> -----Original Message----- > From: Alex Williamson <alex.williamson@xxxxxxxxxx> > Sent: Thursday, February 2, 2023 12:15 PM > To: Liu, Yi L <yi.l.liu@xxxxxxxxx> > Cc: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>; pbonzini@xxxxxxxxxx; > jgg@xxxxxxxxxx; cohuck@xxxxxxxxxx; farman@xxxxxxxxxxxxx; > pmorel@xxxxxxxxxxxxx; borntraeger@xxxxxxxxxxxxx; > frankja@xxxxxxxxxxxxx; imbrenda@xxxxxxxxxxxxx; david@xxxxxxxxxx; > akrowiak@xxxxxxxxxxxxx; jjherne@xxxxxxxxxxxxx; pasic@xxxxxxxxxxxxx; > zhenyuw@xxxxxxxxxxxxxxx; Wang, Zhi A <zhi.a.wang@xxxxxxxxx>; > Christopherson,, Sean <seanjc@xxxxxxxxxx>; Tian, Kevin > <kevin.tian@xxxxxxxxx>; linux-s390@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; > intel-gvt-dev@xxxxxxxxxxxxxxxxxxxxx; intel-gfx@xxxxxxxxxxxxxxxxxxxxx; linux- > kernel@xxxxxxxxxxxxxxx > Subject: Re: [PATCH v2] vfio: fix deadlock between group lock and kvm lock > > On Thu, 2 Feb 2023 03:46:59 +0000 > "Liu, Yi L" <yi.l.liu@xxxxxxxxx> wrote: > > > > From: Alex Williamson <alex.williamson@xxxxxxxxxx> > > > Sent: Thursday, February 2, 2023 7:28 AM > > > > > > On Wed, 1 Feb 2023 14:20:10 -0500 > > > Matthew Rosato <mjrosato@xxxxxxxxxxxxx> wrote: > > > > > > > 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 <akrowiak@xxxxxxxxxxxxx> > > > > Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx> > > > > Signed-off-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxx> > > > > --- > > > > Changes from v1: > > > > * use spin_lock instead of spin_lock_irqsave (Jason) > > > > * clear device->kvm_put as part of vfio_kvm_put_kvm (Yi) > > > > * Re-arrange code to avoid referencing the group contents from > within > > > > vfio_main (Kevin) which meant moving most of the code in this patch > > > > to group.c along with getting/dropping of the dev_set lock > > > > --- > > > > drivers/vfio/group.c | 90 > > > +++++++++++++++++++++++++++++++++++++--- > > > > drivers/vfio/vfio.h | 1 + > > > > drivers/vfio/vfio_main.c | 11 ++--- > > > > include/linux/vfio.h | 2 +- > > > > 4 files changed, 91 insertions(+), 13 deletions(-) > > > > > > > > diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c > > > > index bb24b2f0271e..52f434861294 100644 > > > > --- a/drivers/vfio/group.c > > > > +++ b/drivers/vfio/group.c > > > > @@ -13,6 +13,9 @@ > > > > #include <linux/vfio.h> > > > > #include <linux/iommufd.h> > > > > #include <linux/anon_inodes.h> > > > > +#ifdef CONFIG_HAVE_KVM > > > > +#include <linux/kvm_host.h> > > > > +#endif > > > > #include "vfio.h" > > > > > > > > static struct vfio { > > > > @@ -154,6 +157,55 @@ static int > vfio_group_ioctl_set_container(struct > > > vfio_group *group, > > > > return ret; > > > > } > > > > > > > > +#ifdef CONFIG_HAVE_KVM > > > > +static bool vfio_kvm_get_kvm_safe(struct vfio_device *device, > struct > > > kvm *kvm) > > > > > > I'm tempted to name these vfio_device_get_kvm_safe() and only pass > the > > > vfio_device, where of course we can get the kvm pointer from the > group > > > internally. > > > > > > > +{ > > > > + void (*pfn)(struct kvm *kvm); > > > > + bool (*fn)(struct kvm *kvm); > > > > + bool ret; > > > > + > > > > > > We should assert_lockdep_held(&device->dev_set->lock) in both of > these > > > since that seems to be what's protecting device->kvm and > > > device->put_kvm. > > > > > > If we change as above to get the kvm pointer from the group within this > > > function, we can also move the kvm_ref_lock here, which seems to > > > simplify the caller quite a bit. > > > > > > > + 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; > > > > > > It simplifies the caller if we can use this even in the !device->kvm > > > case. > > > > > > > + > > > > + device->put_kvm(device->kvm); > > > > + device->put_kvm = NULL; > > > > + 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_group_open(struct vfio_device *device) > > > > { > > > > int ret; > > > > @@ -164,14 +216,32 @@ static int vfio_device_group_open(struct > > > vfio_device *device) > > > > goto out_unlock; > > > > } > > > > > > > > + mutex_lock(&device->dev_set->lock); > > > > + > > > > /* > > > > - * 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. > > > > + * Before the first device open, 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 again. Save > > > > + * the pointer in the device for use by drivers. > > > > */ > > > > + if (device->open_count == 0) { > > > > + spin_lock(&device->group->kvm_ref_lock); > > > > + if (device->group->kvm && > > > > + vfio_kvm_get_kvm_safe(device, device->group->kvm)) > > > > + device->kvm = device->group->kvm; > > > > + spin_unlock(&device->group->kvm_ref_lock); > > > > + } > > > > + > > > > ret = vfio_device_open(device, device->group->iommufd, > > > > device->group->kvm); > > > > > > We're using device->group->kvm outside of kvm_ref_lock here, it > should > > > be using device->kvm. > > > > Existing code set device->kvm in the vfio_device_first_open() which is > > called by vfio_device_open(). After above change, seems not necessary > > to pass kvm pointer into the call chain. Isn't it? > > Yes, we can get it from the device. I didn't check how much this > bloats the patch though. As a fix, it might make sense to save that > refactoring for a follow-on patch. Thanks, 65 lines diff file. 😊 follow-on path works well. diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c index bb24b2f0271e..9e04e55c838f 100644 --- a/drivers/vfio/group.c +++ b/drivers/vfio/group.c @@ -169,8 +169,7 @@ static int vfio_device_group_open(struct vfio_device *device) * 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->iommufd); out_unlock: mutex_unlock(&device->group->group_lock); diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h index f8219a438bfb..4ece6cb4cf2e 100644 --- a/drivers/vfio/vfio.h +++ b/drivers/vfio/vfio.h @@ -19,7 +19,7 @@ struct vfio_container; 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); + struct iommufd_ctx *iommufd); void vfio_device_close(struct vfio_device *device, struct iommufd_ctx *iommufd); diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c index 5177bb061b17..45a7d6d38e2e 100644 --- a/drivers/vfio/vfio_main.c +++ b/drivers/vfio/vfio_main.c @@ -345,7 +345,7 @@ static bool vfio_assert_device_open(struct vfio_device *device) } static int vfio_device_first_open(struct vfio_device *device, - struct iommufd_ctx *iommufd, struct kvm *kvm) + struct iommufd_ctx *iommufd) { int ret; @@ -361,7 +361,6 @@ static int vfio_device_first_open(struct vfio_device *device, if (ret) goto err_module_put; - device->kvm = kvm; if (device->ops->open_device) { ret = device->ops->open_device(device); if (ret) @@ -396,14 +395,14 @@ static void vfio_device_last_close(struct vfio_device *device, } int vfio_device_open(struct vfio_device *device, - struct iommufd_ctx *iommufd, struct kvm *kvm) + struct iommufd_ctx *iommufd) { 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, iommufd); if (ret) device->open_count--; }