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

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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?

btw, perhaps it is possible not to call rcu_dereference from the write paths.


--
error compiling committee.c: too many arguments to function

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