After 51cdc8bc120e, we have another deadlock scenario between the
kvm->lock and the vfio group_lock with two different codepaths acquiring
the locks in different order. Specifically in vfio_open_device, vfio
holds the vfio group_lock when issuing device->ops->open_device but some
drivers (like vfio-ap) need to acquire kvm->lock during their open_device
routine; Meanwhile, kvm_vfio_release will acquire the kvm->lock first
before calling vfio_file_set_kvm which will acquire the vfio group_lock.
To resolve this, let's remove the need for the vfio group_lock from the
kvm_vfio_release codepath. This is done by introducing a new spinlock to
protect modifications to the vfio group kvm pointer, and acquiring a kvm
ref from within vfio while holding this spinlock, with the reference held
until the last close for the device in question.
Fixes: 51cdc8bc120e ("kvm/vfio: Fix potential deadlock on vfio group_lock")
Reported-by: Anthony Krowiak <[email protected]>
Suggested-by: Jason Gunthorpe <[email protected]>
Signed-off-by: Matthew Rosato <[email protected]>
---
drivers/vfio/group.c | 14 +++----
drivers/vfio/vfio.h | 5 ++-
drivers/vfio/vfio_main.c | 88 ++++++++++++++++++++++++++++++++++++----
include/linux/vfio.h | 2 +-
4 files changed, 88 insertions(+), 21 deletions(-)
diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
index bb24b2f0271e..8f67d33e0e8d 100644
--- a/drivers/vfio/group.c
+++ b/drivers/vfio/group.c
@@ -164,13 +164,7 @@ static int vfio_device_group_open(struct vfio_device *device)
goto out_unlock;
}
- /*
- * Here we pass the KVM pointer with the group under the lock. If the
- * device driver will use it, it must obtain a reference and release it
- * during close_device.
- */
- ret = vfio_device_open(device, device->group->iommufd,
- device->group->kvm);
+ ret = vfio_device_open(device, device->group);
out_unlock:
mutex_unlock(&device->group->group_lock);
@@ -450,6 +444,7 @@ static struct vfio_group *vfio_group_alloc(struct iommu_group *iommu_group,
refcount_set(&group->drivers, 1);
mutex_init(&group->group_lock);
+ spin_lock_init(&group->kvm_ref_lock);
INIT_LIST_HEAD(&group->device_list);
mutex_init(&group->device_lock);
group->iommu_group = iommu_group;
@@ -799,13 +794,14 @@ EXPORT_SYMBOL_GPL(vfio_file_enforced_coherent);
void vfio_file_set_kvm(struct file *file, struct kvm *kvm)
{
struct vfio_group *group = file->private_data;
+ unsigned long flags;
if (!vfio_file_is_group(file))
return;
- mutex_lock(&group->group_lock);
+ spin_lock_irqsave(&group->kvm_ref_lock, flags);
group->kvm = kvm;
- mutex_unlock(&group->group_lock);
+ spin_unlock_irqrestore(&group->kvm_ref_lock, flags);
}
EXPORT_SYMBOL_GPL(vfio_file_set_kvm);
diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
index f8219a438bfb..d153ad35de24 100644
--- a/drivers/vfio/vfio.h
+++ b/drivers/vfio/vfio.h
@@ -15,11 +15,11 @@ struct iommufd_ctx;
struct iommu_group;
struct vfio_device;
struct vfio_container;
+struct vfio_group;
void vfio_device_put_registration(struct vfio_device *device);
bool vfio_device_try_get_registration(struct vfio_device *device);
-int vfio_device_open(struct vfio_device *device,
- struct iommufd_ctx *iommufd, struct kvm *kvm);
+int vfio_device_open(struct vfio_device *device, struct vfio_group *group);
void vfio_device_close(struct vfio_device *device,
struct iommufd_ctx *iommufd);
@@ -74,6 +74,7 @@ struct vfio_group {
struct file *opened_file;
struct blocking_notifier_head notifier;
struct iommufd_ctx *iommufd;
