linux-kernel - Re: [PATCH] KVM: use unified srcu interface function

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Message-ID: <365fe273-ba11-eb12-4d80-a2e6a17bf0fa@redhat.com>
Date:   Fri, 23 Dec 2022 16:32:53 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Hao Peng <flyingpenghao@...il.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: use unified srcu interface function

On 12/20/22 08:47, Hao Peng wrote:
>>> +       old = srcu_dereference_check(kvm->irq_routing, &kvm->irq_srcu,
>>> +                                       lockdep_is_held(&kvm->irq_lock));
>> Readers of irq_routing are protected via kvm->irq_srcu, but this writer is never
>> called with kvm->irq_srcu held.  I do like the of replacing '1' with
>> lockdep_is_held(&kvm->irq_lock) to document the protection, so what about just
>> doing that?  I.e.
>>
> Sorry for the long delay in replying. Although kvm->irq_srcu is not required
> to protect irq_routing here, this interface function srcu_dereference_check
> indicates that irq_routing is protected by kvm->irq_srcu in the kvm subsystem.
> Thanks.
> 

I agree, the last two arguments basically are alternative conditions to 
satisfy the check:

#define srcu_dereference_check(p, ssp, c) \
         __rcu_dereference_check((p), __UNIQUE_ID(rcu), \
                                 (c) || srcu_read_lock_held(ssp), __rcu)

The idea is to share the code between readers and writers, so what do 
you think of adding a

#define kvm_get_irq_routing(kvm) srcu_dereference_check(...)

macro at the top of virt/kvm/irqchip.c?

Thanks,

Paolo