linux-kernel - Re: [PATCH] KVM: Deal with nested sleeps in kvm_vcpu

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Message-ID: <CABgObfby+9JNwrJnjPRp6pty05CqRUfKBA3AB=TNwq4q0KjBTg@mail.gmail.com>
Date:   Wed, 30 Nov 2022 17:48:51 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Space Meyer <spm@...gle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        kpsingh@...nel.org
Subject: Re: [PATCH] KVM: Deal with nested sleeps in kvm_vcpu_block()

On Wed, Nov 30, 2022 at 5:20 PM Space Meyer <spm@...gle.com> wrote:
> Previously this code assumed nothing would mess with current->state
> between the set_current_state() and schedule(). However the call to
> kvm_vcpu_check_block() in between might end up requiring locks or other
> actions, which would change current->state

This would be a bug (in particular kvm_arch_vcpu_runnable() and
kvm_cpu_has_pending_timer() should not need any lock). Do you
have a specific call stack in mind?

Paolo

>
> Signed-off-by: Space Meyer <spm@...gle.com>
> ---
>  virt/kvm/kvm_main.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index fab4d37905785..64e10d73f2a92 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -32,6 +32,7 @@
>  #include <linux/sched/signal.h>
>  #include <linux/sched/mm.h>
>  #include <linux/sched/stat.h>
> +#include <linux/wait.h>
>  #include <linux/cpumask.h>
>  #include <linux/smp.h>
>  #include <linux/anon_inodes.h>
> @@ -3426,6 +3427,7 @@ static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
>   */
>  bool kvm_vcpu_block(struct kvm_vcpu *vcpu)
>  {
> +       DEFINE_WAIT_FUNC(vcpu_block_wait, woken_wake_function);
>         struct rcuwait *wait = kvm_arch_vcpu_get_wait(vcpu);
>         bool waited = false;
>
> @@ -3437,13 +3439,11 @@ bool kvm_vcpu_block(struct kvm_vcpu *vcpu)
>         preempt_enable();
>
>         for (;;) {
> -               set_current_state(TASK_INTERRUPTIBLE);
> -
>                 if (kvm_vcpu_check_block(vcpu) < 0)
>                         break;
>
>                 waited = true;
> -               schedule();
> +               wait_woken(&vcpu_block_wait, TASK_INTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
>         }
>
>         preempt_disable();
> --
> 2.38.1.584.g0f3c55d4c2-goog
>