scsi: zfcp: implicitly refresh port-data diagnostics when reading sysfs

This patch adds implicit updates to the sysfs entries that read the
diagnostic data stored in the "caching buffer" for Exchange Port Data.

An update is triggered once the buffer is older than ZFCP_DIAG_MAX_AGE
milliseconds (5s). This entails sending an Exchange Port Data command to
the FCP-Channel, and during its ingress path updating the cached data and
the timestamp. To prevent multiple concurrent userspace-applications from
triggering this update in parallel we synchronize all of them using a
wait-queue (waiting threads are interruptible; the updating thread is not).

Link: https://lore.kernel.org/r/c145b5cfc99a63b6a018b1184fbd27bb09c955f5.1572018132.git.bblock@linux.ibm.com
Reviewed-by: Steffen Maier <maier@linux.ibm.com>
Signed-off-by: Benjamin Block <bblock@linux.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/s390/scsi/zfcp_diag.c

@@ -22,6 +22,8 @@
 /* Max age of data in a diagnostics buffer before it needs a refresh (in ms). */
 #define ZFCP_DIAG_MAX_AGE (5 * 1000)
 
+static DECLARE_WAIT_QUEUE_HEAD(__zfcp_diag_publish_wait);
+
 /**
  * zfcp_diag_adapter_setup() - Setup storage for adapter diagnostics.
  * @adapter: the adapter to setup diagnostics for.
@@ -143,3 +145,130 @@ void zfcp_diag_update_xdata(struct zfcp_diag_header *const hdr,
 out:
 	spin_unlock_irqrestore(&hdr->access_lock, flags);
 }
+
+/**
+ * zfcp_diag_update_port_data_buffer() - Implementation of
+ *					 &typedef zfcp_diag_update_buffer_func
+ *					 to collect and update Port Data.
+ * @adapter: Adapter to collect Port Data from.
+ *
+ * This call is SYNCHRONOUS ! It blocks till the respective command has
+ * finished completely, or has failed in some way.
+ *
+ * Return:
+ * * 0		- Successfully retrieved new Diagnostics and Updated the buffer;
+ *		  this also includes cases where data was retrieved, but
+ *		  incomplete; you'll have to check the flag ``incomplete``
+ *		  of &struct zfcp_diag_header.
+ * * see zfcp_fsf_exchange_port_data_sync() for possible error-codes (
+ *   excluding -EAGAIN)
+ */
+int zfcp_diag_update_port_data_buffer(struct zfcp_adapter *const adapter)
+{
+	int rc;
+
+	rc = zfcp_fsf_exchange_port_data_sync(adapter->qdio, NULL);
+	if (rc == -EAGAIN)
+		rc = 0; /* signaling incomplete via struct zfcp_diag_header */
+
+	/* buffer-data was updated in zfcp_fsf_exchange_port_data_handler() */
+
+	return rc;
+}
+
+static int __zfcp_diag_update_buffer(struct zfcp_adapter *const adapter,
+				     struct zfcp_diag_header *const hdr,
+				     zfcp_diag_update_buffer_func buffer_update,
+				     unsigned long *const flags)
+	__must_hold(hdr->access_lock)
+{
+	int rc;
+
+	if (hdr->updating == 1) {
+		rc = wait_event_interruptible_lock_irq(__zfcp_diag_publish_wait,
+						       hdr->updating == 0,
+						       hdr->access_lock);
+		rc = (rc == 0 ? -EAGAIN : -EINTR);
+	} else {
+		hdr->updating = 1;
+		spin_unlock_irqrestore(&hdr->access_lock, *flags);
+
+		/* unlocked, because update function sleeps */
+		rc = buffer_update(adapter);
+
+		spin_lock_irqsave(&hdr->access_lock, *flags);
+		hdr->updating = 0;
+
+		/*
+		 * every thread waiting here went via an interruptible wait,
+		 * so its fine to only wake those
+		 */
+		wake_up_interruptible_all(&__zfcp_diag_publish_wait);
+	}
+
+	return rc;
+}
+
+static bool
+__zfcp_diag_test_buffer_age_isfresh(const struct zfcp_diag_header *const hdr)
+	__must_hold(hdr->access_lock)
+{
+	const unsigned long now = jiffies;
+
+	/*
+	 * Should not happen (data is from the future).. if it does, still
+	 * signal that it needs refresh
+	 */
+	if (!time_after_eq(now, hdr->timestamp))
+		return false;
+
+	if (jiffies_to_msecs(now - hdr->timestamp) >= ZFCP_DIAG_MAX_AGE)
+		return false;
+
+	return true;
+}
+
+/**
+ * zfcp_diag_update_buffer_limited() - Collect diagnostics and update a
+ *				       diagnostics buffer rate limited.
+ * @adapter: Adapter to collect the diagnostics from.
+ * @hdr: buffer-header for which to update with the collected diagnostics.
+ * @buffer_update: Specific implementation for collecting and updating.
+ *
+ * This function will cause an update of the given @hdr by calling the also
+ * given @buffer_update function. If called by multiple sources at the same
+ * time, it will synchornize the update by only allowing one source to call
+ * @buffer_update and the others to wait for that source to complete instead
+ * (the wait is interruptible).
+ *
+ * Additionally this version is rate-limited and will only exit if either the
+ * buffer is fresh enough (within the limit) - it will do nothing if the buffer
+ * is fresh enough to begin with -, or if the source/thread that started this
+ * update is the one that made the update (to prevent endless loops).
+ *
+ * Return:
+ * * 0		- If the update was successfully published and/or the buffer is
+ *		  fresh enough
+ * * -EINTR	- If the thread went into the wait-state and was interrupted
+ * * whatever @buffer_update returns
+ */
+int zfcp_diag_update_buffer_limited(struct zfcp_adapter *const adapter,
+				    struct zfcp_diag_header *const hdr,
+				    zfcp_diag_update_buffer_func buffer_update)
+{
+	unsigned long flags;
+	int rc;
+
+	spin_lock_irqsave(&hdr->access_lock, flags);
+
+	for (rc = 0; !__zfcp_diag_test_buffer_age_isfresh(hdr); rc = 0) {
+		rc = __zfcp_diag_update_buffer(adapter, hdr, buffer_update,
+					       &flags);
+		if (rc != -EAGAIN)
+			break;
+	}
+
+	spin_unlock_irqrestore(&hdr->access_lock, flags);
+
+	return rc;
+}

