Re: [PATCH v3] PCI: Lock the pci_cfg_wait queue for the consistency of data

[Xiang Zheng]

On 2020/6/29 0:14, Bjorn Helgaas wrote:
> On Sun, Jun 28, 2020 at 12:18:10PM +0800, Xiang Zheng wrote:
>> On 2020/6/26 7:24, Bjorn Helgaas wrote:
>>> On Wed, Jun 24, 2020 at 06:23:09PM -0500, Bjorn Helgaas wrote:
>>>> On Tue, Dec 10, 2019 at 11:15:27AM +0800, Xiang Zheng wrote:
>>>>> 7ea7e98fd8d0 ("PCI: Block on access to temporarily unavailable pci
>>>>> device") suggests that the "pci_lock" is sufficient, and all the
>>>>> callers of pci_wait_cfg() are wrapped with the "pci_lock".
>>>>>
>>>>> However, since the commit cdcb33f98244 ("PCI: Avoid possible deadlock on
>>>>> pci_lock and p->pi_lock") merged, the accesses to the pci_cfg_wait queue
>>>>> are not safe anymore. This would cause kernel panic in a very low chance
>>>>> (See more detailed information from the below link). A "pci_lock" is
>>>>> insufficient and we need to hold an additional queue lock while read/write
>>>>> the wait queue.
>>>>>
>>>>> So let's use the add_wait_queue()/remove_wait_queue() instead of
>>>>> __add_wait_queue()/__remove_wait_queue(). Also move the wait queue
>>>>> functionality around the "schedule()" function to avoid reintroducing
>>>>> the deadlock addressed by "cdcb33f98244".
>>>>
>>>> I see that add_wait_queue() acquires the wq_head->lock, while
>>>> __add_wait_queue() does not.
>>>>
>>>> But I don't understand why the existing pci_lock is insufficient.  
>>>> pci_cfg_wait is only used in pci_wait_cfg() and
>>>> pci_cfg_access_unlock().
>>>>
>>>> In pci_wait_cfg(), both __add_wait_queue() and __remove_wait_queue()
>>>> are called while holding pci_lock, so that doesn't seem like the
>>>> problem.
>>>>
>>>> In pci_cfg_access_unlock(), we have:
>>>>
>>>>   pci_cfg_access_unlock
>>>>     wake_up_all(&pci_cfg_wait)
>>>>       __wake_up(&pci_cfg_wait, ...)
>>>>         __wake_up_common_lock(&pci_cfg_wait, ...)
>>>> 	  spin_lock(&pci_cfg_wait->lock)
>>>> 	  __wake_up_common(&pci_cfg_wait, ...)
>>>> 	    list_for_each_entry_safe_from(...)
>>>> 	      list_add_tail(...)                <-- problem?
>>>> 	  spin_unlock(&pci_cfg_wait->lock)
>>>>
>>>> Is the problem that the wake_up_all() modifies the pci_cfg_wait list
>>>> without holding pci_lock?
>>>>
>>>> If so, I don't quite see how the patch below fixes it.  Oh, wait,
>>>> maybe I do ... by using add_wait_queue(), we protect the list using
>>>> the *same* lock used by __wake_up_common_lock.  Is that it?
>>>
>>> Any reaction to the following?  Certainly not as optimized, but also a
>>> little less magic and more in the mainstream of wait_event/wake_up
>>> usage.
>>>
>>> I don't claim any real wait queue knowledge and haven't tested it.
>>> There are only a handful of __add_wait_queue() users compared with
>>> over 1600 users of wait_event() and variants, and I don't like being
>>> such a special case.
>>
>> I think the following patch is OK, even though I prefer mine. :)
> 
> Possibility A:
> 
>         do {
>                 set_current_state(TASK_UNINTERRUPTIBLE);
>                 raw_spin_unlock_irq(&pci_lock);
>                 add_wait_queue(&pci_cfg_wait, &wait);
>                 schedule();
>                 remove_wait_queue(&pci_cfg_wait, &wait);
>                 raw_spin_lock_irq(&pci_lock);
>         } while (dev->block_cfg_access);
> 
> Possibility B:
> 
>         do {
>                 raw_spin_unlock_irq(&pci_lock);
>                 wait_event(pci_cfg_wait, !dev->block_cfg_access);
>                 raw_spin_lock_irq(&pci_lock);
>         } while (dev->block_cfg_access);
> 
> I think both ways probably work.  
> 
> I prefer B because there's less chance for error -- it requires less
> knowledge of the internals of wait/wake_up and we don't have to worry
> about the ordering of set_current_state(), raw_spin_unlock_irq(),
> add_wait_queue(), schedule(), and remove_wait_queue().
> 
> I really don't know much about wait queues, so I'm interested in why
> you prefer A.
> 

Hmm...I also think B is much better than A as you describe above.
I'am not sure that whether "dev->block_cfg_access" is safe, at least
the "do{...}while" cannot be removed.

