include/block/aio.h

   1 /*
   2  * QEMU aio implementation
   3  *
   4  * Copyright IBM, Corp. 2008
   5  *
   6  * Authors:
   7  *  Anthony Liguori   <aliguori@us.ibm.com>
   8  *
   9  * This work is licensed under the terms of the GNU GPL, version 2.  See
  10  * the COPYING file in the top-level directory.
  11  *
  12  */
  13
  14 #ifndef QEMU_AIO_H
  15 #define QEMU_AIO_H
  16
  17 #include "qemu/typedefs.h"
  18 #include "qemu-common.h"
  19 #include "qemu/queue.h"
  20 #include "qemu/event_notifier.h"
  21 #include "qemu/thread.h"
  22 #include "qemu/rfifolock.h"
  23 #include "qemu/timer.h"
  24
  25 typedef struct BlockAIOCB BlockAIOCB;
  26 typedef void BlockCompletionFunc(void *opaque, int ret);
  27
  28 typedef struct AIOCBInfo {
  29     void (*cancel_async)(BlockAIOCB *acb);
  30     AioContext *(*get_aio_context)(BlockAIOCB *acb);
  31     size_t aiocb_size;
  32 } AIOCBInfo;
  33
  34 struct BlockAIOCB {
  35     const AIOCBInfo *aiocb_info;
  36     BlockDriverState *bs;
  37     BlockCompletionFunc *cb;
  38     void *opaque;
  39     int refcnt;
  40 };
  41
  42 void *qemu_aio_get(const AIOCBInfo *aiocb_info, BlockDriverState *bs,
  43                    BlockCompletionFunc *cb, void *opaque);
  44 void qemu_aio_unref(void *p);
  45 void qemu_aio_ref(void *p);
  46
  47 typedef struct AioHandler AioHandler;
  48 typedef void QEMUBHFunc(void *opaque);
  49 typedef void IOHandler(void *opaque);
  50
  51 struct AioContext {
  52     GSource source;
  53
  54     /* Protects all fields from multi-threaded access */
  55     RFifoLock lock;
  56
  57     /* The list of registered AIO handlers */
  58     QLIST_HEAD(, AioHandler) aio_handlers;
  59
  60     /* This is a simple lock used to protect the aio_handlers list.
  61      * Specifically, it's used to ensure that no callbacks are removed while
  62      * we're walking and dispatching callbacks.
  63      */
  64     int walking_handlers;
  65
  66     /* Used to avoid unnecessary event_notifier_set calls in aio_notify;
  67      * accessed with atomic primitives.  If this field is 0, everything
  68      * (file descriptors, bottom halves, timers) will be re-evaluated
  69      * before the next blocking poll(), thus the event_notifier_set call
  70      * can be skipped.  If it is non-zero, you may need to wake up a
  71      * concurrent aio_poll or the glib main event loop, making
  72      * event_notifier_set necessary.
  73      *
  74      * Bit 0 is reserved for GSource usage of the AioContext, and is 1
  75      * between a call to aio_ctx_check and the next call to aio_ctx_dispatch.
  76      * Bits 1-31 simply count the number of active calls to aio_poll
  77      * that are in the prepare or poll phase.
  78      *
  79      * The GSource and aio_poll must use a different mechanism because
  80      * there is no certainty that a call to GSource's prepare callback
  81      * (via g_main_context_prepare) is indeed followed by check and
  82      * dispatch.  It's not clear whether this would be a bug, but let's
  83      * play safe and allow it---it will just cause extra calls to
  84      * event_notifier_set until the next call to dispatch.
  85      *
  86      * Instead, the aio_poll calls include both the prepare and the
  87      * dispatch phase, hence a simple counter is enough for them.
  88      */
  89     uint32_t notify_me;
  90
  91     /* lock to protect between bh's adders and deleter */
  92     QemuMutex bh_lock;
  93
  94     /* Anchor of the list of Bottom Halves belonging to the context */
  95     struct QEMUBH *first_bh;
  96
  97     /* A simple lock used to protect the first_bh list, and ensure that
  98      * no callbacks are removed while we're walking and dispatching callbacks.
  99      */
 100     int walking_bh;
 101
 102     /* Used by aio_notify.
 103      *
 104      * "notified" is used to avoid expensive event_notifier_test_and_clear
 105      * calls.  When it is clear, the EventNotifier is clear, or one thread
 106      * is going to clear "notified" before processing more events.  False
 107      * positives are possible, i.e. "notified" could be set even though the
 108      * EventNotifier is clear.
 109      *
 110      * Note that event_notifier_set *cannot* be optimized the same way.  For
 111      * more information on the problem that would result, see "#ifdef BUG2"
 112      * in the docs/aio_notify_accept.promela formal model.
 113      */
 114     bool notified;
 115     EventNotifier notifier;
 116
 117     /* Thread pool for performing work and receiving completion callbacks */
 118     struct ThreadPool *thread_pool;
 119
 120     /* TimerLists for calling timers - one per clock type */
 121     QEMUTimerListGroup tlg;
 122 };
 123
 124 /**
 125  * aio_context_new: Allocate a new AioContext.
