linux-kernel - Re: [PATCH v3 2/2] kvm/vfio: avoid bouncing the mutex when adding and deleting groups

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Message-ID: <ZLlqb8Hk0S3AkEsf@google.com>
Date:   Thu, 20 Jul 2023 10:10:07 -0700
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "Bradescu, Roxana" <roxabee@...gle.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/2] kvm/vfio: avoid bouncing the mutex when adding
 and deleting groups

On Thu, Jul 20, 2023 at 02:36:16AM +0000, Tian, Kevin wrote:
> > From: Dmitry Torokhov <dmitry.torokhov@...il.com>
> > Sent: Wednesday, July 19, 2023 2:11 PM
> > 
> > On Wed, Jul 19, 2023 at 05:32:27AM +0000, Tian, Kevin wrote:
> > > > From: Dmitry Torokhov <dmitry.torokhov@...il.com>
> > > > Sent: Saturday, July 15, 2023 6:46 AM
> > > >
> > > > @@ -165,30 +161,26 @@ static int kvm_vfio_group_add(struct
> > kvm_device
> > > > *dev, unsigned int fd)
> > > >  	list_for_each_entry(kvg, &kv->group_list, node) {
> > > >  		if (kvg->file == filp) {
> > > >  			ret = -EEXIST;
> > > > -			goto err_unlock;
> > > > +			goto out_unlock;
> > > >  		}
> > > >  	}
> > > >
> > > >  	kvg = kzalloc(sizeof(*kvg), GFP_KERNEL_ACCOUNT);
> > > >  	if (!kvg) {
> > > >  		ret = -ENOMEM;
> > > > -		goto err_unlock;
> > > > +		goto out_unlock;
> > > >  	}
> > > >
> > > > -	kvg->file = filp;
> > > > +	kvg->file = get_file(filp);
> > >
> > > Why is another reference required here?
> > 
> > Because the function now has a single exit point and the original
> > reference is dropped unconditionally on exit. It looks cleaner than
> > checking for non-zero "ret" and deciding whether the reference should be
> > dropped or kept.
> > 
> 
> A comment is appreciated. otherwise,
> 
> Reviewed-by: Kevin Tian <kevin.tian@...el.com>

Thank you for the review! However I do not think any comment is needed,
if one is looking at the final source and not the patch form, the reason
for taking another reference is plain to see.

Thanks!

-- 
Dmitry