+ spinlock_t kvm_ref_lock;
};
int vfio_device_set_group(struct vfio_device *device,
diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
index 5177bb061b17..b0defb0d0d87 100644
--- a/drivers/vfio/vfio_main.c
+++ b/drivers/vfio/vfio_main.c
@@ -16,6 +16,9 @@
#include <linux/fs.h>
#include <linux/idr.h>
#include <linux/iommu.h>
+#ifdef CONFIG_HAVE_KVM
+#include <linux/kvm_host.h>
+#endif
#include <linux/list.h>
#include <linux/miscdevice.h>
#include <linux/module.h>
@@ -344,9 +347,59 @@ static bool vfio_assert_device_open(struct vfio_device *device)
return !WARN_ON_ONCE(!READ_ONCE(device->open_count));
}
+#ifdef CONFIG_HAVE_KVM
+static bool vfio_kvm_get_kvm_safe(struct vfio_device *device, struct kvm *kvm)
+{
+ void (*pfn)(struct kvm *kvm);
+ bool (*fn)(struct kvm *kvm);
+ bool ret;
+
+ pfn = symbol_get(kvm_put_kvm);
+ if (WARN_ON(!pfn))
+ return false;
+
+ fn = symbol_get(kvm_get_kvm_safe);
+ if (WARN_ON(!fn)) {
+ symbol_put(kvm_put_kvm);
+ return false;
+ }
+
+ ret = fn(kvm);
+ if (ret)
+ device->put_kvm = pfn;
+ else
+ symbol_put(kvm_put_kvm);
+
+ symbol_put(kvm_get_kvm_safe);
+
+ return ret;
+}
+
+static void vfio_kvm_put_kvm(struct vfio_device *device)
+{
+ if (WARN_ON(!device->kvm || !device->put_kvm))
+ return;
+
+ device->put_kvm(device->kvm);
+ symbol_put(kvm_put_kvm);
+}
+
+#else
+static bool vfio_kvm_get_kvm_safe(struct vfio_device *device, struct kvm *kvm)
+{
+ return false;
+}
+
+static void vfio_kvm_put_kvm(struct vfio_device *device)
+{
+}
+#endif
+
static int vfio_device_first_open(struct vfio_device *device,
- struct iommufd_ctx *iommufd, struct kvm *kvm)
+ struct vfio_group *group)
{
+ struct iommufd_ctx *iommufd = group->iommufd;
+ unsigned long flags;
int ret;
lockdep_assert_held(&device->dev_set->lock);
@@ -361,16 +414,30 @@ static int vfio_device_first_open(struct vfio_device *device,
if (ret)
goto err_module_put;
- device->kvm = kvm;
+ /*
+ * Get the KVM pointer currently associated with the group, if there
+ * is one, and obtain a reference now that will be held until the
+ * open_count reaches 0. Save the pointer in the device for use by
+ * drivers.
+ */
+ spin_lock_irqsave(&group->kvm_ref_lock, flags);
+ if (group->kvm && vfio_kvm_get_kvm_safe(device, group->kvm))
+ device->kvm = group->kvm;
+ spin_unlock_irqrestore(&group->kvm_ref_lock, flags);
+
if (device->ops->open_device) {
ret = device->ops->open_device(device);
if (ret)
- goto err_unuse_iommu;
+ goto err_put_kvm;
}
return 0;
-err_unuse_iommu:
- device->kvm = NULL;
+err_put_kvm:
+ if (device->kvm) {
+ vfio_kvm_put_kvm(device);
+ device->put_kvm = NULL;
+ device->kvm = NULL;
+ }
if (iommufd)
vfio_iommufd_unbind(device);
else
@@ -387,7 +454,11 @@ static void vfio_device_last_close(struct vfio_device *device,
if (device->ops->close_device)
device->ops->close_device(device);
- device->kvm = NULL;
+ if (device->kvm) {
+ vfio_kvm_put_kvm(device);
+ device->kvm = NULL;
+ device->put_kvm = NULL;
+ }
if (iommufd)
vfio_iommufd_unbind(device);
else
@@ -395,15 +466,14 @@ static void vfio_device_last_close(struct vfio_device *device,
module_put(device->dev->driver->owner);
}
-int vfio_device_open(struct vfio_device *device,
- struct iommufd_ctx *iommufd, struct kvm *kvm)
+int vfio_device_open(struct vfio_device *device, struct vfio_group *group)
{
int ret = 0;
mutex_lock(&device->dev_set->lock);
device->open_count++;
if (device->open_count == 1) {
- ret = vfio_device_first_open(device, iommufd, kvm);
