s390/pci: introduce lock to synchronize state of zpci_dev's

There's a number of tasks that need the state of a zpci device
to be stable. Other tasks need to be synchronized as they change the state.

State changes could be generated by the system as availability or error
events, or be requested by the user through manipulations in sysfs.
Some other actions accessible through sysfs - like device resets - need the
state to be stable.

Unsynchronized state handling could lead to unusable devices. This has
been observed in cases of concurrent state changes through systemd udev
rules and DPM boot control. Some breakage can be provoked by artificial
tests, e.g. through repetitively injecting "recover" on a PCI function
through sysfs while running a "hotplug remove/add" in a loop through a
PCI slot's "power" attribute in sysfs. After a few iterations this could
result in a kernel oops.

So introduce a new mutex "state_lock" to guard the state property of the
struct zpci_dev. Acquire this lock in all task that modify the state:

- hotplug add and remove, through the PCI hotplug slot entry,
- avaiability events, as reported by the platform,
- error events, as reported by the platform,
- during device resets, explicit through sysfs requests or
implict through the common PCI layer.

Break out an inner _do_recover() routine out of recover_store() to
separte the necessary synchronizations from the actual manipulations of
the zpci_dev required for the reset.

With the following changes I was able to run the inject loops for hours
without hitting an error.

Signed-off-by: Gerd Bayer <[e-mail]>
Reviewed-by: Niklas Schnelle <[e-mail]>
Signed-off-by: Heiko Carstens <[e-mail]>

arch/s390/include/asm/pci.h

···
 	struct rcu_head rcu;
 	struct hotplug_slot hotplug_slot;
 
+	struct mutex state_lock;	/* protect state changes */
 	enum zpci_state state;
 	u32		fid;		/* function ID, used by sclp */
 	u32		fh;		/* function handle, used by insn's */

arch/s390/pci/pci.c

···
 #include <linux/jump_label.h>
 #include <linux/pci.h>
 #include <linux/printk.h>
+#include <linux/lockdep.h>
 
 #include <asm/isc.h>
 #include <asm/airq.h>
···
  * equivalent to its state during boot when first probing a driver.
  * Consequently after reset the PCI function requires re-initialization via the
  * common PCI code including re-enabling IRQs via pci_alloc_irq_vectors()
- * and enabling the function via e.g.pci_enablde_device_flags().The caller
+ * and enabling the function via e.g. pci_enable_device_flags(). The caller
  * must guard against concurrent reset attempts.
  *
  * In most cases this function should not be called directly but through
  * pci_reset_function() or pci_reset_bus() which handle the save/restore and
- * locking.
+ * locking - asserted by lockdep.
  *
  * Return: 0 on success and an error value otherwise
  */
···
 	u8 status;
 	int rc;
 
+	lockdep_assert_held(&zdev->state_lock);
 	zpci_dbg(3, "rst fid:%x, fh:%x\n", zdev->fid, zdev->fh);
 	if (zdev_enabled(zdev)) {
 		/* Disables device access, DMAs and IRQs (reset state) */
···
 	zdev->state =  state;
 
 	kref_init(&zdev->kref);
+	mutex_init(&zdev->state_lock);
 	mutex_init(&zdev->fmb_lock);
 	mutex_init(&zdev->kzdev_lock);
 
···
 int zpci_deconfigure_device(struct zpci_dev *zdev)
 {
 	int rc;
+
+	lockdep_assert_held(&zdev->state_lock);
+	if (zdev->state != ZPCI_FN_STATE_CONFIGURED)
+		return 0;
 
 	if (zdev->zbus->bus)
 		zpci_bus_remove_device(zdev, false);

arch/s390/pci/pci_event.c

···
 	zpci_err_hex(ccdf, sizeof(*ccdf));
 
 	if (zdev) {
+		mutex_lock(&zdev->state_lock);
 		zpci_update_fh(zdev, ccdf->fh);
 		if (zdev->zbus->bus)
 			pdev = pci_get_slot(zdev->zbus->bus, zdev->devfn);
···
 	}
 	pci_dev_put(pdev);
 no_pdev:
+	if (zdev)
+		mutex_unlock(&zdev->state_lock);
 	zpci_zdev_put(zdev);
 }
 
