Revert "PCI: Avoid race while enabling upstream bridges"

This reverts commit 40f11adc7cd9281227f0a6a627d966dd0a5f0cd9.

Jens found that iwlwifi firmware loading failed on a Lenovo X1 Carbon,
gen4:

iwlwifi 0000:04:00.0: Direct firmware load for iwlwifi-8000C-34.ucode failed with error -2
iwlwifi 0000:04:00.0: Direct firmware load for iwlwifi-8000C-33.ucode failed with error -2
iwlwifi 0000:04:00.0: Direct firmware load for iwlwifi-8000C-32.ucode failed with error -2
iwlwifi 0000:04:00.0: loaded firmware version 31.532993.0 op_mode iwlmvm
iwlwifi 0000:04:00.0: Detected Intel(R) Dual Band Wireless AC 8260, REV=0x208
...
iwlwifi 0000:04:00.0: Failed to load firmware chunk!
iwlwifi 0000:04:00.0: Could not load the [0] uCode section
iwlwifi 0000:04:00.0: Failed to start INIT ucode: -110
iwlwifi 0000:04:00.0: Failed to run INIT ucode: -110

He bisected it to 40f11adc7cd9 ("PCI: Avoid race while enabling upstream
bridges"). Revert that commit to fix the regression.

Link: http://lkml.kernel.org/r/[e-mail]
Fixes: 40f11adc7cd9 ("PCI: Avoid race while enabling upstream bridges")
Signed-off-by: Bjorn Helgaas <[e-mail]>
CC: Srinath Mannam <[e-mail]>
CC: Jens Axboe <[e-mail]>
CC: Luca Coelho <[e-mail]>
CC: Johannes Berg <[e-mail]>
CC: Emmanuel Grumbach <[e-mail]>

drivers/pci/pci.c

···
 static LIST_HEAD(pci_pme_list);
 static DEFINE_MUTEX(pci_pme_list_mutex);
 static DECLARE_DELAYED_WORK(pci_pme_work, pci_pme_list_scan);
-static DEFINE_MUTEX(pci_bridge_mutex);
 
 struct pci_pme_device {
 	struct list_head list;
···
 	if (bridge)
 		pci_enable_bridge(bridge);
 
-	/*
-	 * Hold pci_bridge_mutex to prevent a race when enabling two
-	 * devices below the bridge simultaneously.  The race may cause a
-	 * PCI_COMMAND_MEMORY update to be lost (see changelog).
-	 */
-	mutex_lock(&pci_bridge_mutex);
 	if (pci_is_enabled(dev)) {
 		if (!dev->is_busmaster)
 			pci_set_master(dev);
-		goto end;
+		return;
 	}
 
 	retval = pci_enable_device(dev);
···
 		dev_err(&dev->dev, "Error enabling bridge (%d), continuing\n",
 			retval);
 	pci_set_master(dev);
-end:
-	mutex_unlock(&pci_bridge_mutex);
 }
 
 static int pci_enable_device_flags(struct pci_dev *dev, unsigned long flags)
···
 		return 0;		/* already enabled */
 
 	bridge = pci_upstream_bridge(dev);
-	if (bridge && !pci_is_enabled(bridge))
+	if (bridge)
 		pci_enable_bridge(bridge);
 
 	/* only skip sriov related */