Re: [PATCH] KVM: fix rcu warning on VM_CREATE errors

["Radim Krčmář" <rkrcmar@redhat.com>]

2017-09-13 14:17+0200, Christian Borntraeger:
> commit 3898da947bba ("KVM: avoid using rcu_dereference_protected") can
> trigger the following lockdep/rcu splat if the VM_CREATE ioctl fails,
> for example if kvm_arch_init_vm fails:
> 
> WARNING: suspicious RCU usage
> 4.13.0+ #105 Not tainted
> -----------------------------
> ./include/linux/kvm_host.h:481 suspicious rcu_dereference_check() usage!
> 
> other info that might help us debug this:
> 
> rcu_scheduler_active = 2, debug_locks = 1
> no locks held by qemu-system-s39/79.
> stack backtrace:
> CPU: 0 PID: 79 Comm: qemu-system-s39 Not tainted 4.13.0+ #105
> Hardware name: IBM 2964 NC9 704 (KVM/Linux)
> Call Trace:
> ([<00000000001140b2>] show_stack+0xea/0xf0)
>  [<00000000008a68a4>] dump_stack+0x94/0xd8
>  [<0000000000134c12>] kvm_dev_ioctl+0x372/0x7a0
>  [<000000000038f940>] do_vfs_ioctl+0xa8/0x6c8
>  [<0000000000390004>] SyS_ioctl+0xa4/0xb8
>  [<00000000008c7a8c>] system_call+0xc4/0x27c
> no locks held by qemu-system-s39/79.
> 
> We have to reset the just created users_count back to 0 to
> tell the check to not trigger.
> 
> Reported-by: Stefan Haberland <sth@linux.vnet.ibm.com>
> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
> Fixes: 3898da947bba ("KVM: avoid using rcu_dereference_protected")
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> ---

Applied, thanks.