Re: [PATCH] powerpc/kvm/book3s64/vio: fix some RCU-list locks

[Paul Mackerras <paulus@ozlabs.org>]

On Sun, May 10, 2020 at 01:18:34AM -0400, Qian Cai wrote:
> It is unsafe to traverse kvm->arch.spapr_tce_tables and
> stt->iommu_tables without the RCU read lock held. Also, add
> cond_resched_rcu() in places with the RCU read lock held that could take
> a while to finish.
> 
>  arch/powerpc/kvm/book3s_64_vio.c:76 RCU-list traversed in non-reader section!!
> 
>  other info that might help us debug this:
> 
>  rcu_scheduler_active = 2, debug_locks = 1
>  no locks held by qemu-kvm/4265.
> 
>  stack backtrace:
>  CPU: 96 PID: 4265 Comm: qemu-kvm Not tainted 5.7.0-rc4-next-20200508+ #2
>  Call Trace:
>  [c000201a8690f720] [c000000000715948] dump_stack+0xfc/0x174 (unreliable)
>  [c000201a8690f770] [c0000000001d9470] lockdep_rcu_suspicious+0x140/0x164
>  [c000201a8690f7f0] [c008000010b9fb48] kvm_spapr_tce_release_iommu_group+0x1f0/0x220 [kvm]
>  [c000201a8690f870] [c008000010b8462c] kvm_spapr_tce_release_vfio_group+0x54/0xb0 [kvm]
>  [c000201a8690f8a0] [c008000010b84710] kvm_vfio_destroy+0x88/0x140 [kvm]
>  [c000201a8690f8f0] [c008000010b7d488] kvm_put_kvm+0x370/0x600 [kvm]
>  [c000201a8690f990] [c008000010b7e3c0] kvm_vm_release+0x38/0x60 [kvm]
>  [c000201a8690f9c0] [c0000000005223f4] __fput+0x124/0x330
>  [c000201a8690fa20] [c000000000151cd8] task_work_run+0xb8/0x130
>  [c000201a8690fa70] [c0000000001197e8] do_exit+0x4e8/0xfa0
>  [c000201a8690fb70] [c00000000011a374] do_group_exit+0x64/0xd0
>  [c000201a8690fbb0] [c000000000132c90] get_signal+0x1f0/0x1200
>  [c000201a8690fcc0] [c000000000020690] do_notify_resume+0x130/0x3c0
>  [c000201a8690fda0] [c000000000038d64] syscall_exit_prepare+0x1a4/0x280
>  [c000201a8690fe20] [c00000000000c8f8] system_call_common+0xf8/0x278
> 
>  ====
>  arch/powerpc/kvm/book3s_64_vio.c:368 RCU-list traversed in non-reader section!!
> 
>  other info that might help us debug this:
> 
>  rcu_scheduler_active = 2, debug_locks = 1
>  2 locks held by qemu-kvm/4264:
>   #0: c000201ae2d000d8 (&vcpu->mutex){+.+.}-{3:3}, at: kvm_vcpu_ioctl+0xdc/0x950 [kvm]
>   #1: c000200c9ed0c468 (&kvm->srcu){....}-{0:0}, at: kvmppc_h_put_tce+0x88/0x340 [kvm]
> 
>  ====
>  arch/powerpc/kvm/book3s_64_vio.c:108 RCU-list traversed in non-reader section!!
> 
>  other info that might help us debug this:
> 
>  rcu_scheduler_active = 2, debug_locks = 1
>  1 lock held by qemu-kvm/4257:
>   #0: c000200b1b363a40 (&kv->lock){+.+.}-{3:3}, at: kvm_vfio_set_attr+0x598/0x6c0 [kvm]
> 
>  ====
>  arch/powerpc/kvm/book3s_64_vio.c:146 RCU-list traversed in non-reader section!!
> 
>  other info that might help us debug this:
> 
>  rcu_scheduler_active = 2, debug_locks = 1
>  1 lock held by qemu-kvm/4257:
>   #0: c000200b1b363a40 (&kv->lock){+.+.}-{3:3}, at: kvm_vfio_set_attr+0x598/0x6c0 [kvm]
> 
> Signed-off-by: Qian Cai <cai@lca.pw>

Thanks, applied to my kvm-ppc-next branch, with the cond_resched_rcu()
in kvmppc_tce_validate removed.

Paul.