Currently it is possible that the final put of a KVM reference comes from
vfio during its device close operation. This occurs while the vfio group
lock is held; however, if the vfio device is still in the kvm device list,
then the following call chain could result in a deadlock:
kvm_put_kvm
-> kvm_destroy_vm
-> kvm_destroy_devices
-> kvm_vfio_destroy
-> kvm_vfio_file_set_kvm
-> vfio_file_set_kvm
-> group->group_lock/group_rwsem
Avoid this scenario by having vfio core code acquire a KVM reference
the first time a device is opened and hold that reference until right
after the group lock is released after the last device is closed.
Fixes: 421cfe6596f6 ("vfio: remove VFIO_GROUP_NOTIFY_SET_KVM")
Reported-by: Alex Williamson <[email protected]>
Signed-off-by: Matthew Rosato <[email protected]>
---
Changes from v3:
* Can't check for open_count after the group lock has been dropped because
it would be possible for the count to change again once the group lock
is dropped (Jason)
Solve this by stashing a copy of the kvm and put_kvm while the group
lock is held, nullifying the device copies of these in device_close()
as soon as the open_count reaches 0, and then checking to see if the
device->kvm changed before dropping the group lock. If it changed
during close, we can drop the reference using the stashed kvm and put
function after dropping the group lock.
Changes from v2:
* Re-arrange vfio_kvm_set_kvm_safe error path to still trigger
device_open with device->kvm=NULL (Alex)
* get device->dev_set->lock when checking device->open_count (Alex)
* but don't hold it over the kvm_put_kvm (Jason)
* get kvm_put symbol upfront and stash it in device until close (Jason)
* check CONFIG_HAVE_KVM to avoid build errors on architectures without
KVM support
Changes from v1:
* Re-write using symbol get logic to get kvm ref during first device
open, release the ref during device fd close after group lock is
released
* Drop kvm get/put changes to drivers; now that vfio core holds a
kvm ref until sometime after the device_close op is called, it
should be fine for drivers to get and put their own references to it.
---
drivers/vfio/group.c | 23 +++++++++++++--
drivers/vfio/vfio.h | 9 ++++++
drivers/vfio/vfio_main.c | 61 +++++++++++++++++++++++++++++++++++++---
include/linux/vfio.h | 2 +-
4 files changed, 87 insertions(+), 8 deletions(-)
diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
index bb24b2f0271e..b396c17d7390 100644
--- a/drivers/vfio/group.c
+++ b/drivers/vfio/group.c
@@ -165,9 +165,9 @@ static int vfio_device_group_open(struct vfio_device *device)
}
/*
- * 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.
+ * Here we pass the KVM pointer with the group under the lock. A
+ * reference will be obtained the first time the device is opened and
+ * will be held until the open_count reaches 0.
*/
ret = vfio_device_open(device, device->group->iommufd,
device->group->kvm);
@@ -179,9 +179,26 @@ static int vfio_device_group_open(struct vfio_device *device)
void vfio_device_group_close(struct vfio_device *device)
{
+ void (*put_kvm)(struct kvm *kvm);
+ struct kvm *kvm;
+
mutex_lock(&device->group->group_lock);
+ kvm = device->kvm;
+ put_kvm = device->put_kvm;
vfio_device_close(device, device->group->iommufd);
+ if (kvm == device->kvm)
+ kvm = NULL;
mutex_unlock(&device->group->group_lock);
+
+ /*
+ * The last kvm reference will trigger kvm_destroy_vm, which can in
+ * turn re-enter vfio and attempt to acquire the group lock. Therefore
+ * we get a copy of the kvm pointer and the put function under the
+ * group lock but wait to put that reference until after releasing the
+ * lock.
