Re: [PATCH 2/6] PCI/AER: introduce pci_bus_ops_get() function to avoid a small race condition window

[Yijing Wang <wangyijing@huawei.com>]

On 2012/9/19 13:52, Huang Ying wrote:
> On Wed, 2012-09-19 at 10:40 +0800, Yijing Wang wrote:
>> When we rmmod aer_inject module, there is a race condition window between pci_bus_ops_pop()
>> and pci_bus_set_ops() in aer_inject_exit, eg. pci_read_aer/pci_write_aer was called between
>> them. So introduce pci_bus_ops_get() to avoid this.
>>
>> Signed-off-by: Yijing Wang <wangyijing@huawei.com>
>> ---
>>  drivers/pci/pcie/aer/aer_inject.c |   21 ++++++++++++++++++---
>>  1 files changed, 18 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/pci/pcie/aer/aer_inject.c b/drivers/pci/pcie/aer/aer_inject.c
>> index 0f00a27..442147b 100644
>> --- a/drivers/pci/pcie/aer/aer_inject.c
>> +++ b/drivers/pci/pcie/aer/aer_inject.c
>> @@ -67,6 +67,8 @@ struct pci_bus_ops {
>>  	struct pci_ops *ops;
>>  };
>>  
>> +#define to_pci_bus_ops(n) container_of(n, struct pci_bus_ops, list)
>> +
>>  static LIST_HEAD(einjected);
>>  
>>  static LIST_HEAD(pci_bus_ops_list);
>> @@ -160,6 +162,18 @@ static struct pci_bus_ops *pci_bus_ops_pop(void)
>>  	return bus_ops;
>>  }
>>  
>> +static struct pci_bus_ops *pci_bus_ops_get(struct pci_bus_ops *from)
>> +{
>> +	struct pci_bus_ops *bus_ops = NULL;
>> +	struct list_head *n;
>> +
>> +	n = from ? from->list.next : pci_bus_ops_list.next;
>> +	if (n != &pci_bus_ops_list)
>> +		bus_ops = to_pci_bus_ops(n);
>> +
>> +	return bus_ops;
>> +}
>> +
>>  static u32 *find_pci_config_dword(struct aer_error *err, int where,
>>  				  int *prw1cs)
>>  {
>> @@ -540,14 +554,15 @@ static void __exit aer_inject_exit(void)
>>  {
>>  	struct aer_error *err, *err_next;
>>  	unsigned long flags;
>> -	struct pci_bus_ops *bus_ops;
>> +	struct pci_bus_ops *bus_ops = NULL;
>>  
>>  	misc_deregister(&aer_inject_device);
>>  
>> -	while ((bus_ops = pci_bus_ops_pop())) {
>> +	while ((bus_ops = pci_bus_ops_get(bus_ops)))
>>  		pci_bus_set_ops(bus_ops->bus, bus_ops->ops);
> 
> In fact, this is
> 
> list_for_each_entry(&pci_bus_ops_list)
> 	pci_bus_set_ops()
> 
> Because we are in module exit path, there will be no new user of
> pci_bus_ops_list, it appears safe to do that without lock.
> 
> But the bus_ops may be deleted from the list when accessed via
> pci_ops_aer.  So It may be better to wait for all pci_ops_aer functions

Hi Huang Ying,
   I have some confusions about this, can you explain this? Thanks very much!
In my idea, if pci_ops_aer be called, it hold the pci_lock, so pci_bus_set_ops will wait for
pci_ops_aer functions to exit.So in my idea, after pci_bus_set_ops loop completed. pci_ops_aer functions
have been exit, and will never be called again(because all pci_ops_aer).

> return before delete them.  synchronize_rcu() should be sufficient for
> that, because all pci_ops_aer functions are called with spinlock held.
> 


> Best Regards,
> Huang Ying
> 
>> +
>> +	while ((bus_ops = pci_bus_ops_pop()))
>>  		kfree(bus_ops);
>> -	}
>>  
>>  	spin_lock_irqsave(&inject_lock, flags);
>>  	list_for_each_entry_safe(err, err_next, &einjected, list) {
> 
> 
> 
> .
> 


-- 
Thanks!
Yijing