>> I can test your patch on my testcase(with hacked 300ms delay before
>> "curr->func" in __wake_up_common()). And if James has more efficient
>> testcase or measure for this problem, then go with James.
> 
> That would be great, thank you!  Let me know how it goes.

I need to make some hacking codes to test your patch, some like:

--- a/drivers/pci/access.c
+++ b/drivers/pci/access.c
@@ -206,19 +206,12 @@ static DECLARE_WAIT_QUEUE_HEAD(pci_cfg_wait);

 static noinline void pci_wait_cfg(struct pci_dev *dev)
 {
-       DECLARE_WAITQUEUE(wait, current);
-       wait.flags = WQ_FLAG_PCI;
-
-       __add_wait_queue(&pci_cfg_wait, &wait);
        do {
                set_current_state(TASK_UNINTERRUPTIBLE);
                raw_spin_unlock_irq(&pci_lock);
-               schedule();
+               wait_event_flags(pci_cfg_wait, !dev->block_cfg_access, WQ_FLAG_PCI);
                raw_spin_lock_irq(&pci_lock);
        } while (dev->block_cfg_access);
-       __remove_wait_queue(&pci_cfg_wait, &wait);
 }
--- a/kernel/sched/wait.c
+++ b/kernel/sched/wait.c
@@ -4,8 +4,12 @@
  *
  * (C) 2004 Nadia Yvette Chambers, Oracle
  */
+#include <linux/delay.h>
+
 #include "sched.h"

+unsigned long wake_up_delay_ms;
+
 void __init_waitqueue_head(struct wait_queue_head *wq_head, const char *name, struct lock_class_key *k
ey)
 {
        spin_lock_init(&wq_head->lock);
@@ -90,6 +94,10 @@ static int __wake_up_common(struct wait_queue_head *wq_head, unsigned int mode,
                if (flags & WQ_FLAG_BOOKMARK)
                        continue;

+               if (flags & WQ_FLAG_PCI && wake_up_delay_ms) {
+                       mdelay(wake_up_delay_ms);
+               }
+
                ret = curr->func(curr, mode, wake_flags, key);
                if (ret < 0)
                        break;


I tested it both on 4.19+ and mainline(5.8.0-rc3+). It's much difficult
to reproduce the kernel panic on mainline(I don't know why).

Anyway, all is well with your patch.

Tested-by: Xiang Zheng <zhengxiang9@xxxxxxxxxx>

> 
>>> diff --git a/drivers/pci/access.c b/drivers/pci/access.c
>>> index 79c4a2ef269a..7c2222bddbff 100644
>>> --- a/drivers/pci/access.c
>>> +++ b/drivers/pci/access.c
>>> @@ -205,16 +205,11 @@ static DECLARE_WAIT_QUEUE_HEAD(pci_cfg_wait);
>>>  
>>>  static noinline void pci_wait_cfg(struct pci_dev *dev)
>>>  {
>>> -	DECLARE_WAITQUEUE(wait, current);
>>> -
>>> -	__add_wait_queue(&pci_cfg_wait, &wait);
>>>  	do {
>>> -		set_current_state(TASK_UNINTERRUPTIBLE);
>>>  		raw_spin_unlock_irq(&pci_lock);
>>> -		schedule();
>>> +		wait_event(pci_cfg_wait, !dev->block_cfg_access);
>>>  		raw_spin_lock_irq(&pci_lock);
>>>  	} while (dev->block_cfg_access);
>>> -	__remove_wait_queue(&pci_cfg_wait, &wait);
>>>  }
>>>  
>>>  /* Returns 0 on success, negative values indicate error. */
>>>
>>> .
>>>
>>
>> -- 
>> Thanks,
>> Xiang
>>
> 
> .
> 

-- 
Thanks,
Xiang