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     /* Scheduling this BH forces the event loop it iterate */
 118     QEMUBH *notify_dummy_bh;
 119
 120     /* Thread pool for performing work and receiving completion callbacks */
 121     struct ThreadPool *thread_pool;
 122
 123     /* TimerLists for calling timers - one per clock type */
 124     QEMUTimerListGroup tlg;
 125 };
 126
 127 /**
 128  * aio_context_new: Allocate a new AioContext.
 129  *
 130  * AioContext provide a mini event-loop that can be waited on synchronously.
 131  * They also provide bottom halves, a service to execute a piece of code
 132  * as soon as possible.
 133  */
 134 AioContext *aio_context_new(Error **errp);
 135
 136 /**
 137  * aio_context_ref:
 138  * @ctx: The AioContext to operate on.
 139  *
 140  * Add a reference to an AioContext.
 141  */
 142 void aio_context_ref(AioContext *ctx);
 143
 144 /**
 145  * aio_context_unref:
 146  * @ctx: The AioContext to operate on.
 147  *
 148  * Drop a reference to an AioContext.
 149  */
 150 void aio_context_unref(AioContext *ctx);
 151
 152 /* Take ownership of the AioContext.  If the AioContext will be shared between
 153  * threads, and a thread does not want to be interrupted, it will have to
 154  * take ownership around calls to aio_poll().  Otherwise, aio_poll()
 155  * automatically takes care of calling aio_context_acquire and
 156  * aio_context_release.
 157  *
 158  * Access to timers and BHs from a thread that has not acquired AioContext
 159  * is possible.  Access to callbacks for now must be done while the AioContext
 160  * is owned by the thread (FIXME).
 161  */
 162 void aio_context_acquire(AioContext *ctx);
 163
 164 /* Relinquish ownership of the AioContext. */
 165 void aio_context_release(AioContext *ctx);
 166
 167 /**
 168  * aio_bh_new: Allocate a new bottom half structure.
 169  *
 170  * Bottom halves are lightweight callbacks whose invocation is guaranteed
 171  * to be wait-free, thread-safe and signal-safe.  The #QEMUBH structure
 172  * is opaque and must be allocated prior to its use.
 173  */
 174 QEMUBH *aio_bh_new(AioContext *ctx, QEMUBHFunc *cb, void *opaque);
 175
 176 /**
 177  * aio_notify: Force processing of pending events.
 178  *
 179  * Similar to signaling a condition variable, aio_notify forces
 180  * aio_wait to exit, so that the next call will re-examine pending events.
 181  * The caller of aio_notify will usually call aio_wait again very soon,
 182  * or go through another iteration of the GLib main loop.  Hence, aio_notify
 183  * also has the side effect of recalculating the sets of file descriptors
 184  * that the main loop waits for.
 185  *
 186  * Calling aio_notify is rarely necessary, because for example scheduling
 187  * a bottom half calls it already.
 188  */
 189 void aio_notify(AioContext *ctx);
 190
 191 /**
 192  * aio_notify_accept: Acknowledge receiving an aio_notify.
 193  *
 194  * aio_notify() uses an EventNotifier in order to wake up a sleeping
 195  * aio_poll() or g_main_context_iteration().  Calls to aio_notify() are
 196  * usually rare, but the AioContext has to clear the EventNotifier on
 197  * every aio_poll() or g_main_context_iteration() in order to avoid
 198  * busy waiting.  This event_notifier_test_and_clear() cannot be done
 199  * using the usual aio_context_set_event_notifier(), because it must
 200  * be done before processing all events (file descriptors, bottom halves,
 201  * timers).
 202  *
 203  * aio_notify_accept() is an optimized event_notifier_test_and_clear()
 204  * that is specific to an AioContext's notifier; it is used internally
 205  * to clear the EventNotifier only if aio_notify() had been called.
 206  */
 207 void aio_notify_accept(AioContext *ctx);
 208
 209 /**
 210  * aio_bh_poll: Poll bottom halves for an AioContext.
 211  *
 212  * These are internal functions used by the QEMU main loop.
 213  * And notice that multiple occurrences of aio_bh_poll cannot
 214  * be called concurrently
 215  */
 216 int aio_bh_poll(AioContext *ctx);
 217
 218 /**
 219  * qemu_bh_schedule: Schedule a bottom half.
 220  *
 221  * Scheduling a bottom half interrupts the main loop and causes the
 222  * execution of the callback that was passed to qemu_bh_new.
 223  *
 224  * Bottom halves that are scheduled from a bottom half handler are instantly
 225  * invoked.  This can create an infinite loop if a bottom half handler
 226  * schedules itself.
