214 lines
7.2 KiB
Plaintext
214 lines
7.2 KiB
Plaintext
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ASYNCHRONOUS OPERATIONS HANDLING
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================================
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By: David Howells <dhowells@redhat.com>
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Contents:
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(*) Overview.
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(*) Operation record initialisation.
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(*) Parameters.
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(*) Procedure.
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(*) Asynchronous callback.
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========
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OVERVIEW
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========
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FS-Cache has an asynchronous operations handling facility that it uses for its
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data storage and retrieval routines. Its operations are represented by
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fscache_operation structs, though these are usually embedded into some other
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structure.
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This facility is available to and expected to be be used by the cache backends,
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and FS-Cache will create operations and pass them off to the appropriate cache
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backend for completion.
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To make use of this facility, <linux/fscache-cache.h> should be #included.
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===============================
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OPERATION RECORD INITIALISATION
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===============================
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An operation is recorded in an fscache_operation struct:
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struct fscache_operation {
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union {
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struct work_struct fast_work;
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struct slow_work slow_work;
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};
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unsigned long flags;
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fscache_operation_processor_t processor;
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...
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};
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Someone wanting to issue an operation should allocate something with this
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struct embedded in it. They should initialise it by calling:
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void fscache_operation_init(struct fscache_operation *op,
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fscache_operation_release_t release);
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with the operation to be initialised and the release function to use.
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The op->flags parameter should be set to indicate the CPU time provision and
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the exclusivity (see the Parameters section).
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The op->fast_work, op->slow_work and op->processor flags should be set as
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appropriate for the CPU time provision (see the Parameters section).
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FSCACHE_OP_WAITING may be set in op->flags prior to each submission of the
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operation and waited for afterwards.
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==========
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PARAMETERS
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==========
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There are a number of parameters that can be set in the operation record's flag
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parameter. There are three options for the provision of CPU time in these
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operations:
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(1) The operation may be done synchronously (FSCACHE_OP_MYTHREAD). A thread
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may decide it wants to handle an operation itself without deferring it to
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another thread.
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This is, for example, used in read operations for calling readpages() on
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the backing filesystem in CacheFiles. Although readpages() does an
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asynchronous data fetch, the determination of whether pages exist is done
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synchronously - and the netfs does not proceed until this has been
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determined.
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If this option is to be used, FSCACHE_OP_WAITING must be set in op->flags
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before submitting the operation, and the operating thread must wait for it
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to be cleared before proceeding:
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wait_on_bit(&op->flags, FSCACHE_OP_WAITING,
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TASK_UNINTERRUPTIBLE);
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(2) The operation may be fast asynchronous (FSCACHE_OP_FAST), in which case it
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will be given to keventd to process. Such an operation is not permitted
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to sleep on I/O.
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This is, for example, used by CacheFiles to copy data from a backing fs
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page to a netfs page after the backing fs has read the page in.
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If this option is used, op->fast_work and op->processor must be
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initialised before submitting the operation:
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INIT_WORK(&op->fast_work, do_some_work);
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(3) The operation may be slow asynchronous (FSCACHE_OP_SLOW), in which case it
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will be given to the slow work facility to process. Such an operation is
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permitted to sleep on I/O.
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This is, for example, used by FS-Cache to handle background writes of
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pages that have just been fetched from a remote server.
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If this option is used, op->slow_work and op->processor must be
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initialised before submitting the operation:
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fscache_operation_init_slow(op, processor)
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Furthermore, operations may be one of two types:
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(1) Exclusive (FSCACHE_OP_EXCLUSIVE). Operations of this type may not run in
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conjunction with any other operation on the object being operated upon.
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An example of this is the attribute change operation, in which the file
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being written to may need truncation.
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(2) Shareable. Operations of this type may be running simultaneously. It's
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up to the operation implementation to prevent interference between other
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operations running at the same time.
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=========
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PROCEDURE
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=========
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Operations are used through the following procedure:
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(1) The submitting thread must allocate the operation and initialise it
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itself. Normally this would be part of a more specific structure with the
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generic op embedded within.
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(2) The submitting thread must then submit the operation for processing using
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one of the following two functions:
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int fscache_submit_op(struct fscache_object *object,
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struct fscache_operation *op);
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int fscache_submit_exclusive_op(struct fscache_object *object,
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struct fscache_operation *op);
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The first function should be used to submit non-exclusive ops and the
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second to submit exclusive ones. The caller must still set the
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FSCACHE_OP_EXCLUSIVE flag.
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If successful, both functions will assign the operation to the specified
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object and return 0. -ENOBUFS will be returned if the object specified is
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permanently unavailable.
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The operation manager will defer operations on an object that is still
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undergoing lookup or creation. The operation will also be deferred if an
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operation of conflicting exclusivity is in progress on the object.
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If the operation is asynchronous, the manager will retain a reference to
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it, so the caller should put their reference to it by passing it to:
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void fscache_put_operation(struct fscache_operation *op);
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(3) If the submitting thread wants to do the work itself, and has marked the
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operation with FSCACHE_OP_MYTHREAD, then it should monitor
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FSCACHE_OP_WAITING as described above and check the state of the object if
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necessary (the object might have died whilst the thread was waiting).
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When it has finished doing its processing, it should call
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fscache_op_complete() and fscache_put_operation() on it.
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(4) The operation holds an effective lock upon the object, preventing other
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exclusive ops conflicting until it is released. The operation can be
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enqueued for further immediate asynchronous processing by adjusting the
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CPU time provisioning option if necessary, eg:
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op->flags &= ~FSCACHE_OP_TYPE;
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op->flags |= ~FSCACHE_OP_FAST;
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and calling:
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void fscache_enqueue_operation(struct fscache_operation *op)
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This can be used to allow other things to have use of the worker thread
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pools.
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=====================
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ASYNCHRONOUS CALLBACK
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=====================
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When used in asynchronous mode, the worker thread pool will invoke the
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processor method with a pointer to the operation. This should then get at the
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container struct by using container_of():
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static void fscache_write_op(struct fscache_operation *_op)
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{
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struct fscache_storage *op =
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container_of(_op, struct fscache_storage, op);
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...
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}
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The caller holds a reference on the operation, and will invoke
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fscache_put_operation() when the processor function returns. The processor
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function is at liberty to call fscache_enqueue_operation() or to take extra
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references.
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