linux-kernel - RE: [PATCH] kvm/eventfd: fix the misleading comment in kvm_irqfd

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Date:   Fri, 3 Dec 2021 03:03:35 +0000
From:   "Longpeng (Mike, Cloud Infrastructure Service Product Dept.)" 
        <longpeng2@...wei.com>
To:     Christian Borntraeger <borntraeger@...ux.ibm.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>
CC:     "cornelia.huck@...ibm.com" <cornelia.huck@...ibm.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Gonglei (Arei)" <arei.gonglei@...wei.com>
Subject: RE: [PATCH] kvm/eventfd: fix the misleading comment in
 kvm_irqfd_assign



> -----Original Message-----
> From: Christian Borntraeger [mailto:borntraeger@...ux.ibm.com]
> Sent: Tuesday, November 30, 2021 1:14 AM
> To: Longpeng (Mike, Cloud Infrastructure Service Product Dept.)
> <longpeng2@...wei.com>; pbonzini@...hat.com
> Cc: cornelia.huck@...ibm.com; kvm@...r.kernel.org;
> linux-kernel@...r.kernel.org; Gonglei (Arei) <arei.gonglei@...wei.com>
> Subject: Re: [PATCH] kvm/eventfd: fix the misleading comment in kvm_irqfd_assign
> 
> 
> 
> Am 29.11.21 um 04:43 schrieb Longpeng(Mike):
> > From: Longpeng <longpeng2@...wei.com>
> >
> > The comment above the invocation of vfs_poll() is misleading, move
> > it to the right place.
> >
> I think that the current variant is better.
> events is only used in that function to check for EPOLLIN, so the
> assignment and the if belong together from a "what am I doing here" perspective.
> 

Hi Christian,

I think that add the irqfd->wait to the file's wait queue is much more
important, the current variant may lead to ignoring it.

Both of these two variants are supported in the current kernel:

[1] https://elixir.bootlin.com/linux/latest/source/drivers/vfio/virqfd.c#L169
```
	events = vfs_poll(irqfd.file, &virqfd->pt);

	/*
	 * Check if there was an event already pending on the eventfd
	 * before we registered and trigger it as if we didn't miss it.
	 */
	if (events & EPOLLIN) {
		if ((!handler || handler(opaque, data)) && thread)
			schedule_work(&virqfd->inject);
	}
```

[2] https://elixir.bootlin.com/linux/latest/source/drivers/virt/acrn/irqfd.c#L161
```
	/* Check the pending event in this stage */
	events = vfs_poll(f.file, &irqfd->pt);

	if (events & EPOLLIN)
		acrn_irqfd_inject(irqfd);
```

Since there's no any code changes, I agree to drop this unmeaning change.

Thanks.

> > Fixes: 684a0b719ddb ("KVM: eventfd: Fix lock order inversion")
> > Signed-off-by: Longpeng <longpeng2@...wei.com>
> > ---
> >   virt/kvm/eventfd.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
> > index 2ad013b..cd01814 100644
> > --- a/virt/kvm/eventfd.c
> > +++ b/virt/kvm/eventfd.c
> > @@ -406,12 +406,12 @@ bool __attribute__((weak))
> kvm_arch_irqfd_route_changed(
> >
> >   	spin_unlock_irq(&kvm->irqfds.lock);
> >
> > +	events = vfs_poll(f.file, &irqfd->pt);
> > +
> >   	/*
> >   	 * Check if there was an event already pending on the eventfd
> >   	 * before we registered, and trigger it as if we didn't miss it.
> >   	 */
> > -	events = vfs_poll(f.file, &irqfd->pt);
> > -
> >   	if (events & EPOLLIN)
> >   		schedule_work(&irqfd->inject);
> >