[PATCH review for 4.9 50/50] PCI: Avoid possible deadlock on pci_lock and p->pi_lock

[Levin, Alexander (Sasha Levin)]

From: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>

[ Upstream commit cdcb33f9824429a926b971bf041a6cec238f91ff ]

pci_lock is an IRQ-safe spinlock that protects all accesses to PCI
configuration space (see PCI_OP_READ() and PCI_OP_WRITE() in pci/access.c).

The pci_cfg_access_unlock() path acquires pci_lock, then p->pi_lock (inside
wake_up_all()).  According to lockdep, there is a possible path involving
snbep_uncore_pci_read_counter() that could acquire them in the reverse
order: acquiring p->pi_lock, then pci_lock, which could result in a
deadlock.  Lockdep details are in the bugzilla below.

Avoid the possible deadlock by dropping pci_lock before waking up any
config access waiters.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=192901
Signed-off-by: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxx>
---
 drivers/pci/access.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/access.c b/drivers/pci/access.c
index d11cdbb8fba3..7b5cf6d1181a 100644
--- a/drivers/pci/access.c
+++ b/drivers/pci/access.c
@@ -672,8 +672,9 @@ void pci_cfg_access_unlock(struct pci_dev *dev)
 	WARN_ON(!dev->block_cfg_access);
 
 	dev->block_cfg_access = 0;
-	wake_up_all(&pci_cfg_wait);
 	raw_spin_unlock_irqrestore(&pci_lock, flags);
+
+	wake_up_all(&pci_cfg_wait);
 }
 EXPORT_SYMBOL_GPL(pci_cfg_access_unlock);
 
-- 
2.11.0