+ */
+ if (kvm)
+ vfio_kvm_put_kvm(put_kvm, kvm);
}
static struct file *vfio_device_open_file(struct vfio_device *device)
diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
index f8219a438bfb..08a5a23d6fef 100644
--- a/drivers/vfio/vfio.h
+++ b/drivers/vfio/vfio.h
@@ -251,4 +251,13 @@ extern bool vfio_noiommu __read_mostly;
enum { vfio_noiommu = false };
#endif
+#ifdef CONFIG_HAVE_KVM
+void vfio_kvm_put_kvm(void (*put)(struct kvm *kvm), struct kvm *kvm);
+#else
+static inline void vfio_kvm_put_kvm(void (*put)(struct kvm *kvm),
+ struct kvm *kvm)
+{
+}
+#endif
+
#endif
diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
index 5177bb061b17..c6bb07af46b8 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>
@@ -344,6 +347,49 @@ 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;
+}
+
+void vfio_kvm_put_kvm(void (*put)(struct kvm *kvm), struct kvm *kvm)
+{
+ if (WARN_ON(!put))
+ return;
+
+ put(kvm);
+ symbol_put(kvm_put_kvm);
+}
+#else
+static bool vfio_kvm_get_kvm_safe(struct vfio_device *device, struct kvm *kvm)
+{
+ return false;
+}
+#endif
+
static int vfio_device_first_open(struct vfio_device *device,
struct iommufd_ctx *iommufd, struct kvm *kvm)
{
@@ -361,16 +407,22 @@ static int vfio_device_first_open(struct vfio_device *device,
if (ret)
goto err_module_put;
- device->kvm = kvm;
+ if (kvm && vfio_kvm_get_kvm_safe(device, kvm))
+ device->kvm = kvm;
+
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->put_kvm, device->kvm);
+ device->put_kvm = NULL;
+ device->kvm = NULL;
+ }
if (iommufd)
vfio_iommufd_unbind(device);
else
@@ -388,6 +440,7 @@ static void vfio_device_last_close(struct vfio_device *device,
if (device->ops->close_device)
device->ops->close_device(device);
device->kvm = NULL;
+ device->put_kvm = NULL;
if (iommufd)
vfio_iommufd_unbind(device);
else
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;
--
2.39.0
On Fri, Jan 13, 2023 at 07:03:51PM -0500, Matthew Rosato wrote:
> Currently it is possible that the final put of a KVM reference comes from
> vfio during its device close operation. This occurs while the vfio group
> lock is held; however, if the vfio device is still in the kvm device list,
> then the following call chain could result in a deadlock:
>
> kvm_put_kvm
> -> kvm_destroy_vm
> -> kvm_destroy_devices
> -> kvm_vfio_destroy
> -> kvm_vfio_file_set_kvm
> -> vfio_file_set_kvm
> -> group->group_lock/group_rwsem
>
> Avoid this scenario by having vfio core code acquire a KVM reference
> the first time a device is opened and hold that reference until right
> after the group lock is released after the last device is closed.
>
> Fixes: 421cfe6596f6 ("vfio: remove VFIO_GROUP_NOTIFY_SET_KVM")
> Reported-by: Alex Williamson <[email protected]>
> Signed-off-by: Matthew Rosato <[email protected]>
> ---
Reviewed-by: Jason Gunthorpe <[email protected]>
Jason
> From: Matthew Rosato <[email protected]>
> Sent: Saturday, January 14, 2023 8:04 AM
>
> void vfio_device_group_close(struct vfio_device *device)
> {
> + void (*put_kvm)(struct kvm *kvm);
> + struct kvm *kvm;
> +
> mutex_lock(&device->group->group_lock);
> + kvm = device->kvm;
> + put_kvm = device->put_kvm;
> vfio_device_close(device, device->group->iommufd);
> + if (kvm == device->kvm)
> + kvm = NULL;
Add a simple comment that this check is to detect the last close
> +void vfio_kvm_put_kvm(void (*put)(struct kvm *kvm), struct kvm *kvm)
> +{
> + if (WARN_ON(!put))
> + return;
also WARN_ON(!kvm)?
