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

[Huang Ying <ying.huang@intel.com>]

On Wed, 2012-09-19 at 14:42 +0800, Yijing Wang wrote:
> 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).

Yes. You are right,  waiting is not necessary here.

Best Regards,
Huang Ying

> > 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) {
> > 
> > 
> > 
> > .
> > 
> 
>