Re: [PATCH] kvm: use the correct RCU API

[Lai Jiangshan <laijs@cn.fujitsu.com>]

Paul E. McKenney wrote:
> On Mon, Apr 19, 2010 at 12:49:04PM +0300, Avi Kivity wrote:
>> On 04/19/2010 12:41 PM, Lai Jiangshan wrote:
>>> The RCU/SRCU API have already changed for proving RCU usage.
>>>
>>> I got the following dmesg when PROVE_RCU=y because we used incorrect API.
>>> This patch coverts rcu_deference() to srcu_dereference() or family API.
>>>
>>> ===================================================
>>> [ INFO: suspicious rcu_dereference_check() usage. ]
>>> ---------------------------------------------------
>>> arch/x86/kvm/mmu.c:3020 invoked rcu_dereference_check() without protection!
>>>
>>> other info that might help us debug this:
>>>
>>>
>>> rcu_scheduler_active = 1, debug_locks = 0
>>> 2 locks held by qemu-system-x86/8550:
>>>  #0:  (&kvm->slots_lock){+.+.+.}, at: [<ffffffffa011a6ac>] kvm_set_memory_region+0x29/0x50 [kvm]
>>>  #1:  (&(&kvm->mmu_lock)->rlock){+.+...}, at: [<ffffffffa012262d>] kvm_arch_commit_memory_region+0xa6/0xe2 [kvm]
>>>
>>> stack backtrace:
>>> Pid: 8550, comm: qemu-system-x86 Not tainted 2.6.34-rc4-tip-01028-g939eab1 #27
>>> Call Trace:
>>>  [<ffffffff8106c59e>] lockdep_rcu_dereference+0xaa/0xb3
>>>  [<ffffffffa012f6c1>] kvm_mmu_calculate_mmu_pages+0x44/0x7d [kvm]
>>>  [<ffffffffa012263e>] kvm_arch_commit_memory_region+0xb7/0xe2 [kvm]
>>>  [<ffffffffa011a5d7>] __kvm_set_memory_region+0x636/0x6e2 [kvm]
>>>  [<ffffffffa011a6ba>] kvm_set_memory_region+0x37/0x50 [kvm]
>>>  [<ffffffffa015e956>] vmx_set_tss_addr+0x46/0x5a [kvm_intel]
>>>  [<ffffffffa0126592>] kvm_arch_vm_ioctl+0x17a/0xcf8 [kvm]
>>>  [<ffffffff810a8692>] ? unlock_page+0x27/0x2c
>>>  [<ffffffff810bf879>] ? __do_fault+0x3a9/0x3e1
>>>  [<ffffffffa011b12f>] kvm_vm_ioctl+0x364/0x38d [kvm]
>>>  [<ffffffff81060cfa>] ? up_read+0x23/0x3d
>>>  [<ffffffff810f3587>] vfs_ioctl+0x32/0xa6
>>>  [<ffffffff810f3b19>] do_vfs_ioctl+0x495/0x4db
>>>  [<ffffffff810e6b2f>] ? fget_light+0xc2/0x241
>>>  [<ffffffff810e416c>] ? do_sys_open+0x104/0x116
>>>  [<ffffffff81382d6d>] ? retint_swapgs+0xe/0x13
>>>  [<ffffffff810f3ba6>] sys_ioctl+0x47/0x6a
>>>  [<ffffffff810021db>] system_call_fastpath+0x16/0x1b
>>>
>>>
>>>
>>> +static inline struct kvm_memslots *kvm_memslots(struct kvm *kvm)
>>> +{
>>> +	return rcu_dereference_check(kvm->memslots,
>>> +			srcu_read_lock_held(&kvm->srcu)
>>> +			|| lockdep_is_held(&kvm->slots_lock));
>>> +}
>>> +
>>
>> This open-codes srcu_dereference().  I guess we need an
>> srcu_dereference_check().  Paul?
> 

rcu_dereference_check() is useful when rcu_dereference(),
rcu_dereference_bh(), rcu_dereference_sched() and srcu_dereference()
are not appropriate.

I think we don't need srcu_dereference_check() nor rcu_dereference_bh_check()
nor rcu_dereference_sched_check().

> One is coming in Arnd's sparse-based patchset.  It is probably best
> to open-code this in the meantime and clean up later, but I will
> double-check with Arnd.
> 
>> btw, perhaps it is possible not to call rcu_dereference from the
>> write paths.
> 
> There is an rcu_dereference_protected() on its way to mainline to handle
> the case where the reference is always protected by a lock.  Why not
> just access it directly?  Because if you do that, the sparse-based checks
> will yell at you.
> 
> There is also an rcu_access_pointer() on its way to mainline for cases
> where you only want to test the pointer itself, not dereference it.
> 
> 						Thanx, Paul
> 

I reviewed the code, the functions can be called from the srcu-read-site
or update-site, rcu_dereference_check() can simplify the code.

If we use rcu_dereference_protected(), we may need duplicate the functions.

I think there is very small overhead of using rcu_dereference(), so we can
call it from write paths.

Thanks,
Lai