 126  *
 127  * AioContext provide a mini event-loop that can be waited on synchronously.
 128  * They also provide bottom halves, a service to execute a piece of code
 129  * as soon as possible.
 130  */
 131 AioContext *aio_context_new(Error **errp);
 132
 133 /**
 134  * aio_context_ref:
 135  * @ctx: The AioContext to operate on.
 136  *
 137  * Add a reference to an AioContext.
 138  */
 139 void aio_context_ref(AioContext *ctx);
 140
 141 /**
 142  * aio_context_unref:
 143  * @ctx: The AioContext to operate on.
 144  *
 145  * Drop a reference to an AioContext.
 146  */
 147 void aio_context_unref(AioContext *ctx);
 148
 149 /* Take ownership of the AioContext.  If the AioContext will be shared between
 150  * threads, and a thread does not want to be interrupted, it will have to
 151  * take ownership around calls to aio_poll().  Otherwise, aio_poll()
 152  * automatically takes care of calling aio_context_acquire and
 153  * aio_context_release.
 154  *
 155  * Access to timers and BHs from a thread that has not acquired AioContext
 156  * is possible.  Access to callbacks for now must be done while the AioContext
 157  * is owned by the thread (FIXME).
 158  */
 159 void aio_context_acquire(AioContext *ctx);
 160
 161 /* Relinquish ownership of the AioContext. */
 162 void aio_context_release(AioContext *ctx);
 163
 164 /**
 165  * aio_bh_new: Allocate a new bottom half structure.
 166  *
 167  * Bottom halves are lightweight callbacks whose invocation is guaranteed
 168  * to be wait-free, thread-safe and signal-safe.  The #QEMUBH structure
 169  * is opaque and must be allocated prior to its use.
 170  */
 171 QEMUBH *aio_bh_new(AioContext *ctx, QEMUBHFunc *cb, void *opaque);
 172
 173 /**
 174  * aio_notify: Force processing of pending events.
 175  *
 176  * Similar to signaling a condition variable, aio_notify forces
 177  * aio_wait to exit, so that the next call will re-examine pending events.
 178  * The caller of aio_notify will usually call aio_wait again very soon,
 179  * or go through another iteration of the GLib main loop.  Hence, aio_notify
 180  * also has the side effect of recalculating the sets of file descriptors
 181  * that the main loop waits for.
 182  *
 183  * Calling aio_notify is rarely necessary, because for example scheduling
 184  * a bottom half calls it already.
 185  */
 186 void aio_notify(AioContext *ctx);
 187
 188 /**
 189  * aio_notify_accept: Acknowledge receiving an aio_notify.
 190  *
 191  * aio_notify() uses an EventNotifier in order to wake up a sleeping
 192  * aio_poll() or g_main_context_iteration().  Calls to aio_notify() are
 193  * usually rare, but the AioContext has to clear the EventNotifier on
 194  * every aio_poll() or g_main_context_iteration() in order to avoid
 195  * busy waiting.  This event_notifier_test_and_clear() cannot be done
 196  * using the usual aio_context_set_event_notifier(), because it must
 197  * be done before processing all events (file descriptors, bottom halves,
 198  * timers).
 199  *
 200  * aio_notify_accept() is an optimized event_notifier_test_and_clear()
 201  * that is specific to an AioContext's notifier; it is used internally
 202  * to clear the EventNotifier only if aio_notify() had been called.
 203  */
 204 void aio_notify_accept(AioContext *ctx);
 205
 206 /**
 207  * aio_bh_poll: Poll bottom halves for an AioContext.
 208  *
 209  * These are internal functions used by the QEMU main loop.
 210  * And notice that multiple occurrences of aio_bh_poll cannot
 211  * be called concurrently
 212  */
 213 int aio_bh_poll(AioContext *ctx);
 214
 215 /**
 216  * qemu_bh_schedule: Schedule a bottom half.
 217  *
 218  * Scheduling a bottom half interrupts the main loop and causes the
 219  * execution of the callback that was passed to qemu_bh_new.
 220  *
 221  * Bottom halves that are scheduled from a bottom half handler are instantly
 222  * invoked.  This can create an infinite loop if a bottom half handler
 223  * schedules itself.
 224  *
 225  * @bh: The bottom half to be scheduled.
 226  */
 227 void qemu_bh_schedule(QEMUBH *bh);
 228
 229 /**
 230  * qemu_bh_cancel: Cancel execution of a bottom half.
 231  *
 232  * Canceling execution of a bottom half undoes the effect of calls to
 233  * qemu_bh_schedule without freeing its resources yet.  While cancellation
 234  * itself is also wait-free and thread-safe, it can of course race with the
 235  * loop that executes bottom halves unless you are holding the iothread
 236  * mutex.  This makes it mostly useless if you are not holding the mutex.
 237  *
 238  * @bh: The bottom half to be canceled.
 239  */
 240 void qemu_bh_cancel(QEMUBH *bh);
 241
 242 /**
 243  *qemu_bh_delete: Cancel execution of a bottom half and free its resources.
