FS-Cache: Permit cache retrieval ops to be interrupted in the initial wait phase

Permit the operations to retrieve data from the cache or to allocate space in
the cache for future writes to be interrupted whilst they're waiting for
permission for the operation to proceed.  Typically this wait occurs whilst the
cache object is being looked up on disk in the background.

If an interruption occurs, and the operation has not yet been given the
go-ahead to run, the operation is dequeued and cancelled, and control returns
to the read operation of the netfs routine with none of the requested pages
having been read or in any way marked as known by the cache.

This means that the initial wait is done interruptibly rather than
uninterruptibly.

In addition, extra stats values are made available to show the number of ops
cancelled and the number of cache space allocations interrupted.

Signed-off-by: David Howells <dhowells@redhat.com>

1 files changed, 54 insertions, 28 deletions

diff --git a/fs/fscache/operation.c b/fs/fscache/operation.c
index 09e43b6e822f..296492efb81b 100644
--- a/fs/fscache/operation.c
+++ b/fs/fscache/operation.c
@@ -34,32 +34,31 @@ void fscache_enqueue_operation(struct fscache_operation *op)
 
         fscache_set_op_state(op, "EnQ");
 
+        ASSERT(list_empty(&op->pend_link));
         ASSERT(op->processor != NULL);
         ASSERTCMP(op->object->state, >=, FSCACHE_OBJECT_AVAILABLE);
         ASSERTCMP(atomic_read(&op->usage), >, 0);
 
-        if (list_empty(&op->pend_link)) {
-                switch (op->flags & FSCACHE_OP_TYPE) {
-                case FSCACHE_OP_FAST:
-                        _debug("queue fast");
-                        atomic_inc(&op->usage);
-                        if (!schedule_work(&op->fast_work))
-                                fscache_put_operation(op);
-                        break;
-                case FSCACHE_OP_SLOW:
-                        _debug("queue slow");
-                        slow_work_enqueue(&op->slow_work);
-                        break;
-                case FSCACHE_OP_MYTHREAD:
-                        _debug("queue for caller's attention");
-                        break;
-                default:
-                        printk(KERN_ERR "FS-Cache: Unexpected op type %lx",
-                               op->flags);
-                        BUG();
-                        break;
-                }
-                fscache_stat(&fscache_n_op_enqueue);
+        fscache_stat(&fscache_n_op_enqueue);
+        switch (op->flags & FSCACHE_OP_TYPE) {
+        case FSCACHE_OP_FAST:
+                _debug("queue fast");
+                atomic_inc(&op->usage);
+                if (!schedule_work(&op->fast_work))
+                        fscache_put_operation(op);
+                break;
+        case FSCACHE_OP_SLOW:
+                _debug("queue slow");
+                slow_work_enqueue(&op->slow_work);
+                break;
+        case FSCACHE_OP_MYTHREAD:
+                _debug("queue for caller's attention");
+                break;
+        default:
+                printk(KERN_ERR "FS-Cache: Unexpected op type %lx",
+                       op->flags);
+                BUG();
+                break;
         }
 }
 EXPORT_SYMBOL(fscache_enqueue_operation);
@@ -97,6 +96,7 @@ int fscache_submit_exclusive_op(struct fscache_object *object,
         spin_lock(&object->lock);
         ASSERTCMP(object->n_ops, >=, object->n_in_progress);
         ASSERTCMP(object->n_ops, >=, object->n_exclusive);
+        ASSERT(list_empty(&op->pend_link));
 
         ret = -ENOBUFS;
         if (fscache_object_is_active(object)) {
@@ -202,6 +202,7 @@ int fscache_submit_op(struct fscache_object *object,
         spin_lock(&object->lock);
         ASSERTCMP(object->n_ops, >=, object->n_in_progress);
         ASSERTCMP(object->n_ops, >=, object->n_exclusive);
+        ASSERT(list_empty(&op->pend_link));
 
         ostate = object->state;
         smp_rmb();
@@ -273,12 +274,7 @@ void fscache_start_operations(struct fscache_object *object)
                         stop = true;
                 }
                 list_del_init(&op->pend_link);
-                object->n_in_progress++;
-
-                if (test_and_clear_bit(FSCACHE_OP_WAITING, &op->flags))
-                        wake_up_bit(&op->flags, FSCACHE_OP_WAITING);
-                if (op->processor)
-                        fscache_enqueue_operation(op);
+                fscache_run_op(object, op);
 
                 /* the pending queue was holding a ref on the object */
                 fscache_put_operation(op);
@@ -291,6 +287,36 @@ void fscache_start_operations(struct fscache_object *object)
 }
 
 /*
+ * cancel an operation that's pending on an object
+ */
+int fscache_cancel_op(struct fscache_operation *op)
+{
+        struct fscache_object *object = op->object;
+        int ret;
+
+        _enter("OBJ%x OP%x}", op->object->debug_id, op->debug_id);
+
+        spin_lock(&object->lock);
+
+        ret = -EBUSY;
+        if (!list_empty(&op->pend_link)) {
+                fscache_stat(&fscache_n_op_cancelled);
+                list_del_init(&op->pend_link);
+                object->n_ops--;
+                if (test_bit(FSCACHE_OP_EXCLUSIVE, &op->flags))
+                        object->n_exclusive--;
+                if (test_and_clear_bit(FSCACHE_OP_WAITING, &op->flags))
+                        wake_up_bit(&op->flags, FSCACHE_OP_WAITING);
+                fscache_put_operation(op);
+                ret = 0;
+        }
+
+        spin_unlock(&object->lock);
+        _leave(" = %d", ret);
+        return ret;
+}
+
+/*
  * release an operation
  * - queues pending ops if this is the last in-progress op
  */