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Message-ID: <67ec09bf-cb24-34e3-6ec4-1ae87b0738bd@linux.ibm.com>
Date: Tue, 31 Jan 2023 09:27:54 -0500
From: Anthony Krowiak <akrowiak@...ux.ibm.com>
To: Yi Liu <yi.l.liu@...el.com>, alex.williamson@...hat.com,
pbonzini@...hat.com, mjrosato@...ux.ibm.com
Cc: jgg@...dia.com, kevin.tian@...el.com, cohuck@...hat.com,
farman@...ux.ibm.com, pmorel@...ux.ibm.com,
borntraeger@...ux.ibm.com, frankja@...ux.ibm.com,
imbrenda@...ux.ibm.com, david@...hat.com, jjherne@...ux.ibm.com,
pasic@...ux.ibm.com, zhenyuw@...ux.intel.com, zhi.a.wang@...el.com,
seanjc@...gle.com, linux-s390@...r.kernel.org, kvm@...r.kernel.org,
intel-gvt-dev@...ts.freedesktop.org,
intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kvm/vfio: Fix potential deadlock on vfio group_lock
I encountered a lockdep splat while running some regression tests today
(see below). I suspected it might be this patch so I reverted it,
rebuilt the kernel and ran the regression tests again; this time, the
test ran cleanly. It looks like this patch may not have fixed the
problem for which it was intended. Here is the relevant dmesg output:
[ 579.471402] hades[1099]: Start test run
[ 579.473486] hades[1099]: Start
'tests.test_vfio_ap.VfioAPAssignMdevToGuestTest.runTest' test
[ 579.505804] vfio_ap matrix: MDEV: Registered
[ 579.604024] vfio_ap_mdev 529654a9-bea4-461a-b64d-9d9c63df0deb: Adding
to iommu group 0
[ 585.043898] ======================================================
[ 585.043900] WARNING: possible circular locking dependency detected
[ 585.043902] 6.2.0-rc6-00057-g41c03ba9beea-dirty #18 Not tainted
[ 585.043904] ------------------------------------------------------
[ 585.043905] CPU 0/KVM/1173 is trying to acquire lock:
[ 585.043907] 000000008cfb24b0 (&group->group_lock){+.+.}-{3:3}, at:
vfio_file_set_kvm+0x50/0x68 [vfio]
[ 585.043919]
but task is already holding lock:
[ 585.043920] 00000000b2960ba0 (&kvm->lock){+.+.}-{3:3}, at:
kvm_device_release+0x4a/0xb8 [kvm]
[ 585.043960]
which lock already depends on the new lock.
[ 585.043962]
the existing dependency chain (in reverse order) is:
[ 585.043963]
-> #3 (&kvm->lock){+.+.}-{3:3}:
[ 585.043967] __lock_acquire+0x3e2/0x750
[ 585.043974] lock_acquire.part.0+0xe2/0x250
[ 585.043977] lock_acquire+0xac/0x1d0
[ 585.043980] __mutex_lock+0x9e/0x868
[ 585.043985] mutex_lock_nested+0x32/0x40
[ 585.043988] vfio_ap_mdev_open_device+0x9a/0x198 [vfio_ap]
[ 585.043991] vfio_device_open+0x122/0x168 [vfio]
[ 585.043995] vfio_device_open_file+0x64/0x120 [vfio]
[ 585.043999] vfio_group_fops_unl_ioctl+0xd4/0x1e0 [vfio]
[ 585.044002] __s390x_sys_ioctl+0xc0/0x100
[ 585.044007] do_syscall+0xee/0x118
[ 585.044032] __do_syscall+0xd2/0x120
[ 585.044035] system_call+0x82/0xb0
[ 585.044037]
-> #2 (&matrix_dev->guests_lock){+.+.}-{3:3}:
[ 585.044041] __lock_acquire+0x3e2/0x750
[ 585.044044] lock_acquire.part.0+0xe2/0x250
[ 585.044047] lock_acquire+0xac/0x1d0
[ 585.044049] __mutex_lock+0x9e/0x868
[ 585.044052] mutex_lock_nested+0x32/0x40
[ 585.044054] vfio_ap_mdev_open_device+0x8c/0x198 [vfio_ap]
[ 585.044057] vfio_device_open+0x122/0x168 [vfio]
[ 585.044060] vfio_device_open_file+0x64/0x120 [vfio]
[ 585.044064] vfio_group_fops_unl_ioctl+0xd4/0x1e0 [vfio]
[ 585.044068] __s390x_sys_ioctl+0xc0/0x100
[ 585.044070] do_syscall+0xee/0x118
[ 585.044072] __do_syscall+0xd2/0x120
[ 585.044074] system_call+0x82/0xb0
[ 585.044076]
-> #1 (&new_dev_set->lock){+.+.}-{3:3}:
[ 585.044080] __lock_acquire+0x3e2/0x750
[ 585.044082] lock_acquire.part.0+0xe2/0x250
