linux-kernel - Re: [PATCH 7/7] kvm,mips: Fix potential swait

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Date:   Wed, 13 Sep 2017 22:35:47 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Davidlohr Bueso <dave@...olabs.net>, mingo@...nel.org,
        peterz@...radead.org
Cc:     npiggin@...il.com, paulmck@...ux.vnet.ibm.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 7/7] kvm,mips: Fix potential swait_active() races

On 13/09/2017 22:08, Davidlohr Bueso wrote:
> For example, the following could occur, making us miss a wakeup:
> 
> CPU0					CPU1
> kvm_vcpu_block				kvm_mips_comparecount_func
> 					  [L] swait_active(&vcpu->wq)
>   [S] prepare_to_swait(&vcpu->wq)
>   [L] if (!kvm_vcpu_has_pending_timer(vcpu))
>          schedule()                       [S] queue_timer_int(vcpu)
> 
> Ensure that the swait_active() check is not hoisted over the interrupt.
> 
> Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
> ---
>  arch/mips/kvm/mips.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/mips/kvm/mips.c b/arch/mips/kvm/mips.c
> index bce2a6431430..d535edc01434 100644
> --- a/arch/mips/kvm/mips.c
> +++ b/arch/mips/kvm/mips.c
> @@ -514,7 +514,7 @@ int kvm_vcpu_ioctl_interrupt(struct kvm_vcpu *vcpu,
>  
>  	dvcpu->arch.wait = 0;
>  
> -	if (swait_active(&dvcpu->wq))
> +	if (swq_has_sleeper(&dvcpu->wq))
>  		swake_up(&dvcpu->wq);
>  
>  	return 0;
> @@ -1179,7 +1179,7 @@ static void kvm_mips_comparecount_func(unsigned long data)
>  	kvm_mips_callbacks->queue_timer_int(vcpu);
>  
>  	vcpu->arch.wait = 0;
> -	if (swait_active(&vcpu->wq))
> +	if (swq_has_sleeper(&vcpu->wq))
>  		swake_up(&vcpu->wq);
>  }
>  
> 

has_sleeper*s*.  Can fix when committing.

Paolo