 244  *
 245  * Deleting a bottom half frees the memory that was allocated for it by
 246  * qemu_bh_new.  It also implies canceling the bottom half if it was
 247  * scheduled.
 248  * This func is async. The bottom half will do the delete action at the finial
 249  * end.
 250  *
 251  * @bh: The bottom half to be deleted.
 252  */
 253 void qemu_bh_delete(QEMUBH *bh);
 254
 255 /* Return whether there are any pending callbacks from the GSource
 256  * attached to the AioContext, before g_poll is invoked.
 257  *
 258  * This is used internally in the implementation of the GSource.
 259  */
 260 bool aio_prepare(AioContext *ctx);
 261
 262 /* Return whether there are any pending callbacks from the GSource
 263  * attached to the AioContext, after g_poll is invoked.
 264  *
 265  * This is used internally in the implementation of the GSource.
 266  */
 267 bool aio_pending(AioContext *ctx);
 268
 269 /* Dispatch any pending callbacks from the GSource attached to the AioContext.
 270  *
 271  * This is used internally in the implementation of the GSource.
 272  */
 273 bool aio_dispatch(AioContext *ctx);
 274
 275 /* Progress in completing AIO work to occur.  This can issue new pending
 276  * aio as a result of executing I/O completion or bh callbacks.
 277  *
 278  * Return whether any progress was made by executing AIO or bottom half
 279  * handlers.  If @blocking == true, this should always be true except
 280  * if someone called aio_notify.
 281  *
 282  * If there are no pending bottom halves, but there are pending AIO
 283  * operations, it may not be possible to make any progress without
 284  * blocking.  If @blocking is true, this function will wait until one
 285  * or more AIO events have completed, to ensure something has moved
 286  * before returning.
 287  */
 288 bool aio_poll(AioContext *ctx, bool blocking);
 289
 290 /* Register a file descriptor and associated callbacks.  Behaves very similarly
 291  * to qemu_set_fd_handler.  Unlike qemu_set_fd_handler, these callbacks will
 292  * be invoked when using aio_poll().
 293  *
 294  * Code that invokes AIO completion functions should rely on this function
 295  * instead of qemu_set_fd_handler[2].
 296  */
 297 void aio_set_fd_handler(AioContext *ctx,
 298                         int fd,
 299                         IOHandler *io_read,
 300                         IOHandler *io_write,
 301                         void *opaque);
 302
 303 /* Register an event notifier and associated callbacks.  Behaves very similarly
 304  * to event_notifier_set_handler.  Unlike event_notifier_set_handler, these callbacks
 305  * will be invoked when using aio_poll().
 306  *
 307  * Code that invokes AIO completion functions should rely on this function
 308  * instead of event_notifier_set_handler.
 309  */
 310 void aio_set_event_notifier(AioContext *ctx,
 311                             EventNotifier *notifier,
 312                             EventNotifierHandler *io_read);
 313
 314 /* Return a GSource that lets the main loop poll the file descriptors attached
 315  * to this AioContext.
 316  */
 317 GSource *aio_get_g_source(AioContext *ctx);
 318
 319 /* Return the ThreadPool bound to this AioContext */
 320 struct ThreadPool *aio_get_thread_pool(AioContext *ctx);
 321
 322 /**
 323  * aio_timer_new:
 324  * @ctx: the aio context
 325  * @type: the clock type
 326  * @scale: the scale
 327  * @cb: the callback to call on timer expiry
 328  * @opaque: the opaque pointer to pass to the callback
 329  *
 330  * Allocate a new timer attached to the context @ctx.
 331  * The function is responsible for memory allocation.
 332  *
 333  * The preferred interface is aio_timer_init. Use that
 334  * unless you really need dynamic memory allocation.
 335  *
 336  * Returns: a pointer to the new timer
 337  */
 338 static inline QEMUTimer *aio_timer_new(AioContext *ctx, QEMUClockType type,
 339                                        int scale,
 340                                        QEMUTimerCB *cb, void *opaque)
 341 {
 342     return timer_new_tl(ctx->tlg.tl[type], scale, cb, opaque);
 343 }
 344
 345 /**
 346  * aio_timer_init:
 347  * @ctx: the aio context
 348  * @ts: the timer
 349  * @type: the clock type
 350  * @scale: the scale
 351  * @cb: the callback to call on timer expiry
 352  * @opaque: the opaque pointer to pass to the callback
 353  *
 354  * Initialise a new timer attached to the context @ctx.
 355  * The caller is responsible for memory allocation.
 356  */
 357 static inline void aio_timer_init(AioContext *ctx,
 358                                   QEMUTimer *ts, QEMUClockType type,
 359                                   int scale,
 360                                   QEMUTimerCB *cb, void *opaque)
 361 {
 362     timer_init_tl(ts, ctx->tlg.tl[type], scale, cb, opaque);
 363 }
 364
 365 /**
 366  * aio_compute_timeout:
 367  * @ctx: the aio context
 368  *
 369  * Compute the timeout that a blocking aio_poll should use.
 370  */
 371 int64_t aio_compute_timeout(AioContext *ctx);
 372
 373 #endif