otherwise this looks good to me:
Reviewed-by: Kevin Tian <[email protected]>
On Fri, 13 Jan 2023 19:03:51 -0500
Matthew Rosato <[email protected]> wrote:
> Currently it is possible that the final put of a KVM reference comes from
> vfio during its device close operation. This occurs while the vfio group
> lock is held; however, if the vfio device is still in the kvm device list,
> then the following call chain could result in a deadlock:
>
> kvm_put_kvm
> -> kvm_destroy_vm
> -> kvm_destroy_devices
> -> kvm_vfio_destroy
> -> kvm_vfio_file_set_kvm
> -> vfio_file_set_kvm
> -> group->group_lock/group_rwsem
>
> Avoid this scenario by having vfio core code acquire a KVM reference
> the first time a device is opened and hold that reference until right
> after the group lock is released after the last device is closed.
>
> Fixes: 421cfe6596f6 ("vfio: remove VFIO_GROUP_NOTIFY_SET_KVM")
> Reported-by: Alex Williamson <[email protected]>
> Signed-off-by: Matthew Rosato <[email protected]>
> ---
> Changes from v3:
> * Can't check for open_count after the group lock has been dropped because
> it would be possible for the count to change again once the group lock
> is dropped (Jason)
> Solve this by stashing a copy of the kvm and put_kvm while the group
> lock is held, nullifying the device copies of these in device_close()
> as soon as the open_count reaches 0, and then checking to see if the
> device->kvm changed before dropping the group lock. If it changed
> during close, we can drop the reference using the stashed kvm and put
> function after dropping the group lock.
>
> Changes from v2:
> * Re-arrange vfio_kvm_set_kvm_safe error path to still trigger
> device_open with device->kvm=NULL (Alex)
> * get device->dev_set->lock when checking device->open_count (Alex)
> * but don't hold it over the kvm_put_kvm (Jason)
> * get kvm_put symbol upfront and stash it in device until close (Jason)
> * check CONFIG_HAVE_KVM to avoid build errors on architectures without
> KVM support
>
> Changes from v1:
> * Re-write using symbol get logic to get kvm ref during first device
> open, release the ref during device fd close after group lock is
> released
> * Drop kvm get/put changes to drivers; now that vfio core holds a
> kvm ref until sometime after the device_close op is called, it
> should be fine for drivers to get and put their own references to it.
> ---
> drivers/vfio/group.c | 23 +++++++++++++--
> drivers/vfio/vfio.h | 9 ++++++
> drivers/vfio/vfio_main.c | 61 +++++++++++++++++++++++++++++++++++++---
> include/linux/vfio.h | 2 +-
> 4 files changed, 87 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
> index bb24b2f0271e..b396c17d7390 100644
> --- a/drivers/vfio/group.c
> +++ b/drivers/vfio/group.c
> @@ -165,9 +165,9 @@ static int vfio_device_group_open(struct vfio_device *device)
> }
>
> /*
> - * 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.
> + * Here we pass the KVM pointer with the group under the lock. A
> + * reference will be obtained the first time the device is opened and
> + * will be held until the open_count reaches 0.
> */
> ret = vfio_device_open(device, device->group->iommufd,
> device->group->kvm);
> @@ -179,9 +179,26 @@ static int vfio_device_group_open(struct vfio_device *device)
>
> void vfio_device_group_close(struct vfio_device *device)
> {
> + void (*put_kvm)(struct kvm *kvm);
> + struct kvm *kvm;
> +
> mutex_lock(&device->group->group_lock);
> + kvm = device->kvm;
> + put_kvm = device->put_kvm;
> vfio_device_close(device, device->group->iommufd);
> + if (kvm == device->kvm)
> + kvm = NULL;
Hmm, so we're using whether the device->kvm pointer gets cleared in
last_close to detect whether we should put the kvm reference. That's a
bit obscure. Our get and put is also asymmetric.
