Orangefs: update orangefs.txt

Al Viro has cleaned up the way ops are processed and waited for,
now orangefs.txt has an overview of how it works. Several recent
related commits have added to the comments in the code as well.

Signed-off-by: Mike Marshall <[e-mail]>

Documentation/filesystems/orangefs.txt

···
 
 The slab allocator is used to keep a cache of op structures handy.
 
-The life cycle of a typical op goes like this:
+At init time the kernel module defines and initializes a request list
+and an in_progress hash table to keep track of all the ops that are
+in flight at any given time.
 
-  - obtain and initialize an op structure from the op_cache.
+Ops are stateful:
 
-  - queue the op to the pvfs device so that its upcall data can be
-    read by userspace.
+ * unknown  - op was just initialized
+ * waiting  - op is on request_list (upward bound)
+ * inprogr  - op is in progress (waiting for downcall)
+ * serviced - op has matching downcall; ok
+ * purged   - op has to start a timer since client-core
+              exited uncleanly before servicing op
+ * given up - submitter has given up waiting for it
 
-  - wait for userspace to write downcall data back to the pvfs device.
+When some arbitrary userspace program needs to perform a
+filesystem operation on Orangefs (readdir, I/O, create, whatever)
+an op structure is initialized and tagged with a distinguishing ID
+number. The upcall part of the op is filled out, and the op is
+passed to the "service_operation" function.
 
-  - consume the downcall and return the op struct to the op_cache.
+Service_operation changes the op's state to "waiting", puts
+it on the request list, and signals the Orangefs file_operations.poll
+function through a wait queue. Userspace is polling the pseudo-device
+and thus becomes aware of the upcall request that needs to be read.
 
-Some ops are atypical with respect to their payloads: readdir and io ops.
+When the Orangefs file_operations.read function is triggered, the
+request list is searched for an op that seems ready-to-process.
+The op is removed from the request list. The tag from the op and
+the filled-out upcall struct are copy_to_user'ed back to userspace.
+
+If any of these (and some additional protocol) copy_to_users fail,
+the op's state is set to "waiting" and the op is added back to
+the request list. Otherwise, the op's state is changed to "in progress",
+and the op is hashed on its tag and put onto the end of a list in the
+in_progress hash table at the index the tag hashed to.
+
+When userspace has assembled the response to the upcall, it
+writes the response, which includes the distinguishing tag, back to
+the pseudo device in a series of io_vecs. This triggers the Orangefs
+file_operations.write_iter function to find the op with the associated
+tag and remove it from the in_progress hash table. As long as the op's
+state is not "canceled" or "given up", its state is set to "serviced".
+The file_operations.write_iter function returns to the waiting vfs,
+and back to service_operation through wait_for_matching_downcall.
+
+Service operation returns to its caller with the op's downcall
+part (the response to the upcall) filled out.
+
+The "client-core" is the bridge between the kernel module and
+userspace. The client-core is a daemon. The client-core has an
+associated watchdog daemon. If the client-core is ever signaled
+to die, the watchdog daemon restarts the client-core. Even though
+the client-core is restarted "right away", there is a period of
+time during such an event that the client-core is dead. A dead client-core
+can't be triggered by the Orangefs file_operations.poll function.
+Ops that pass through service_operation during a "dead spell" can timeout
+on the wait queue and one attempt is made to recycle them. Obviously,
+if the client-core stays dead too long, the arbitrary userspace processes
+trying to use Orangefs will be negatively affected. Waiting ops
+that can't be serviced will be removed from the request list and
+have their states set to "given up". In-progress ops that can't 
+be serviced will be removed from the in_progress hash table and
+have their states set to "given up".
+
+Readdir and I/O ops are atypical with respect to their payloads.
 
   - readdir ops use the smaller of the two pre-allocated pre-partitioned
     memory buffers. The readdir buffer is only available to userspace.
···
     jamb everything needed to represent a pvfs2_readdir_response_t into
     the readdir buffer descriptor specified in the upcall.
 
-writev() on /dev/pvfs2-req is used to pass responses to the requests
+Userspace uses writev() on /dev/pvfs2-req to pass responses to the requests
 made by the kernel side.
 
 A buffer_list containing:

fs/orangefs/devorangefs-req.c

···
 
 wakeup:
 	/*
-	 * tell the vfs op waiting on a waitqueue
-	 * that this op is done
+	 * Return to vfs waitqueue, and back to service_operation
+	 * through wait_for_matching_downcall. 
 	 */
 	spin_lock(&op->lock);
 	if (unlikely(op_is_cancel(op))) {