--- /dev/null
+ ================================
+ ASYNCHRONOUS OPERATIONS HANDLING
+ ================================
+
+By: David Howells <dhowells@redhat.com>
+
+Contents:
+
+ (*) Overview.
+
+ (*) Operation record initialisation.
+
+ (*) Parameters.
+
+ (*) Procedure.
+
+ (*) Asynchronous callback.
+
+
+========
+OVERVIEW
+========
+
+FS-Cache has an asynchronous operations handling facility that it uses for its
+data storage and retrieval routines. Its operations are represented by
+fscache_operation structs, though these are usually embedded into some other
+structure.
+
+This facility is available to and expected to be be used by the cache backends,
+and FS-Cache will create operations and pass them off to the appropriate cache
+backend for completion.
+
+To make use of this facility, <linux/fscache-cache.h> should be #included.
+
+
+===============================
+OPERATION RECORD INITIALISATION
+===============================
+
+An operation is recorded in an fscache_operation struct:
+
+ struct fscache_operation {
+ union {
+ struct work_struct fast_work;
+ struct slow_work slow_work;
+ };
+ unsigned long flags;
+ fscache_operation_processor_t processor;
+ ...
+ };
+
+Someone wanting to issue an operation should allocate something with this
+struct embedded in it. They should initialise it by calling:
+
+ void fscache_operation_init(struct fscache_operation *op,
+ fscache_operation_release_t release);
+
+with the operation to be initialised and the release function to use.
+
+The op->flags parameter should be set to indicate the CPU time provision and
+the exclusivity (see the Parameters section).
+
+The op->fast_work, op->slow_work and op->processor flags should be set as
+appropriate for the CPU time provision (see the Parameters section).
+
+FSCACHE_OP_WAITING may be set in op->flags prior to each submission of the
+operation and waited for afterwards.
+
+
+==========
+PARAMETERS
+==========
+
+There are a number of parameters that can be set in the operation record's flag
+parameter. There are three options for the provision of CPU time in these
+operations:
+
+ (1) The operation may be done synchronously (FSCACHE_OP_MYTHREAD). A thread
+ may decide it wants to handle an operation itself without deferring it to
+ another thread.
+
+ This is, for example, used in read operations for calling readpages() on
+ the backing filesystem in CacheFiles. Although readpages() does an
+ asynchronous data fetch, the determination of whether pages exist is done
+ synchronously - and the netfs does not proceed until this has been
+ determined.
+
+ If this option is to be used, FSCACHE_OP_WAITING must be set in op->flags
+ before submitting the operation, and the operating thread must wait for it
+ to be cleared before proceeding:
+
+ wait_on_bit(&op->flags, FSCACHE_OP_WAITING,
+ fscache_wait_bit, TASK_UNINTERRUPTIBLE);
+
+
+ (2) The operation may be fast asynchronous (FSCACHE_OP_FAST), in which case it
+ will be given to keventd to process. Such an operation is not permitted
+ to sleep on I/O.
+
+ This is, for example, used by CacheFiles to copy data from a backing fs
+ page to a netfs page after the backing fs has read the page in.
+
+ If this option is used, op->fast_work and op->processor must be
+ initialised before submitting the operation:
+
+ INIT_WORK(&op->fast_work, do_some_work);
+
+
+ (3) The operation may be slow asynchronous (FSCACHE_OP_SLOW), in which case it
+ will be given to the slow work facility to process. Such an operation is
+ permitted to sleep on I/O.
+
+ This is, for example, used by FS-Cache to handle background writes of
+ pages that have just been fetched from a remote server.
+
+ If this option is used, op->slow_work and op->processor must be
+ initialised before submitting the operation:
+
+ fscache_operation_init_slow(op, processor)
+
+
+Furthermore, operations may be one of two types:
+
+ (1) Exclusive (FSCACHE_OP_EXCLUSIVE). Operations of this type may not run in
+ conjunction with any other operation on the object being operated upon.
+
+ An example of this is the attribute change operation, in which the file
+ being written to may need truncation.
+
+ (2) Shareable. Operations of this type may be running simultaneously. It's
+ up to the operation implementation to prevent interference between other
+ operations running at the same time.
