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

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Date:   Fri, 14 Jul 2023 15:45:33 -0700
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Roxana Bradescu <roxabee@...gle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH v3 2/2] kvm/vfio: avoid bouncing the mutex when adding and deleting groups

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@...hat.com>
Reviewed-by: Alex Williamson <alex.williamson@...hat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@...il.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(-)

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;
 }
 
-- 
2.41.0.255.g8b1d071c50-goog