Did we decide that we couldn't do this via a schedule_work() from the
last_close function, ie. implementing our own version of an async put?
It seems like that potentially has a cleaner implementation, symmetric
call points, handling all the storing and clearing of kvm related
pointers within the get/put wrappers, passing only a vfio_device to the
put wrapper, using the "vfio_device_" prefix for both. Potentially
we'd just want an unconditional flush outside of lock here for
deterministic release.
What's the downside? Thanks,
Alex
> mutex_unlock(&device->group->group_lock);
> +
> + /*
> + * The last kvm reference will trigger kvm_destroy_vm, which
> can in
> + * turn re-enter vfio and attempt to acquire the group lock.
> Therefore
> + * we get a copy of the kvm pointer and the put function
> under the
> + * group lock but wait to put that reference until after
> releasing the
> + * lock.
> + */
> + if (kvm)
> + vfio_kvm_put_kvm(put_kvm, kvm);
> }
>
> static struct file *vfio_device_open_file(struct vfio_device *device)
> diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
> index f8219a438bfb..08a5a23d6fef 100644
> --- a/drivers/vfio/vfio.h
> +++ b/drivers/vfio/vfio.h
> @@ -251,4 +251,13 @@ extern bool vfio_noiommu __read_mostly;
> enum { vfio_noiommu = false };
> #endif
>
> +#ifdef CONFIG_HAVE_KVM
> +void vfio_kvm_put_kvm(void (*put)(struct kvm *kvm), struct kvm *kvm);
> +#else
> +static inline void vfio_kvm_put_kvm(void (*put)(struct kvm *kvm),
> + struct kvm *kvm)
> +{
> +}
> +#endif
> +
> #endif
> diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> index 5177bb061b17..c6bb07af46b8 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>
> @@ -344,6 +347,49 @@ 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;
> +}
> +
> +void vfio_kvm_put_kvm(void (*put)(struct kvm *kvm), struct kvm *kvm)
> +{
> + if (WARN_ON(!put))
> + return;
> +
> + put(kvm);
> + symbol_put(kvm_put_kvm);
> +}
> +#else
> +static bool vfio_kvm_get_kvm_safe(struct vfio_device *device, struct
> kvm *kvm) +{
> + return false;
> +}
> +#endif
> +
> static int vfio_device_first_open(struct vfio_device *device,
> struct iommufd_ctx *iommufd,
> struct kvm *kvm) {
> @@ -361,16 +407,22 @@ static int vfio_device_first_open(struct
> vfio_device *device, if (ret)
> goto err_module_put;
>
> - device->kvm = kvm;
> + if (kvm && vfio_kvm_get_kvm_safe(device, kvm))
> + device->kvm = kvm;
> +
> 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->put_kvm, device->kvm);
> + device->put_kvm = NULL;
> + device->kvm = NULL;
> + }
> if (iommufd)
> vfio_iommufd_unbind(device);
> else
> @@ -388,6 +440,7 @@ static void vfio_device_last_close(struct
> vfio_device *device, if (device->ops->close_device)
> device->ops->close_device(device);
> device->kvm = NULL;
> + device->put_kvm = NULL;
> if (iommufd)
> vfio_iommufd_unbind(device);
> else
> 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;
> From: Alex Williamson
> Sent: Wednesday, January 18, 2023 5:23 AM
>
> On Fri, 13 Jan 2023 19:03:51 -0500
> Matthew Rosato <[email protected]> wrote:
>
> > void vfio_device_group_close(struct vfio_device *device)
> > {
> > + void (*put_kvm)(struct kvm *kvm);
> > + struct kvm *kvm;
> > +
> > mutex_lock(&device->group->group_lock);
> > + kvm = device->kvm;
> > + put_kvm = device->put_kvm;
> > vfio_device_close(device, device->group->iommufd);
> > + if (kvm == device->kvm)
> > + kvm = NULL;
>
> Hmm, so we're using whether the device->kvm pointer gets cleared in
> last_close to detect whether we should put the kvm reference. That's a
> bit obscure. Our get and put is also asymmetric.