[ 585.044085] lock_acquire+0xac/0x1d0
[ 585.044088] __mutex_lock+0x9e/0x868
[ 585.044090] mutex_lock_nested+0x32/0x40
[ 585.044093] vfio_device_open+0x3e/0x168 [vfio]
[ 585.044096] vfio_device_open_file+0x64/0x120 [vfio]
[ 585.044100] vfio_group_fops_unl_ioctl+0xd4/0x1e0 [vfio]
[ 585.044104] __s390x_sys_ioctl+0xc0/0x100
[ 585.044106] do_syscall+0xee/0x118
[ 585.044108] __do_syscall+0xd2/0x120
[ 585.044110] system_call+0x82/0xb0
[ 585.044112]
-> #0 (&group->group_lock){+.+.}-{3:3}:
[ 585.044115] check_prev_add+0xd4/0xf10
[ 585.044118] validate_chain+0x698/0x8e8
[ 585.044120] __lock_acquire+0x3e2/0x750
[ 585.044123] lock_acquire.part.0+0xe2/0x250
[ 585.044125] lock_acquire+0xac/0x1d0
[ 585.044128] __mutex_lock+0x9e/0x868
[ 585.044130] mutex_lock_nested+0x32/0x40
[ 585.044133] vfio_file_set_kvm+0x50/0x68 [vfio]
[ 585.044137] kvm_vfio_release+0x5e/0xf8 [kvm]
[ 585.044156] kvm_device_release+0x90/0xb8 [kvm]
[ 585.044175] __fput+0xaa/0x2a0
[ 585.044180] task_work_run+0x76/0xd0
[ 585.044183] do_exit+0x248/0x538
[ 585.044186] do_group_exit+0x40/0xb0
[ 585.044188] get_signal+0x614/0x698
[ 585.044192] arch_do_signal_or_restart+0x58/0x370
[ 585.044195] exit_to_user_mode_loop+0xe8/0x1b8
[ 585.044200] exit_to_user_mode_prepare+0x164/0x190
[ 585.044203] __do_syscall+0xd2/0x120
[ 585.044205] system_call+0x82/0xb0
[ 585.044207]
other info that might help us debug this:
[ 585.044209] Chain exists of:
&group->group_lock --> &matrix_dev->guests_lock -->
&kvm->lock
[ 585.044213] Possible unsafe locking scenario:
[ 585.044214] CPU0 CPU1
[ 585.044216] ---- ----
[ 585.044217] lock(&kvm->lock);
[ 585.044219] lock(&matrix_dev->guests_lock);
[ 585.044221] lock(&kvm->lock);
[ 585.044223] lock(&group->group_lock);
[ 585.044225]
*** DEADLOCK ***
[ 585.044227] 1 lock held by CPU 0/KVM/1173:
[ 585.044228] #0: 00000000b2960ba0 (&kvm->lock){+.+.}-{3:3}, at:
kvm_device_release+0x4a/0xb8 [kvm]
[ 585.044251]
stack backtrace:
[ 585.044253] CPU: 3 PID: 1173 Comm: CPU 0/KVM Not tainted
6.2.0-rc6-00057-g41c03ba9beea-dirty #18
[ 585.044256] Hardware name: IBM 8561 T01 772 (LPAR)
[ 585.044257] Call Trace:
[ 585.044258] [<000000011a818936>] dump_stack_lvl+0x8e/0xc8
[ 585.044261] [<0000000119aca3f2>] check_noncircular+0x132/0x158
[ 585.044264] [<0000000119acba44>] check_prev_add+0xd4/0xf10
[ 585.044267] [<0000000119accf18>] validate_chain+0x698/0x8e8
[ 585.044270] [<0000000119ace70a>] __lock_acquire+0x3e2/0x750
[ 585.044273] [<0000000119acf682>] lock_acquire.part.0+0xe2/0x250
[ 585.044276] [<0000000119acf89c>] lock_acquire+0xac/0x1d0
[ 585.044279] [<000000011a823c66>] __mutex_lock+0x9e/0x868
[ 585.044282] [<000000011a824462>] mutex_lock_nested+0x32/0x40
[ 585.044285] [<000003ff7fbcd6a0>] vfio_file_set_kvm+0x50/0x68 [vfio]
[ 585.044289] [<000003ff7feacab6>] kvm_vfio_release+0x5e/0xf8 [kvm]
[ 585.044308] [<000003ff7fea6d58>] kvm_device_release+0x90/0xb8 [kvm]
[ 585.044328] [<0000000119dbb83a>] __fput+0xaa/0x2a0
[ 585.044331] [<0000000119a67c66>] task_work_run+0x76/0xd0
[ 585.044333] [<0000000119a3ec18>] do_exit+0x248/0x538
[ 585.044335] [<0000000119a3f0c8>] do_group_exit+0x40/0xb0
[ 585.044338] [<0000000119a50dec>] get_signal+0x614/0x698
[ 585.044340] [<00000001199ea030>] arch_do_signal_or_restart+0x58/0x370
[ 585.044343] [<0000000119b0bb50>] exit_to_user_mode_loop+0xe8/0x1b8
[ 585.044346] [<0000000119b0bd84>] exit_to_user_mode_prepare+0x164/0x190
[ 585.044349] [<000000011a818d2a>] __do_syscall+0xd2/0x120
[ 585.044351] [<000000011a82c462>] system_call+0x82/0xb0
[ 585.044354] INFO: lockdep is turned off.
