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Subject: Re: [PATCH v2 17/18] kvm/eventfd: Use sequence counter with
 associated spinlock
To:     "Ahmed S. Darwish" <a.darwish@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        "Sebastian A. Siewior" <bigeasy@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        LKML <linux-kernel@vger.kernel.org>, kvm@vger.kernel.org
References: <20200519214547.352050-1-a.darwish@linutronix.de>
 <20200608005729.1874024-1-a.darwish@linutronix.de>
 <20200608005729.1874024-18-a.darwish@linutronix.de>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <cc198b70-7521-47be-0f1d-ff5ac754cb69@redhat.com>
Date:   Mon, 8 Jun 2020 14:57:20 +0200
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On 08/06/20 02:57, Ahmed S. Darwish wrote:
> A sequence counter write side critical section must be protected by some
> form of locking to serialize writers. A plain seqcount_t does not
> contain the information of which lock must be held when entering a write
> side critical section.
> 
> Use the new seqcount_spinlock_t data type, which allows to associate a
> spinlock with the sequence counter. This enables lockdep to verify that
> the spinlock used for writer serialization is held when the write side
> critical section is entered.
> 
> If lockdep is disabled this lock association is compiled out and has
> neither storage size nor runtime overhead.
> 
> Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
> ---
>  include/linux/kvm_irqfd.h | 2 +-
>  virt/kvm/eventfd.c        | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/kvm_irqfd.h b/include/linux/kvm_irqfd.h
> index dc1da020305b..dac047abdba7 100644
> --- a/include/linux/kvm_irqfd.h
> +++ b/include/linux/kvm_irqfd.h
> @@ -42,7 +42,7 @@ struct kvm_kernel_irqfd {
>  	wait_queue_entry_t wait;
>  	/* Update side is protected by irqfds.lock */
>  	struct kvm_kernel_irq_routing_entry irq_entry;
> -	seqcount_t irq_entry_sc;
> +	seqcount_spinlock_t irq_entry_sc;
>  	/* Used for level IRQ fast-path */
>  	int gsi;
>  	struct work_struct inject;
> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
> index 67b6fc153e9c..8694a2920ea9 100644
> --- a/virt/kvm/eventfd.c
> +++ b/virt/kvm/eventfd.c
> @@ -303,7 +303,7 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
>  	INIT_LIST_HEAD(&irqfd->list);
>  	INIT_WORK(&irqfd->inject, irqfd_inject);
>  	INIT_WORK(&irqfd->shutdown, irqfd_shutdown);
> -	seqcount_init(&irqfd->irq_entry_sc);
> +	seqcount_spinlock_init(&irqfd->irq_entry_sc, &kvm->irqfds.lock);
>  
>  	f = fdget(args->fd);
>  	if (!f.file) {
> 

Acked-by: Paolo Bonzini <pbonzini@redhat.com>