>
> Did we decide that we couldn't do this via a schedule_work() from the
> last_close function, ie. implementing our own version of an async put?
> It seems like that potentially has a cleaner implementation, symmetric
> call points, handling all the storing and clearing of kvm related
> pointers within the get/put wrappers, passing only a vfio_device to the
> put wrapper, using the "vfio_device_" prefix for both. Potentially
> we'd just want an unconditional flush outside of lock here for
> deterministic release.
>
> What's the downside? Thanks,
>
btw I guess this can be also fixed by Yi's work here:
https://lore.kernel.org/kvm/[email protected]/
with set_kvm(NULL) moved to the release callback of kvm_vfio device,
such circular lock dependency can be avoided too.
On 1/17/23 4:22 PM, Alex Williamson wrote:
> On Fri, 13 Jan 2023 19:03:51 -0500
> Matthew Rosato <[email protected]> wrote:
>
>> Currently it is possible that the final put of a KVM reference comes from
>> vfio during its device close operation. This occurs while the vfio group
>> lock is held; however, if the vfio device is still in the kvm device list,
>> then the following call chain could result in a deadlock:
>>
>> kvm_put_kvm
>> -> kvm_destroy_vm
>> -> kvm_destroy_devices
>> -> kvm_vfio_destroy
>> -> kvm_vfio_file_set_kvm
>> -> vfio_file_set_kvm
>> -> group->group_lock/group_rwsem
>>
>> Avoid this scenario by having vfio core code acquire a KVM reference
>> the first time a device is opened and hold that reference until right
>> after the group lock is released after the last device is closed.
>>
>> Fixes: 421cfe6596f6 ("vfio: remove VFIO_GROUP_NOTIFY_SET_KVM")
>> Reported-by: Alex Williamson <[email protected]>
>> Signed-off-by: Matthew Rosato <[email protected]>
>> ---
>> Changes from v3:
>> * Can't check for open_count after the group lock has been dropped because
>> it would be possible for the count to change again once the group lock
>> is dropped (Jason)
>> Solve this by stashing a copy of the kvm and put_kvm while the group
>> lock is held, nullifying the device copies of these in device_close()
>> as soon as the open_count reaches 0, and then checking to see if the
>> device->kvm changed before dropping the group lock. If it changed
>> during close, we can drop the reference using the stashed kvm and put
>> function after dropping the group lock.
>>
>> Changes from v2:
>> * Re-arrange vfio_kvm_set_kvm_safe error path to still trigger
>> device_open with device->kvm=NULL (Alex)
>> * get device->dev_set->lock when checking device->open_count (Alex)
>> * but don't hold it over the kvm_put_kvm (Jason)
>> * get kvm_put symbol upfront and stash it in device until close (Jason)
>> * check CONFIG_HAVE_KVM to avoid build errors on architectures without
>> KVM support
>>
>> Changes from v1:
>> * Re-write using symbol get logic to get kvm ref during first device
>> open, release the ref during device fd close after group lock is
>> released
>> * Drop kvm get/put changes to drivers; now that vfio core holds a
>> kvm ref until sometime after the device_close op is called, it
>> should be fine for drivers to get and put their own references to it.
>> ---
>> drivers/vfio/group.c | 23 +++++++++++++--
>> drivers/vfio/vfio.h | 9 ++++++
>> drivers/vfio/vfio_main.c | 61 +++++++++++++++++++++++++++++++++++++---
>> include/linux/vfio.h | 2 +-
>> 4 files changed, 87 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
>> index bb24b2f0271e..b396c17d7390 100644
>> --- a/drivers/vfio/group.c
>> +++ b/drivers/vfio/group.c
>> @@ -165,9 +165,9 @@ static int vfio_device_group_open(struct vfio_device *device)
>> }
>>
>> /*
>> - * 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.