+
+
+=========
+PROCEDURE
+=========
+
+Operations are used through the following procedure:
+
+ (1) The submitting thread must allocate the operation and initialise it
+ itself. Normally this would be part of a more specific structure with the
+ generic op embedded within.
+
+ (2) The submitting thread must then submit the operation for processing using
+ one of the following two functions:
+
+ int fscache_submit_op(struct fscache_object *object,
+ struct fscache_operation *op);
+
+ int fscache_submit_exclusive_op(struct fscache_object *object,
+ struct fscache_operation *op);
+
+ The first function should be used to submit non-exclusive ops and the
+ second to submit exclusive ones. The caller must still set the
+ FSCACHE_OP_EXCLUSIVE flag.
+
+ If successful, both functions will assign the operation to the specified
+ object and return 0. -ENOBUFS will be returned if the object specified is
+ permanently unavailable.
+
+ The operation manager will defer operations on an object that is still
+ undergoing lookup or creation. The operation will also be deferred if an
+ operation of conflicting exclusivity is in progress on the object.
+
+ If the operation is asynchronous, the manager will retain a reference to
+ it, so the caller should put their reference to it by passing it to:
+
+ void fscache_put_operation(struct fscache_operation *op);
+
+ (3) If the submitting thread wants to do the work itself, and has marked the
+ operation with FSCACHE_OP_MYTHREAD, then it should monitor
+ FSCACHE_OP_WAITING as described above and check the state of the object if
+ necessary (the object might have died whilst the thread was waiting).
+
+ When it has finished doing its processing, it should call
+ fscache_put_operation() on it.
+
+ (4) The operation holds an effective lock upon the object, preventing other
+ exclusive ops conflicting until it is released. The operation can be
+ enqueued for further immediate asynchronous processing by adjusting the
+ CPU time provisioning option if necessary, eg:
+
+ op->flags &= ~FSCACHE_OP_TYPE;
+ op->flags |= ~FSCACHE_OP_FAST;
+
+ and calling:
+
+ void fscache_enqueue_operation(struct fscache_operation *op)
+
+ This can be used to allow other things to have use of the worker thread
+ pools.
+
+
+=====================
+ASYNCHRONOUS CALLBACK
+=====================
+
+When used in asynchronous mode, the worker thread pool will invoke the
+processor method with a pointer to the operation. This should then get at the
+container struct by using container_of():
+
+ static void fscache_write_op(struct fscache_operation *_op)
+ {
+ struct fscache_storage *op =
+ container_of(_op, struct fscache_storage, op);
+ ...
+ }
+
+The caller holds a reference on the operation, and will invoke
+fscache_put_operation() when the processor function returns. The processor
+function is at liberty to call fscache_enqueue_operation() or to take extra
+references.
--- /dev/null
+/* FS-Cache worker operation management routines
+ *
+ * Copyright (C) 2008 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version
+ * 2 of the License, or (at your option) any later version.
+ *
+ * See Documentation/filesystems/caching/operations.txt
+ */
+
+#define FSCACHE_DEBUG_LEVEL OPERATION
+#include <linux/module.h>
+#include "internal.h"
+
+atomic_t fscache_op_debug_id;
+EXPORT_SYMBOL(fscache_op_debug_id);
+
+/**
+ * fscache_enqueue_operation - Enqueue an operation for processing
+ * @op: The operation to enqueue
+ *
+ * Enqueue an operation for processing by the FS-Cache thread pool.
+ *
+ * This will get its own ref on the object.