drivers/s390/scsi/zfcp_diag.h

@@ -71,6 +71,17 @@ void zfcp_diag_sysfs_destroy(struct zfcp_adapter *const adapter);
 void zfcp_diag_update_xdata(struct zfcp_diag_header *const hdr,
 			    const void *const data, const bool incomplete);
 
+/*
+ * Function-Type used in zfcp_diag_update_buffer_limited() for the function
+ * that does the buffer-implementation dependent work.
+ */
+typedef int (*zfcp_diag_update_buffer_func)(struct zfcp_adapter *const adapter);
+
+int zfcp_diag_update_port_data_buffer(struct zfcp_adapter *const adapter);
+int zfcp_diag_update_buffer_limited(struct zfcp_adapter *const adapter,
+				    struct zfcp_diag_header *const hdr,
+				    zfcp_diag_update_buffer_func buffer_update);
+
 /**
  * zfcp_diag_support_sfp() - Return %true if the @adapter supports reporting
  *			     SFP Data.

drivers/s390/scsi/zfcp_sysfs.c

@@ -693,6 +693,11 @@ struct device_attribute *zfcp_sysfs_shost_attrs[] = {
 									       \
 		diag_hdr = &adapter->diagnostics->port_data.header;	       \
 									       \
+		rc = zfcp_diag_update_buffer_limited(			       \
+			adapter, diag_hdr, zfcp_diag_update_port_data_buffer); \
+		if (rc != 0)						       \
+			goto out;					       \
+									       \
 		spin_lock_irqsave(&diag_hdr->access_lock, flags);	       \
 		rc = scnprintf(						       \
 			buf, (_prtsize) + 2, _prtfmt "\n",		       \