diff mbox series

[v3,2/2] kvm/vfio: avoid bouncing the mutex when adding and deleting groups

Message ID 20230714224538.404793-2-dmitry.torokhov@gmail.com
State New
Headers
Series [v3,1/2] kvm/vfio: ensure kvg instance stays around in kvm_vfio_group_add() |

Commit Message

Dmitry Torokhov July 14, 2023, 10:45 p.m. UTC 
  Stop taking kv->lock mutex in kvm_vfio_update_coherency() and instead
call it with this mutex held: the callers of the function usually
already have it taken (and released) before calling
kvm_vfio_update_coherency(). This avoid bouncing the lock up and down.

The exception is kvm_vfio_release() where we do not take the lock, but
it is being executed when the very last reference to kvm_device is being
dropped, so there are no concerns about concurrency.

Suggested-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
---

v3: initialize "ret" with 0 (per Alex), added Alex's reviewed-by

v2: new patch.

 virt/kvm/vfio.c | 26 +++++++++-----------------
 1 file changed, 9 insertions(+), 17 deletions(-)

Comments

Tian, Kevin July 19, 2023, 5:32 a.m. UTC | #1 
> From: Dmitry Torokhov <dmitry.torokhov@gmail.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?
Dmitry Torokhov July 19, 2023, 6:10 a.m. UTC | #2 
On Wed, Jul 19, 2023 at 05:32:27AM +0000, Tian, Kevin wrote:
> > From: Dmitry Torokhov <dmitry.torokhov@gmail.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.

Thanks.
Tian, Kevin July 20, 2023, 2:36 a.m. UTC | #3 
> From: Dmitry Torokhov <dmitry.torokhov@gmail.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@gmail.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@intel.com>
Dmitry Torokhov July 20, 2023, 5:10 p.m. UTC | #4 
On Thu, Jul 20, 2023 at 02:36:16AM +0000, Tian, Kevin wrote:
> > From: Dmitry Torokhov <dmitry.torokhov@gmail.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@gmail.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@intel.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!
diff mbox series

Patch

diff --git a/virt/kvm/vfio.c b/virt/kvm/vfio.c
index cd46d7ef98d6..dbf2b855cf78 100644
--- a/virt/kvm/vfio.c
+++ b/virt/kvm/vfio.c
@@ -122,8 +122,6 @@  static void kvm_vfio_update_coherency(struct kvm_device *dev)
 	bool noncoherent = false;
 	struct kvm_vfio_group *kvg;
 
-	mutex_lock(&kv->lock);
-
 	list_for_each_entry(kvg, &kv->group_list, node) {
 		if (!kvm_vfio_file_enforced_coherent(kvg->file)) {
 			noncoherent = true;
@@ -139,8 +137,6 @@  static void kvm_vfio_update_coherency(struct kvm_device *dev)
 		else
 			kvm_arch_unregister_noncoherent_dma(dev->kvm);
 	}
-
-	mutex_unlock(&kv->lock);
 }
 
 static int kvm_vfio_group_add(struct kvm_device *dev, unsigned int fd)
@@ -148,7 +144,7 @@  static int kvm_vfio_group_add(struct kvm_device *dev, unsigned int fd)
 	struct kvm_vfio *kv = dev->private;
 	struct kvm_vfio_group *kvg;
 	struct file *filp;
-	int ret;
+	int ret = 0;
 
 	filp = fget(fd);
 	if (!filp)
@@ -157,7 +153,7 @@  static int kvm_vfio_group_add(struct kvm_device *dev, unsigned int fd)
 	/* Ensure the FD is a vfio group FD.*/
 	if (!kvm_vfio_file_is_group(filp)) {
 		ret = -EINVAL;
-		goto err_fput;
+		goto out_fput;
 	}
 
 	mutex_lock(&kv->lock);
@@ -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);
 	list_add_tail(&kvg->node, &kv->group_list);
 
 	kvm_arch_start_assignment(dev->kvm);
 	kvm_vfio_file_set_kvm(kvg->file, dev->kvm);
-
-	mutex_unlock(&kv->lock);
-
 	kvm_vfio_update_coherency(dev);
 
-	return 0;
-err_unlock:
+out_unlock:
 	mutex_unlock(&kv->lock);
-err_fput:
+out_fput:
 	fput(filp);
 	return ret;
 }
@@ -224,12 +216,12 @@  static int kvm_vfio_group_del(struct kvm_device *dev, unsigned int fd)
 		break;
 	}
 
+	kvm_vfio_update_coherency(dev);
+
 	mutex_unlock(&kv->lock);
 
 	fdput(f);
 
-	kvm_vfio_update_coherency(dev);
-
 	return ret;
 }