>> + * Here we pass the KVM pointer with the group under the lock. A
>> + * reference will be obtained the first time the device is opened and
>> + * will be held until the open_count reaches 0.
>> */
>> ret = vfio_device_open(device, device->group->iommufd,
>> device->group->kvm);
>> @@ -179,9 +179,26 @@ static int vfio_device_group_open(struct vfio_device *device)
>>
>> void vfio_device_group_close(struct vfio_device *device)
>> {
>> + void (*put_kvm)(struct kvm *kvm);
>> + struct kvm *kvm;
>> +
>> mutex_lock(&device->group->group_lock);
>> + kvm = device->kvm;
>> + put_kvm = device->put_kvm;
>> vfio_device_close(device, device->group->iommufd);
>> + if (kvm == device->kvm)
>> + kvm = NULL;
>
> Hmm, so we're using whether the device->kvm pointer gets cleared in
> last_close to detect whether we should put the kvm reference. That's a
> bit obscure. Our get and put is also asymmetric.
>
> Did we decide that we couldn't do this via a schedule_work() from the
> last_close function, ie. implementing our own version of an async put?
> It seems like that potentially has a cleaner implementation, symmetric
> call points, handling all the storing and clearing of kvm related
> pointers within the get/put wrappers, passing only a vfio_device to the
> put wrapper, using the "vfio_device_" prefix for both. Potentially
> we'd just want an unconditional flush outside of lock here for
> deterministic release.
>
> What's the downside? Thanks,
>
I did mention something like this as a possibility when discussing v3.. It's doable, the main issue with doing schedule_work() of an async put is that we can't actually use the device->kvm / device->put_kvm values during the scheduled work task as they become unreliable once we drop the group lock -- e.g. schedule_work() some put call while under the group lock, drop the group lock and then another thread gets the group lock and does a new open_device() before that async put task fires; device->kvm and (less likely) put_kvm might have changed in between.
I think in that case we would need to stash the kvm and put_kvm values in some secondary structure to be processed off a queue by the schedule_work task (an example of what I mean would be bio_dirty_list in block/bio.c). Very unlikely that this queue would ever have more than 1 element in it.
On 1/18/23 4:03 AM, Tian, Kevin wrote:
>> From: Alex Williamson
>> Sent: Wednesday, January 18, 2023 5:23 AM
>>
>> On Fri, 13 Jan 2023 19:03:51 -0500
>> Matthew Rosato <[email protected]> wrote:
>>
>>> void vfio_device_group_close(struct vfio_device *device)
>>> {
>>> + void (*put_kvm)(struct kvm *kvm);
>>> + struct kvm *kvm;
>>> +
>>> mutex_lock(&device->group->group_lock);
>>> + kvm = device->kvm;
>>> + put_kvm = device->put_kvm;
>>> vfio_device_close(device, device->group->iommufd);
>>> + if (kvm == device->kvm)
>>> + kvm = NULL;
>>
>> Hmm, so we're using whether the device->kvm pointer gets cleared in
>> last_close to detect whether we should put the kvm reference. That's a
>> bit obscure. Our get and put is also asymmetric.
>>
>> Did we decide that we couldn't do this via a schedule_work() from the
>> last_close function, ie. implementing our own version of an async put?
>> It seems like that potentially has a cleaner implementation, symmetric
>> call points, handling all the storing and clearing of kvm related
>> pointers within the get/put wrappers, passing only a vfio_device to the
>> put wrapper, using the "vfio_device_" prefix for both. Potentially
>> we'd just want an unconditional flush outside of lock here for
>> deterministic release.
>>
>> What's the downside? Thanks,
>>
>
> btw I guess this can be also fixed by Yi's work here:
>
> https://lore.kernel.org/kvm/[email protected]/
>
> with set_kvm(NULL) moved to the release callback of kvm_vfio device,
> such circular lock dependency can be avoided too.