···
 
 	zpci_dbg(3, "avl fid:%x, fh:%x, pec:%x\n",
 		 ccdf->fid, ccdf->fh, ccdf->pec);
+
+	if (existing_zdev)
+		mutex_lock(&zdev->state_lock);
+
 	switch (ccdf->pec) {
 	case 0x0301: /* Reserved|Standby -> Configured */
 		if (!zdev) {
···
 	default:
 		break;
 	}
-	if (existing_zdev)
+	if (existing_zdev) {
+		mutex_unlock(&zdev->state_lock);
 		zpci_zdev_put(zdev);
+	}
 }
 
 void zpci_event_availability(void *data)

arch/s390/pci/pci_sysfs.c

···
 }
 static DEVICE_ATTR_RO(mio_enabled);
 
+static int _do_recover(struct pci_dev *pdev, struct zpci_dev *zdev)
+{
+	u8 status;
+	int ret;
+
+	pci_stop_and_remove_bus_device(pdev);
+	if (zdev_enabled(zdev)) {
+		ret = zpci_disable_device(zdev);
+		/*
+		 * Due to a z/VM vs LPAR inconsistency in the error
+		 * state the FH may indicate an enabled device but
+		 * disable says the device is already disabled don't
+		 * treat it as an error here.
+		 */
+		if (ret == -EINVAL)
+			ret = 0;
+		if (ret)
+			return ret;
+	}
+
+	ret = zpci_enable_device(zdev);
+	if (ret)
+		return ret;
+
+	if (zdev->dma_table) {
+		ret = zpci_register_ioat(zdev, 0, zdev->start_dma, zdev->end_dma,
+					 virt_to_phys(zdev->dma_table), &status);
+		if (ret)
+			zpci_disable_device(zdev);
+	}
+	return ret;
+}
+
 static ssize_t recover_store(struct device *dev, struct device_attribute *attr,
 			     const char *buf, size_t count)
 {
···
 	struct pci_dev *pdev = to_pci_dev(dev);
 	struct zpci_dev *zdev = to_zpci(pdev);
 	int ret = 0;
-	u8 status;
 
 	/* Can't use device_remove_self() here as that would lead us to lock
 	 * the pci_rescan_remove_lock while holding the device' kernfs lock.
···
 	 */
 	kn = sysfs_break_active_protection(&dev->kobj, &attr->attr);
 	WARN_ON_ONCE(!kn);
+
+	/* Device needs to be configured and state must not change */
+	mutex_lock(&zdev->state_lock);
+	if (zdev->state != ZPCI_FN_STATE_CONFIGURED)
+		goto out;
+
 	/* device_remove_file() serializes concurrent calls ignoring all but
 	 * the first
 	 */
···
 	 */
 	pci_lock_rescan_remove();
 	if (pci_dev_is_added(pdev)) {
-		pci_stop_and_remove_bus_device(pdev);
-		if (zdev_enabled(zdev)) {
-			ret = zpci_disable_device(zdev);
-			/*
-			 * Due to a z/VM vs LPAR inconsistency in the error
-			 * state the FH may indicate an enabled device but
-			 * disable says the device is already disabled don't
-			 * treat it as an error here.
-			 */
-			if (ret == -EINVAL)
-				ret = 0;
-			if (ret)
-				goto out;
-		}
-
-		ret = zpci_enable_device(zdev);
-		if (ret)
-			goto out;
-
-		if (zdev->dma_table) {
-			ret = zpci_register_ioat(zdev, 0, zdev->start_dma, zdev->end_dma,
-						 virt_to_phys(zdev->dma_table), &status);
-			if (ret)
-				zpci_disable_device(zdev);
-		}
+		ret = _do_recover(pdev, zdev);
 	}
-out:
 	pci_rescan_bus(zdev->zbus->bus);
 	pci_unlock_rescan_remove();
+
+out:
+	mutex_unlock(&zdev->state_lock);
 	if (kn)
 		sysfs_unbreak_active_protection(kn);
 	return ret ? ret : count;

drivers/pci/hotplug/s390_pci_hpc.c

···
 					     hotplug_slot);
 	int rc;
 