+ ret = vfio_device_first_open(device, group);
if (ret)
device->open_count--;
}
diff --git a/include/linux/vfio.h b/include/linux/vfio.h
index 35be78e9ae57..87ff862ff555 100644
--- a/include/linux/vfio.h
+++ b/include/linux/vfio.h
@@ -46,7 +46,6 @@ struct vfio_device {
struct vfio_device_set *dev_set;
struct list_head dev_set_list;
unsigned int migration_flags;
- /* Driver must reference the kvm during open_device or never touch it */
struct kvm *kvm;
/* Members below here are private, not for driver use */
@@ -58,6 +57,7 @@ struct vfio_device {
struct list_head group_next;
struct list_head iommu_entry;
struct iommufd_access *iommufd_access;
+ void (*put_kvm)(struct kvm *kvm);
#if IS_ENABLED(CONFIG_IOMMUFD)
struct iommufd_device *iommufd_device;
struct iommufd_ctx *iommufd_ictx;
--
2.39.1
On Tue, Jan 31, 2023 at 03:06:35PM -0500, Matthew Rosato wrote:
> @@ -799,13 +794,14 @@ EXPORT_SYMBOL_GPL(vfio_file_enforced_coherent);
> void vfio_file_set_kvm(struct file *file, struct kvm *kvm)
> {
> struct vfio_group *group = file->private_data;
> + unsigned long flags;
>
> if (!vfio_file_is_group(file))
> return;
>
> - mutex_lock(&group->group_lock);
> + spin_lock_irqsave(&group->kvm_ref_lock, flags);
> group->kvm = kvm;
> - mutex_unlock(&group->group_lock);
> + spin_unlock_irqrestore(&group->kvm_ref_lock, flags);
We know we are in a sleeping context here so these are just
'spin_lock()', same with the other one
Otherwise it seems Ok to me
Reviewed-by: Jason Gunthorpe <[email protected]>
Jason
> From: Matthew Rosato <[email protected]>
> Sent: Wednesday, February 1, 2023 4:07 AM
>
> - device->kvm = kvm;
> + /*
> + * Get the KVM pointer currently associated with the group, if there
> + * is one, and obtain a reference now that will be held until the
> + * open_count reaches 0. Save the pointer in the device for use by
> + * drivers.
> + */
> + spin_lock_irqsave(&group->kvm_ref_lock, flags);
> + if (group->kvm && vfio_kvm_get_kvm_safe(device, group->kvm))
> + device->kvm = group->kvm;
> + spin_unlock_irqrestore(&group->kvm_ref_lock, flags);
> +
No group reference in vfio_main.c.
btw Yi, looks the deadlock issue also exists in your cdev work.
kvm_vfio_release() holds kvm lock and then try to acquire
device->device_set->lock in vfio_device_file_set_kvm().
vfio_device_ioctl_bind_iommufd() holds device->device_set->lock
and then call vfio_device_open() which finally hit kvm lock
acquisition in driver's open_device routine (e.g. vfio-ap).
A similar fix is required in your series.
Thanks
Kevin
> From: Matthew Rosato <[email protected]>
> Sent: Wednesday, February 1, 2023 4:07 AM
>
> After 51cdc8bc120e, we have another deadlock scenario between the
> kvm->lock and the vfio group_lock with two different codepaths acquiring
> the locks in different order. Specifically in vfio_open_device, vfio
> holds the vfio group_lock when issuing device->ops->open_device but
> some
> drivers (like vfio-ap) need to acquire kvm->lock during their open_device
> routine; Meanwhile, kvm_vfio_release will acquire the kvm->lock first
> before calling vfio_file_set_kvm which will acquire the vfio group_lock.
>
> To resolve this, let's remove the need for the vfio group_lock from the
> kvm_vfio_release codepath. This is done by introducing a new spinlock to
> protect modifications to the vfio group kvm pointer, and acquiring a kvm
> ref from within vfio while holding this spinlock, with the reference held
> until the last close for the device in question.
