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

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Message-ID: <BN9PR11MB52763F0990CD23DB63AC36578C3EA@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Thu, 20 Jul 2023 02:36:16 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Dmitry Torokhov <dmitry.torokhov@...il.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

> 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>