Oh, interesting... It seems to me that this would eliminate the reported call chain altogether:
kvm_put_kvm
-> kvm_destroy_vm
-> kvm_destroy_devices
-> kvm_vfio_destroy (starting here -- this would no longer be executed)
-> kvm_vfio_file_set_kvm
-> vfio_file_set_kvm
-> group->group_lock/group_rwsem
because kvm_destroy_devices now can't end up calling kvm_vfio_destroy and friends, it won't try and acquire the group lock a 2nd time making a kvm_put_kvm while the group lock is held OK to do. The vfio_file_set_kvm call will now always come from a separate thread of execution, kvm_vfio_group_add, kvm_vfio_group_del or the release thread:
kvm_device_release (where the group->group_lock would not be held since vfio does not trigger closing of the kvm fd)
-> kvm_vfio_destroy (or, kvm_vfio_release)
-> kvm_vfio_file_set_kvm
-> vfio_file_set_kvm
-> group->group_lock/group_rwsem
> From: Matthew Rosato <[email protected]>
> Sent: Wednesday, January 18, 2023 10:56 PM
>
> On 1/18/23 4:03 AM, Tian, Kevin wrote:
> >> From: Alex Williamson
> >> Sent: Wednesday, January 18, 2023 5:23 AM
> >>
> >> On Fri, 13 Jan 2023 19:03:51 -0500
> >> Matthew Rosato <[email protected]> wrote:
> >>
> >>> void vfio_device_group_close(struct vfio_device *device)
> >>> {
> >>> + void (*put_kvm)(struct kvm *kvm);
> >>> + struct kvm *kvm;
> >>> +
> >>> mutex_lock(&device->group->group_lock);
> >>> + kvm = device->kvm;
> >>> + put_kvm = device->put_kvm;
> >>> vfio_device_close(device, device->group->iommufd);
> >>> + if (kvm == device->kvm)
> >>> + kvm = NULL;
> >>
> >> Hmm, so we're using whether the device->kvm pointer gets cleared in
> >> last_close to detect whether we should put the kvm reference. That's a
> >> bit obscure. Our get and put is also asymmetric.
> >>
> >> Did we decide that we couldn't do this via a schedule_work() from the
> >> last_close function, ie. implementing our own version of an async put?
> >> It seems like that potentially has a cleaner implementation, symmetric
> >> call points, handling all the storing and clearing of kvm related
> >> pointers within the get/put wrappers, passing only a vfio_device to the
> >> put wrapper, using the "vfio_device_" prefix for both. Potentially
> >> we'd just want an unconditional flush outside of lock here for
> >> deterministic release.
> >>
> >> What's the downside? Thanks,
> >>
> >
> > btw I guess this can be also fixed by Yi's work here:
> >
> > https://lore.kernel.org/kvm/[email protected]/
> >
> > with set_kvm(NULL) moved to the release callback of kvm_vfio device,
> > such circular lock dependency can be avoided too.
>
> Oh, interesting... It seems to me that this would eliminate the reported call
> chain altogether:
>
> kvm_put_kvm
> -> kvm_destroy_vm
> -> kvm_destroy_devices
> -> kvm_vfio_destroy (starting here -- this would no longer be executed)
> -> kvm_vfio_file_set_kvm
> -> vfio_file_set_kvm
> -> group->group_lock/group_rwsem
>
> because kvm_destroy_devices now can't end up calling kvm_vfio_destroy
> and friends, it won't try and acquire the group lock a 2nd time making a
> kvm_put_kvm while the group lock is held OK to do. The vfio_file_set_kvm
> call will now always come from a separate thread of execution,
> kvm_vfio_group_add, kvm_vfio_group_del or the release thread:
>
> kvm_device_release (where the group->group_lock would not be held since
> vfio does not trigger closing of the kvm fd)
> -> kvm_vfio_destroy (or, kvm_vfio_release)
> -> kvm_vfio_file_set_kvm
> -> vfio_file_set_kvm
> -> group->group_lock/group_rwsem
Yes, that's my point. If Alex/Jason are also OK with it probably Yi can
send that patch separately as a fix to this issue. It's much simpler. ????