>
> Fixes: 51cdc8bc120e ("kvm/vfio: Fix potential deadlock on vfio group_lock")
> Reported-by: Anthony Krowiak <[email protected]>
> Suggested-by: Jason Gunthorpe <[email protected]>
> Signed-off-by: Matthew Rosato <[email protected]>
> ---
> drivers/vfio/group.c | 14 +++----
> drivers/vfio/vfio.h | 5 ++-
> drivers/vfio/vfio_main.c | 88
> ++++++++++++++++++++++++++++++++++++----
> include/linux/vfio.h | 2 +-
> 4 files changed, 88 insertions(+), 21 deletions(-)
>
> diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
> index bb24b2f0271e..8f67d33e0e8d 100644
> --- a/drivers/vfio/group.c
> +++ b/drivers/vfio/group.c
> @@ -164,13 +164,7 @@ static int vfio_device_group_open(struct
> vfio_device *device)
> goto out_unlock;
> }
>
> - /*
> - * Here we pass the KVM pointer with the group under the lock. If
> the
> - * device driver will use it, it must obtain a reference and release it
> - * during close_device.
> - */
> - ret = vfio_device_open(device, device->group->iommufd,
> - device->group->kvm);
> + ret = vfio_device_open(device, device->group);
>
> out_unlock:
> mutex_unlock(&device->group->group_lock);
> @@ -450,6 +444,7 @@ static struct vfio_group *vfio_group_alloc(struct
> iommu_group *iommu_group,
>
> refcount_set(&group->drivers, 1);
> mutex_init(&group->group_lock);
> + spin_lock_init(&group->kvm_ref_lock);
> INIT_LIST_HEAD(&group->device_list);
> mutex_init(&group->device_lock);
> group->iommu_group = iommu_group;
> @@ -799,13 +794,14 @@
> EXPORT_SYMBOL_GPL(vfio_file_enforced_coherent);
> void vfio_file_set_kvm(struct file *file, struct kvm *kvm)
> {
> struct vfio_group *group = file->private_data;
> + unsigned long flags;
>
> if (!vfio_file_is_group(file))
> return;
>
> - mutex_lock(&group->group_lock);
> + spin_lock_irqsave(&group->kvm_ref_lock, flags);
> group->kvm = kvm;
> - mutex_unlock(&group->group_lock);
> + spin_unlock_irqrestore(&group->kvm_ref_lock, flags);
> }
> EXPORT_SYMBOL_GPL(vfio_file_set_kvm);
>
> diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
> index f8219a438bfb..d153ad35de24 100644
> --- a/drivers/vfio/vfio.h
> +++ b/drivers/vfio/vfio.h
> @@ -15,11 +15,11 @@ struct iommufd_ctx;
> struct iommu_group;
> struct vfio_device;
> struct vfio_container;
> +struct vfio_group;
>
> void vfio_device_put_registration(struct vfio_device *device);
> bool vfio_device_try_get_registration(struct vfio_device *device);
> -int vfio_device_open(struct vfio_device *device,
> - struct iommufd_ctx *iommufd, struct kvm *kvm);
> +int vfio_device_open(struct vfio_device *device, struct vfio_group
> *group);
> void vfio_device_close(struct vfio_device *device,
> struct iommufd_ctx *iommufd);
>
> @@ -74,6 +74,7 @@ struct vfio_group {
> struct file *opened_file;
> struct blocking_notifier_head notifier;
> struct iommufd_ctx *iommufd;
> + spinlock_t kvm_ref_lock;
> };
>
> int vfio_device_set_group(struct vfio_device *device,
> diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> index 5177bb061b17..b0defb0d0d87 100644
> --- a/drivers/vfio/vfio_main.c
> +++ b/drivers/vfio/vfio_main.c
> @@ -16,6 +16,9 @@
> #include <linux/fs.h>
> #include <linux/idr.h>
> #include <linux/iommu.h>
> +#ifdef CONFIG_HAVE_KVM
> +#include <linux/kvm_host.h>
> +#endif
> #include <linux/list.h>
> #include <linux/miscdevice.h>
> #include <linux/module.h>
> @@ -344,9 +347,59 @@ static bool vfio_assert_device_open(struct
> vfio_device *device)
> return !WARN_ON_ONCE(!READ_ONCE(device->open_count));
> }
>
> +#ifdef CONFIG_HAVE_KVM
> +static bool vfio_kvm_get_kvm_safe(struct vfio_device *device, struct kvm
> *kvm)
> +{
