs cpu model
>>       KVM: arm/arm64: Fix isues with GICv2 on GICv3 migration
>>
>> Nothing really stands out to me which would "fix" the issue.
>
> My guess would be an undo of the change that provoked the problem
> in the first place.  Did you try bisecting within the above group
> of commits?
> 
> Either way, CCing Paolo for his thoughts?

There is "kvm: async_pf: fix rcu_irq_enter() with irqs enabled", but it
would have caused splats, not deadlocks.

If you are using nested virtualization, "KVM: async_pf: avoid async pf
injection when in guest mode" can be a wildcard, but only if you have
memory pressure.

My bet is still on the former changing the timing just a little bit.

Paolo