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

[Xiang Zheng]

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".

Signed-off-by: Xiang Zheng <zhengxiang9@xxxxxxxxxx>
Cc: Heyi Guo <guoheyi@xxxxxxxxxx>
Cc: Biaoxiang Ye <yebiaoxiang@xxxxxxxxxx>
Link: https://lore.kernel.org/linux-pci/79827f2f-9b43-4411-1376-b9063b67aee3@xxxxxxxxxx/
---

v3:
  Improve the commit subject and message.

v2:
  Move the wait queue functionality around the "schedule()".

---
 drivers/pci/access.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/pci/access.c b/drivers/pci/access.c
index 2fccb5762c76..09342a74e5ea 100644
--- a/drivers/pci/access.c
+++ b/drivers/pci/access.c
@@ -207,14 +207,14 @@ 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);
+		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);
-	__remove_wait_queue(&pci_cfg_wait, &wait);
 }
 
 /* Returns 0 on success, negative values indicate error. */
-- 
2.19.1