> + void (*pfn)(struct kvm *kvm);
> + bool (*fn)(struct kvm *kvm);
> + bool ret;
> +
> + pfn = symbol_get(kvm_put_kvm);
> + if (WARN_ON(!pfn))
> + return false;
> +
> + fn = symbol_get(kvm_get_kvm_safe);
> + if (WARN_ON(!fn)) {
> + symbol_put(kvm_put_kvm);
> + return false;
> + }
> +
> + ret = fn(kvm);
> + if (ret)
> + device->put_kvm = pfn;
> + else
> + symbol_put(kvm_put_kvm);
> +
> + symbol_put(kvm_get_kvm_safe);
> +
> + return ret;
> +}
> +
> +static void vfio_kvm_put_kvm(struct vfio_device *device)
> +{
> + if (WARN_ON(!device->kvm || !device->put_kvm))
> + return;
> +
> + device->put_kvm(device->kvm);
> + symbol_put(kvm_put_kvm);
> +}
> +
> +#else
> +static bool vfio_kvm_get_kvm_safe(struct vfio_device *device, struct kvm
> *kvm)
> +{
> + return false;
> +}
> +
> +static void vfio_kvm_put_kvm(struct vfio_device *device)
> +{
> +}
> +#endif
> +
> static int vfio_device_first_open(struct vfio_device *device,
> - struct iommufd_ctx *iommufd, struct kvm
> *kvm)
> + struct vfio_group *group)
> {
> + struct iommufd_ctx *iommufd = group->iommufd;
> + unsigned long flags;
> int ret;
>
> lockdep_assert_held(&device->dev_set->lock);
> @@ -361,16 +414,30 @@ static int vfio_device_first_open(struct
> vfio_device *device,
> if (ret)
> goto err_module_put;
>
> - device->kvm = kvm;
> + /*
> + * Get the KVM pointer currently associated with the group, if there
> + * is one, and obtain a reference now that will be held until the
> + * open_count reaches 0. Save the pointer in the device for use by
> + * drivers.
> + */
> + spin_lock_irqsave(&group->kvm_ref_lock, flags);
> + if (group->kvm && vfio_kvm_get_kvm_safe(device, group->kvm))
> + device->kvm = group->kvm;
> + spin_unlock_irqrestore(&group->kvm_ref_lock, flags);
> +
> if (device->ops->open_device) {
> ret = device->ops->open_device(device);
> if (ret)
> - goto err_unuse_iommu;
> + goto err_put_kvm;
> }
> return 0;
>
> -err_unuse_iommu:
> - device->kvm = NULL;
> +err_put_kvm:
> + if (device->kvm) {
> + vfio_kvm_put_kvm(device);
> + device->put_kvm = NULL;
Can "device->put_kvm = NULL;" be part of vfio_kvm_put_kvm()?
Regards,
Yi Liu
> From: Jason Gunthorpe <[email protected]>
> Sent: Wednesday, February 1, 2023 4:26 AM
>
> On Tue, Jan 31, 2023 at 03:06:35PM -0500, Matthew Rosato wrote:
> > @@ -799,13 +794,14 @@
> EXPORT_SYMBOL_GPL(vfio_file_enforced_coherent);
> > void vfio_file_set_kvm(struct file *file, struct kvm *kvm)
> > {
> > struct vfio_group *group = file->private_data;
> > + unsigned long flags;
> >
> > if (!vfio_file_is_group(file))
> > return;
> >
> > - mutex_lock(&group->group_lock);
> > + spin_lock_irqsave(&group->kvm_ref_lock, flags);
> > group->kvm = kvm;
> > - mutex_unlock(&group->group_lock);
> > + spin_unlock_irqrestore(&group->kvm_ref_lock, flags);
>
> We know we are in a sleeping context here so these are just
> 'spin_lock()', same with the other one
a dumb question. Why spinlock is required here? ????
Regards,
Yi Liu
On 2/1/23 1:13 AM, Tian, Kevin wrote:
>> From: Matthew Rosato <[email protected]>
>> Sent: Wednesday, February 1, 2023 4:07 AM
>>
>> - device->kvm = kvm;
>> + /*
>> + * Get the KVM pointer currently associated with the group, if there
>> + * is one, and obtain a reference now that will be held until the
>> + * open_count reaches 0. Save the pointer in the device for use by
>> + * drivers.
>> + */
>> + spin_lock_irqsave(&group->kvm_ref_lock, flags);
>> + if (group->kvm && vfio_kvm_get_kvm_safe(device, group->kvm))
>> + device->kvm = group->kvm;
>> + spin_unlock_irqrestore(&group->kvm_ref_lock, flags);
>> +
>
> No group reference in vfio_main.c.
