Subject: Re: suspicious RCU usage with kvm_pr

From: Thomas Huth <[email-masked]>
Date: Wed, 16 Sep 2015 12:16:01 +0200

On 16/09/15 10:51, Denis Kirjanov wrote:
> Hi,
> 
> I see the following trace on qemu startup (ps700 blade):
> 
> v4.2-11169-g64d1def
> 
> 
> [  143.369638] ===============================
> [  143.369640] [ INFO: suspicious RCU usage. ]
> [  143.369643] 4.2.0-11169-g64d1def #10 Tainted: G S
> [  143.369645] -------------------------------
> [  143.369647] arch/powerpc/kvm/../../../virt/kvm/kvm_main.c:3310
> suspicious rcu_dereference_check() usage!
> [  143.369649]
> other info that might help us debug this:
> 
> [  143.369652]
> rcu_scheduler_active = 1, debug_locks = 1
> [  143.369655] 1 lock held by qemu-system-ppc/2292:
> [  143.369656]  #0:  (&vcpu->mutex){+.+.+.}, at: [<d000000004225bac>]
> .vcpu_load+0x2c/0xb0 [kvm]
> [  143.369672]
> stack backtrace:
> [  143.369675] CPU: 12 PID: 2292 Comm: qemu-system-ppc Tainted: G S
>           4.2.0-11169-g64d1def #10
> [  143.369677] Call Trace:
> [  143.369682] [c0000001d08bf200] [c000000000816dd0]
> .dump_stack+0x98/0xd4 (unreliable)
> [  143.369687] [c0000001d08bf280] [c0000000000f7058]
> .lockdep_rcu_suspicious+0x108/0x170
> [  143.369696] [c0000001d08bf310] [d0000000042296d8]
> .kvm_io_bus_read+0x1d8/0x220 [kvm]
> [  143.369705] [c0000001d08bf3c0] [d00000000422f980]
> .kvmppc_h_logical_ci_load+0x60/0xe0 [kvm]

Could it be that we need to srcu_read_lock(&vcpu->kvm->srcu) before
calling the kvm_io_bus_read/write() function in the
kvmppc_h_logical_ci_load/store() function?

 Thomas