+ */
+void fscache_enqueue_operation(struct fscache_operation *op)
+{
+ _enter("{OBJ%x OP%x,%u}",
+ op->object->debug_id, op->debug_id, atomic_read(&op->usage));
+
+ 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);
+ }
+}
+EXPORT_SYMBOL(fscache_enqueue_operation);
+
+/*
+ * start an op running
+ */
+static void fscache_run_op(struct fscache_object *object,
+ struct fscache_operation *op)
+{
+ 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_stat(&fscache_n_op_run);
+}
+
+/*
+ * submit an exclusive operation for an object
+ * - other ops are excluded from running simultaneously with this one
+ * - this gets any extra refs it needs on an op
+ */
+int fscache_submit_exclusive_op(struct fscache_object *object,
+ struct fscache_operation *op)
+{
+ int ret;
+
+ _enter("{OBJ%x OP%x},", object->debug_id, op->debug_id);
+
+ spin_lock(&object->lock);
+ ASSERTCMP(object->n_ops, >=, object->n_in_progress);
+ ASSERTCMP(object->n_ops, >=, object->n_exclusive);
+
+ ret = -ENOBUFS;
+ if (fscache_object_is_active(object)) {
+ op->object = object;
+ object->n_ops++;
+ object->n_exclusive++; /* reads and writes must wait */
+
+ if (object->n_ops > 0) {
+ atomic_inc(&op->usage);
+ list_add_tail(&op->pend_link, &object->pending_ops);
+ fscache_stat(&fscache_n_op_pend);
+ } else if (!list_empty(&object->pending_ops)) {
+ atomic_inc(&op->usage);
+ list_add_tail(&op->pend_link, &object->pending_ops);
+ fscache_stat(&fscache_n_op_pend);
+ fscache_start_operations(object);
+ } else {
+ ASSERTCMP(object->n_in_progress, ==, 0);
+ fscache_run_op(object, op);
+ }
+
+ /* need to issue a new write op after this */
+ clear_bit(FSCACHE_OBJECT_PENDING_WRITE, &object->flags);
+ ret = 0;
+ } else if (object->state == FSCACHE_OBJECT_CREATING) {
+ op->object = object;
+ object->n_ops++;
+ object->n_exclusive++; /* reads and writes must wait */
+ atomic_inc(&op->usage);
+ list_add_tail(&op->pend_link, &object->pending_ops);
+ fscache_stat(&fscache_n_op_pend);
+ ret = 0;
+ } else {
+ /* not allowed to submit ops in any other state */
+ BUG();
+ }
+
+ spin_unlock(&object->lock);
+ return ret;
+}
+
+/*
+ * report an unexpected submission
+ */
+static void fscache_report_unexpected_submission(struct fscache_object *object,
+ struct fscache_operation *op,
+ unsigned long ostate)
+{
+ static bool once_only;
+ struct fscache_operation *p;
+ unsigned n;
+
+ if (once_only)
+ return;
+ once_only = true;
+
+ kdebug("unexpected submission OP%x [OBJ%x %s]",
+ op->debug_id, object->debug_id,
+ fscache_object_states[object->state]);
+ kdebug("objstate=%s [%s]",
+ fscache_object_states[object->state],
+ fscache_object_states[ostate]);
+ kdebug("objflags=%lx", object->flags);
+ kdebug("objevent=%lx [%lx]", object->events, object->event_mask);
+ kdebug("ops=%u inp=%u exc=%u",
+ object->n_ops, object->n_in_progress, object->n_exclusive);
+
+ if (!list_empty(&object->pending_ops)) {
+ n = 0;
+ list_for_each_entry(p, &object->pending_ops, pend_link) {
+ ASSERTCMP(p->object, ==, object);
+ kdebug("%p %p", op->processor, op->release);
+ n++;
+ }
+
+ kdebug("n=%u", n);
+ }
+
+ dump_stack();
+}
+
+/*
+ * submit an operation for an object
+ * - objects may be submitted only in the following states:
+ * - during object creation (write ops may be submitted)
+ * - whilst the object is active
+ * - after an I/O error incurred in one of the two above states (op rejected)
+ * - this gets any extra refs it needs on an op
