linux-kernel - Re: [patch 2/5] x86/kvm: Handle task

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Message-ID: <20190802120407.GB20111@redhat.com>
Date:   Fri, 2 Aug 2019 14:04:07 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Julia Cartwright <julia@...com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Frederic Weisbecker <fweisbec@...il.com>, kvm@...r.kernel.org,
        Radim Krcmar <rkrcmar@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        John Stultz <john.stultz@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: Re: [patch 2/5] x86/kvm: Handle task_work on VMENTER/EXIT

On 08/01, Thomas Gleixner wrote:
>
> On Thu, 1 Aug 2019, Oleg Nesterov wrote:
> > On 08/01, Thomas Gleixner wrote:
> > >
> > > @@ -8172,6 +8174,10 @@ static int vcpu_run(struct kvm_vcpu *vcp
> > >  			++vcpu->stat.signal_exits;
> > >  			break;
> > >  		}
> > > +
> > > +		if (notify_resume_pending())
> > > +			tracehook_handle_notify_resume();
> >
> > shouldn't you drop kvm->srcu before tracehook_handle_notify_resume() ?
> >
> > I don't understand this code at all, but vcpu_run() does this even before
> > cond_resched().
>
> Yeah, I noticed that it's dropped around cond_resched().
>
> My understanding is that for voluntary giving up the CPU via cond_resched()
> it needs to be dropped.

I am not sure it really needs, but this doesn't matter.

tracehook_handle_notify_resume() can do "anything", say it can run the
works queued by systemtap. I don't think it should delay synchronize_srcu().
And may be this is simply unsafe, even if I don't think a task_work can
ever call synchronize_srcu(kvm->srcu) directly.

Oleg.