On Thu, 19 Jan 2023 03:43:36 +0000
"Tian, Kevin" <[email protected]> wrote:
> > From: Matthew Rosato <[email protected]>
> > Sent: Wednesday, January 18, 2023 10:56 PM
> >
> > On 1/18/23 4:03 AM, Tian, Kevin wrote:
> > >> From: Alex Williamson
> > >> Sent: Wednesday, January 18, 2023 5:23 AM
> > >>
> > >> On Fri, 13 Jan 2023 19:03:51 -0500
> > >> Matthew Rosato <[email protected]> wrote:
> > >>
> > >>> void vfio_device_group_close(struct vfio_device *device)
> > >>> {
> > >>> + void (*put_kvm)(struct kvm *kvm);
> > >>> + struct kvm *kvm;
> > >>> +
> > >>> mutex_lock(&device->group->group_lock);
> > >>> + kvm = device->kvm;
> > >>> + put_kvm = device->put_kvm;
> > >>> vfio_device_close(device, device->group->iommufd);
> > >>> + if (kvm == device->kvm)
> > >>> + kvm = NULL;
> > >>
> > >> Hmm, so we're using whether the device->kvm pointer gets cleared in
> > >> last_close to detect whether we should put the kvm reference. That's a
> > >> bit obscure. Our get and put is also asymmetric.
> > >>
> > >> Did we decide that we couldn't do this via a schedule_work() from the
> > >> last_close function, ie. implementing our own version of an async put?
> > >> It seems like that potentially has a cleaner implementation, symmetric
> > >> call points, handling all the storing and clearing of kvm related
> > >> pointers within the get/put wrappers, passing only a vfio_device to the
> > >> put wrapper, using the "vfio_device_" prefix for both. Potentially
> > >> we'd just want an unconditional flush outside of lock here for
> > >> deterministic release.
> > >>
> > >> What's the downside? Thanks,
> > >>
> > >
> > > btw I guess this can be also fixed by Yi's work here:
> > >
> > > https://lore.kernel.org/kvm/[email protected]/
> > >
> > > with set_kvm(NULL) moved to the release callback of kvm_vfio device,
> > > such circular lock dependency can be avoided too.
> >
> > Oh, interesting... It seems to me that this would eliminate the reported call
> > chain altogether:
> >
> > kvm_put_kvm
> > -> kvm_destroy_vm
> > -> kvm_destroy_devices
> > -> kvm_vfio_destroy (starting here -- this would no longer be executed)
> > -> kvm_vfio_file_set_kvm
> > -> vfio_file_set_kvm
> > -> group->group_lock/group_rwsem
> >
> > because kvm_destroy_devices now can't end up calling kvm_vfio_destroy
> > and friends, it won't try and acquire the group lock a 2nd time making a
> > kvm_put_kvm while the group lock is held OK to do. The vfio_file_set_kvm
> > call will now always come from a separate thread of execution,
> > kvm_vfio_group_add, kvm_vfio_group_del or the release thread:
> >
> > kvm_device_release (where the group->group_lock would not be held since
> > vfio does not trigger closing of the kvm fd)
> > -> kvm_vfio_destroy (or, kvm_vfio_release)
> > -> kvm_vfio_file_set_kvm
> > -> vfio_file_set_kvm
> > -> group->group_lock/group_rwsem
>
> Yes, that's my point. If Alex/Jason are also OK with it probably Yi can
> send that patch separately as a fix to this issue. It's much simpler. ????
If we can extract that flow separate from the cdev refactoring, ideally
something that matches the stable kernel backport rules, then that
sounds like the preferred solution. Thanks,
Alex