+ */
+int fscache_submit_op(struct fscache_object *object,
+ struct fscache_operation *op)
+{
+ unsigned long ostate;
+ int ret;
+
+ _enter("{OBJ%x OP%x},{%u}",
+ object->debug_id, op->debug_id, atomic_read(&op->usage));
+
+ ASSERTCMP(atomic_read(&op->usage), >, 0);
+
+ spin_lock(&object->lock);
+ ASSERTCMP(object->n_ops, >=, object->n_in_progress);
+ ASSERTCMP(object->n_ops, >=, object->n_exclusive);
+
+ ostate = object->state;
+ smp_rmb();
+
+ if (fscache_object_is_active(object)) {
+ op->object = object;
+ object->n_ops++;
+
+ if (object->n_exclusive > 0) {
+ atomic_inc(&op->usage);
+ list_add_tail(&op->pend_link, &object->pending_ops);
+ fscache_stat(&fscache_n_op_pend);
+ } else if (!list_empty(&object->pending_ops)) {
+ atomic_inc(&op->usage);
+ list_add_tail(&op->pend_link, &object->pending_ops);
+ fscache_stat(&fscache_n_op_pend);
+ fscache_start_operations(object);
+ } else {
+ ASSERTCMP(object->n_exclusive, ==, 0);
+ fscache_run_op(object, op);
+ }
+ ret = 0;
+ } else if (object->state == FSCACHE_OBJECT_CREATING) {
+ op->object = object;
+ object->n_ops++;
+ atomic_inc(&op->usage);
+ list_add_tail(&op->pend_link, &object->pending_ops);
+ fscache_stat(&fscache_n_op_pend);
+ ret = 0;
+ } else if (!test_bit(FSCACHE_IOERROR, &object->cache->flags)) {
+ fscache_report_unexpected_submission(object, op, ostate);
+ ASSERT(!fscache_object_is_active(object));
+ ret = -ENOBUFS;
+ } else {
+ ret = -ENOBUFS;
+ }
+
+ spin_unlock(&object->lock);
+ return ret;
+}
+
+/*
+ * queue an object for withdrawal on error, aborting all following asynchronous
+ * operations
+ */
+void fscache_abort_object(struct fscache_object *object)
+{
+ _enter("{OBJ%x}", object->debug_id);
+
+ fscache_raise_event(object, FSCACHE_OBJECT_EV_ERROR);
+}
+
+/*
+ * jump start the operation processing on an object
+ * - caller must hold object->lock
+ */
+void fscache_start_operations(struct fscache_object *object)
+{
+ struct fscache_operation *op;
+ bool stop = false;
+
+ while (!list_empty(&object->pending_ops) && !stop) {
+ op = list_entry(object->pending_ops.next,
+ struct fscache_operation, pend_link);
+
+ if (test_bit(FSCACHE_OP_EXCLUSIVE, &op->flags)) {
+ if (object->n_in_progress > 0)
+ break;
+ 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);
+
+ /* the pending queue was holding a ref on the object */
+ fscache_put_operation(op);
+ }
+
+ ASSERTCMP(object->n_in_progress, <=, object->n_ops);
+
+ _debug("woke %d ops on OBJ%x",
+ object->n_in_progress, object->debug_id);
+}
+
+/*
+ * release an operation
+ * - queues pending ops if this is the last in-progress op
+ */
+void fscache_put_operation(struct fscache_operation *op)
+{
+ struct fscache_object *object;
+ struct fscache_cache *cache;
+
+ _enter("{OBJ%x OP%x,%d}",
+ op->object->debug_id, op->debug_id, atomic_read(&op->usage));
+
+ ASSERTCMP(atomic_read(&op->usage), >, 0);
+
+ if (!atomic_dec_and_test(&op->usage))
+ return;
+
+ _debug("PUT OP");
+ if (test_and_set_bit(FSCACHE_OP_DEAD, &op->flags))
+ BUG();
+
+ fscache_stat(&fscache_n_op_release);
+
+ if (op->release) {
+ op->release(op);
+ op->release = NULL;
+ }
+
+ object = op->object;
+
+ /* now... we may get called with the object spinlock held, so we
+ * complete the cleanup here only if we can immediately acquire the
+ * lock, and defer it otherwise */
+ if (!spin_trylock(&object->lock)) {
+ _debug("defer put");
+ fscache_stat(&fscache_n_op_deferred_release);
+
+ cache = object->cache;