OK -- I think to do that I'll have to move the lock/unlock of the dev_set lock to group.c right before we call vfio_device_{open,close} and check open_count there to make decisions about the kvm ref (before calling vfio_device_open to decide to get kvm ref, after returning from vfio_device_open to see if we to drop ref on error, after close to see if we need to drop ref).
>
> btw Yi, looks the deadlock issue also exists in your cdev work.
>
> kvm_vfio_release() holds kvm lock and then try to acquire
> device->device_set->lock in vfio_device_file_set_kvm().
>
> vfio_device_ioctl_bind_iommufd() holds device->device_set->lock
> and then call vfio_device_open() which finally hit kvm lock
> acquisition in driver's open_device routine (e.g. vfio-ap).
>
> A similar fix is required in your series.
>
> Thanks
> Kevin
On 2/1/23 7:42 AM, Liu, Yi L wrote:
>> From: Matthew Rosato <[email protected]>
...
>> + if (device->kvm) {
>> + vfio_kvm_put_kvm(device);
>> + device->put_kvm = NULL;
>
> Can "device->put_kvm = NULL;" be part of vfio_kvm_put_kvm()?
Sure
On 2/1/23 7:43 AM, Liu, Yi L wrote:
>> From: Jason Gunthorpe <[email protected]>
>> Sent: Wednesday, February 1, 2023 4:26 AM
>>
>> On Tue, Jan 31, 2023 at 03:06:35PM -0500, Matthew Rosato wrote:
>>> @@ -799,13 +794,14 @@
>> EXPORT_SYMBOL_GPL(vfio_file_enforced_coherent);
>>> void vfio_file_set_kvm(struct file *file, struct kvm *kvm)
>>> {
>>> struct vfio_group *group = file->private_data;
>>> + unsigned long flags;
>>>
>>> if (!vfio_file_is_group(file))
>>> return;
>>>
>>> - mutex_lock(&group->group_lock);
>>> + spin_lock_irqsave(&group->kvm_ref_lock, flags);
>>> group->kvm = kvm;
>>> - mutex_unlock(&group->group_lock);
>>> + spin_unlock_irqrestore(&group->kvm_ref_lock, flags);
>>
>> We know we are in a sleeping context here so these are just
>> 'spin_lock()', same with the other one
>
> a dumb question. Why spinlock is required here? ????
>
You mean as opposed to another mutex? I don't think it's required per se (we are replacing a mutex so we could have again used another mutex here), but all current users of this new lock hold it over a very short window (e.g. set a pointer as above, or refcount++ and copy the pointer as in the first device_open)
> From: Matthew Rosato <[email protected]>
> Sent: Wednesday, February 1, 2023 10:43 PM
>
> On 2/1/23 7:43 AM, Liu, Yi L wrote:
> >> From: Jason Gunthorpe <[email protected]>
> >> Sent: Wednesday, February 1, 2023 4:26 AM
> >>
> >> On Tue, Jan 31, 2023 at 03:06:35PM -0500, Matthew Rosato wrote:
> >>> @@ -799,13 +794,14 @@
> >> EXPORT_SYMBOL_GPL(vfio_file_enforced_coherent);
> >>> void vfio_file_set_kvm(struct file *file, struct kvm *kvm)
> >>> {
> >>> struct vfio_group *group = file->private_data;
> >>> + unsigned long flags;
> >>>
> >>> if (!vfio_file_is_group(file))
> >>> return;
> >>>
> >>> - mutex_lock(&group->group_lock);
> >>> + spin_lock_irqsave(&group->kvm_ref_lock, flags);
> >>> group->kvm = kvm;
> >>> - mutex_unlock(&group->group_lock);
> >>> + spin_unlock_irqrestore(&group->kvm_ref_lock, flags);
> >>
> >> We know we are in a sleeping context here so these are just
> >> 'spin_lock()', same with the other one
> >
> > a dumb question. Why spinlock is required here? ????
> >
>
> You mean as opposed to another mutex? I don't think it's required per se
> (we are replacing a mutex so we could have again used another mutex
> here), but all current users of this new lock hold it over a very short window
> (e.g. set a pointer as above, or refcount++ and copy the pointer as in the
> first device_open)
I see. Just not sure if spinlock is required for a special reason.
Regards,
Yi Liu