-	if (zdev->state != ZPCI_FN_STATE_STANDBY)
-		return -EIO;
+	mutex_lock(&zdev->state_lock);
+	if (zdev->state != ZPCI_FN_STATE_STANDBY) {
+		rc = -EIO;
+		goto out;
+	}
 
 	rc = sclp_pci_configure(zdev->fid);
 	zpci_dbg(3, "conf fid:%x, rc:%d\n", zdev->fid, rc);
 	if (rc)
-		return rc;
+		goto out;
 	zdev->state = ZPCI_FN_STATE_CONFIGURED;
 
-	return zpci_scan_configured_device(zdev, zdev->fh);
+	rc = zpci_scan_configured_device(zdev, zdev->fh);
+out:
+	mutex_unlock(&zdev->state_lock);
+	return rc;
 }
 
 static int disable_slot(struct hotplug_slot *hotplug_slot)
 {
 	struct zpci_dev *zdev = container_of(hotplug_slot, struct zpci_dev,
 					     hotplug_slot);
-	struct pci_dev *pdev;
+	struct pci_dev *pdev = NULL;
+	int rc;
 
-	if (zdev->state != ZPCI_FN_STATE_CONFIGURED)
-		return -EIO;
+	mutex_lock(&zdev->state_lock);
+	if (zdev->state != ZPCI_FN_STATE_CONFIGURED) {
+		rc = -EIO;
+		goto out;
+	}
 
 	pdev = pci_get_slot(zdev->zbus->bus, zdev->devfn);
 	if (pdev && pci_num_vf(pdev)) {
 		pci_dev_put(pdev);
-		return -EBUSY;
+		rc = -EBUSY;
+		goto out;
 	}
-	pci_dev_put(pdev);
 
-	return zpci_deconfigure_device(zdev);
+	rc = zpci_deconfigure_device(zdev);
+out:
+	mutex_unlock(&zdev->state_lock);
+	if (pdev)
+		pci_dev_put(pdev);
+	return rc;
 }
 
 static int reset_slot(struct hotplug_slot *hotplug_slot, bool probe)
 {
 	struct zpci_dev *zdev = container_of(hotplug_slot, struct zpci_dev,
 					     hotplug_slot);
+	int rc = -EIO;
 
-	if (zdev->state != ZPCI_FN_STATE_CONFIGURED)
-		return -EIO;
 	/*
-	 * We can't take the zdev->lock as reset_slot may be called during
-	 * probing and/or device removal which already happens under the
-	 * zdev->lock. Instead the user should use the higher level
-	 * pci_reset_function() or pci_bus_reset() which hold the PCI device
-	 * lock preventing concurrent removal. If not using these functions
-	 * holding the PCI device lock is required.
+	 * If we can't get the zdev->state_lock the device state is
+	 * currently undergoing a transition and we bail out - just
+	 * the same as if the device's state is not configured at all.
 	 */
+	if (!mutex_trylock(&zdev->state_lock))
+		return rc;
 
-	/* As long as the function is configured we can reset */
-	if (probe)
-		return 0;
+	/* We can reset only if the function is configured */
+	if (zdev->state != ZPCI_FN_STATE_CONFIGURED)
+		goto out;
 
-	return zpci_hot_reset_device(zdev);
+	if (probe) {
+		rc = 0;
+		goto out;
+	}
+
+	rc = zpci_hot_reset_device(zdev);
+out:
+	mutex_unlock(&zdev->state_lock);
+	return rc;
 }
 
 static int get_power_status(struct hotplug_slot *hotplug_slot, u8 *value)