+ spin_lock(&cache->op_gc_list_lock);
+ list_add_tail(&op->pend_link, &cache->op_gc_list);
+ spin_unlock(&cache->op_gc_list_lock);
+ schedule_work(&cache->op_gc);
+ _leave(" [defer]");
+ return;
+ }
+
+ if (test_bit(FSCACHE_OP_EXCLUSIVE, &op->flags)) {
+ ASSERTCMP(object->n_exclusive, >, 0);
+ object->n_exclusive--;
+ }
+
+ ASSERTCMP(object->n_in_progress, >, 0);
+ object->n_in_progress--;
+ if (object->n_in_progress == 0)
+ fscache_start_operations(object);
+
+ ASSERTCMP(object->n_ops, >, 0);
+ object->n_ops--;
+ if (object->n_ops == 0)
+ fscache_raise_event(object, FSCACHE_OBJECT_EV_CLEARED);
+
+ spin_unlock(&object->lock);
+
+ kfree(op);
+ _leave(" [done]");
+}
+EXPORT_SYMBOL(fscache_put_operation);
+
+/*
+ * garbage collect operations that have had their release deferred
+ */
+void fscache_operation_gc(struct work_struct *work)
+{
+ struct fscache_operation *op;
+ struct fscache_object *object;
+ struct fscache_cache *cache =
+ container_of(work, struct fscache_cache, op_gc);
+ int count = 0;
+
+ _enter("");
+
+ do {
+ spin_lock(&cache->op_gc_list_lock);
+ if (list_empty(&cache->op_gc_list)) {
+ spin_unlock(&cache->op_gc_list_lock);
+ break;
+ }
+
+ op = list_entry(cache->op_gc_list.next,
+ struct fscache_operation, pend_link);
+ list_del(&op->pend_link);
+ spin_unlock(&cache->op_gc_list_lock);
+
+ object = op->object;
+
+ _debug("GC DEFERRED REL OBJ%x OP%x",
+ object->debug_id, op->debug_id);
+ fscache_stat(&fscache_n_op_gc);
+
+ ASSERTCMP(atomic_read(&op->usage), ==, 0);
+
+ spin_lock(&object->lock);
+ if (test_bit(FSCACHE_OP_EXCLUSIVE, &op->flags)) {
+ ASSERTCMP(object->n_exclusive, >, 0);
+ object->n_exclusive--;
+ }
+
+ ASSERTCMP(object->n_in_progress, >, 0);
+ object->n_in_progress--;
+ if (object->n_in_progress == 0)
+ fscache_start_operations(object);
+
+ ASSERTCMP(object->n_ops, >, 0);
+ object->n_ops--;
+ if (object->n_ops == 0)
+ fscache_raise_event(object, FSCACHE_OBJECT_EV_CLEARED);
+
+ spin_unlock(&object->lock);
+
+ } while (count++ < 20);
+
+ if (!list_empty(&cache->op_gc_list))
+ schedule_work(&cache->op_gc);
+
+ _leave("");
+}
+
+/*
+ * allow the slow work item processor to get a ref on an operation
+ */
+static int fscache_op_get_ref(struct slow_work *work)
+{
+ struct fscache_operation *op =
+ container_of(work, struct fscache_operation, slow_work);
+
+ atomic_inc(&op->usage);
+ return 0;
+}
+
+/*
+ * allow the slow work item processor to discard a ref on an operation
+ */
+static void fscache_op_put_ref(struct slow_work *work)
+{
+ struct fscache_operation *op =
+ container_of(work, struct fscache_operation, slow_work);
+
+ fscache_put_operation(op);
+}
+
+/*
+ * execute an operation using the slow thread pool to provide processing context
+ * - the caller holds a ref to this object, so we don't need to hold one
+ */
+static void fscache_op_execute(struct slow_work *work)
+{
+ struct fscache_operation *op =
+ container_of(work, struct fscache_operation, slow_work);
+ unsigned long start;
+
+ _enter("{OBJ%x OP%x,%d}",
+ op->object->debug_id, op->debug_id, atomic_read(&op->usage));
+
+ ASSERT(op->processor != NULL);
+ start = jiffies;
+ op->processor(op);
+ fscache_hist(fscache_ops_histogram, start);
+
+ _leave("");
+}
+
+const struct slow_work_ops fscache_op_slow_work_ops = {
+ .get_ref = fscache_op_get_ref,
+ .put_ref = fscache_op_put_ref,
+ .execute = fscache_op_execute,
+};
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