[ 610.595528] vfio_ap_mdev 529654a9-bea4-461a-b64d-9d9c63df0deb:
Removing from iommu group 0
[ 610.604408] vfio_ap matrix: MDEV: Unregistering
[ 610.826074] hades[1099]: Stop
'tests.test_vfio_ap.VfioAPAssignMdevToGuestTest.runTest'
On 1/20/23 10:05 AM, Yi Liu 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:
>
> VFIO holds group->group_lock/group_rwsem
> -> kvm_put_kvm
> -> kvm_destroy_vm
> -> kvm_destroy_devices
> -> kvm_vfio_destroy
> -> kvm_vfio_file_set_kvm
> -> vfio_file_set_kvm
> -> try to hold group->group_lock/group_rwsem
>
> The key function is the kvm_destroy_devices() which triggers destroy cb
> of kvm_device_ops. It calls back to vfio and try to hold group_lock. So
> if this path doesn't call back to vfio, this dead lock would be fixed.
> Actually, there is a way for it. KVM provides another point to free the
> kvm-vfio device which is the point when the device file descriptor is
> closed. This can be achieved by providing the release cb instead of the
> destroy cb. Also rename kvm_vfio_destroy() to be kvm_vfio_release().
>
> /*
> * Destroy is responsible for freeing dev.
> *
> * Destroy may be called before or after destructors are called
> * on emulated I/O regions, depending on whether a reference is
> * held by a vcpu or other kvm component that gets destroyed
> * after the emulated I/O.
> */
> void (*destroy)(struct kvm_device *dev);
>
> /*
> * Release is an alternative method to free the device. It is
> * called when the device file descriptor is closed. Once
> * release is called, the destroy method will not be called
> * anymore as the device is removed from the device list of
> * the VM. kvm->lock is held.
> */
> void (*release)(struct kvm_device *dev);
>
> Fixes: 421cfe6596f6 ("vfio: remove VFIO_GROUP_NOTIFY_SET_KVM")
> Reported-by: Alex Williamson <alex.williamson@...hat.com>
> Suggested-by: Kevin Tian <kevin.tian@...el.com>
> Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
> Signed-off-by: Yi Liu <yi.l.liu@...el.com>
> ---
> virt/kvm/vfio.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/virt/kvm/vfio.c b/virt/kvm/vfio.c
> index 495ceabffe88..e94f3ea718e5 100644
> --- a/virt/kvm/vfio.c
> +++ b/virt/kvm/vfio.c
> @@ -336,7 +336,7 @@ static int kvm_vfio_has_attr(struct kvm_device *dev,
> return -ENXIO;
> }
>
> -static void kvm_vfio_destroy(struct kvm_device *dev)
> +static void kvm_vfio_release(struct kvm_device *dev)
> {
> struct kvm_vfio *kv = dev->private;
> struct kvm_vfio_group *kvg, *tmp;
> @@ -363,7 +363,7 @@ static int kvm_vfio_create(struct kvm_device *dev, u32 type);
> static struct kvm_device_ops kvm_vfio_ops = {
> .name = "kvm-vfio",
> .create = kvm_vfio_create,
> - .destroy = kvm_vfio_destroy,
> + .release = kvm_vfio_release,
> .set_attr = kvm_vfio_set_attr,
> .has_attr = kvm_vfio_has_attr,
> };
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