 227  *
 228  * @bh: The bottom half to be scheduled.
 229  */
 230 void qemu_bh_schedule(QEMUBH *bh);
 231
 232 /**
 233  * qemu_bh_cancel: Cancel execution of a bottom half.
 234  *
 235  * Canceling execution of a bottom half undoes the effect of calls to
 236  * qemu_bh_schedule without freeing its resources yet.  While cancellation
 237  * itself is also wait-free and thread-safe, it can of course race with the
 238  * loop that executes bottom halves unless you are holding the iothread
 239  * mutex.  This makes it mostly useless if you are not holding the mutex.
 240  *
 241  * @bh: The bottom half to be canceled.
 242  */
 243 void qemu_bh_cancel(QEMUBH *bh);
 244
 245 /**
 246  *qemu_bh_delete: Cancel execution of a bottom half and free its resources.
 247  *
 248  * Deleting a bottom half frees the memory that was allocated for it by
 249  * qemu_bh_new.  It also implies canceling the bottom half if it was
 250  * scheduled.
 251  * This func is async. The bottom half will do the delete action at the finial
 252  * end.
 253  *
 254  * @bh: The bottom half to be deleted.
 255  */
 256 void qemu_bh_delete(QEMUBH *bh);
 257
 258 /* Return whether there are any pending callbacks from the GSource
 259  * attached to the AioContext, before g_poll is invoked.
 260  *
 261  * This is used internally in the implementation of the GSource.
 262  */
 263 bool aio_prepare(AioContext *ctx);
 264
 265 /* Return whether there are any pending callbacks from the GSource
 266  * attached to the AioContext, after g_poll is invoked.
 267  *
 268  * This is used internally in the implementation of the GSource.
 269  */
 270 bool aio_pending(AioContext *ctx);
 271
 272 /* Dispatch any pending callbacks from the GSource attached to the AioContext.
 273  *
 274  * This is used internally in the implementation of the GSource.
 275  */
 276 bool aio_dispatch(AioContext *ctx);
 277
 278 /* Progress in completing AIO work to occur.  This can issue new pending
 279  * aio as a result of executing I/O completion or bh callbacks.
 280  *
 281  * Return whether any progress was made by executing AIO or bottom half
 282  * handlers.  If @blocking == true, this should always be true except
 283  * if someone called aio_notify.
 284  *
 285  * If there are no pending bottom halves, but there are pending AIO
 286  * operations, it may not be possible to make any progress without
 287  * blocking.  If @blocking is true, this function will wait until one
 288  * or more AIO events have completed, to ensure something has moved
 289  * before returning.
 290  */
 291 bool aio_poll(AioContext *ctx, bool blocking);
 292
 293 /* Register a file descriptor and associated callbacks.  Behaves very similarly
 294  * to qemu_set_fd_handler.  Unlike qemu_set_fd_handler, these callbacks will
 295  * be invoked when using aio_poll().
 296  *
 297  * Code that invokes AIO completion functions should rely on this function
 298  * instead of qemu_set_fd_handler[2].
 299  */
 300 void aio_set_fd_handler(AioContext *ctx,
 301                         int fd,
 302                         IOHandler *io_read,
 303                         IOHandler *io_write,
 304                         void *opaque);
 305
 306 /* Register an event notifier and associated callbacks.  Behaves very similarly
 307  * to event_notifier_set_handler.  Unlike event_notifier_set_handler, these callbacks
 308  * will be invoked when using aio_poll().
 309  *
 310  * Code that invokes AIO completion functions should rely on this function
 311  * instead of event_notifier_set_handler.
 312  */
 313 void aio_set_event_notifier(AioContext *ctx,
 314                             EventNotifier *notifier,
 315                             EventNotifierHandler *io_read);
 316
 317 /* Return a GSource that lets the main loop poll the file descriptors attached
 318  * to this AioContext.
 319  */
 320 GSource *aio_get_g_source(AioContext *ctx);
 321
 322 /* Return the ThreadPool bound to this AioContext */
 323 struct ThreadPool *aio_get_thread_pool(AioContext *ctx);
 324
 325 /**
 326  * aio_timer_new:
 327  * @ctx: the aio context
 328  * @type: the clock type
 329  * @scale: the scale
 330  * @cb: the callback to call on timer expiry
 331  * @opaque: the opaque pointer to pass to the callback
 332  *
 333  * Allocate a new timer attached to the context @ctx.
 334  * The function is responsible for memory allocation.
 335  *
 336  * The preferred interface is aio_timer_init. Use that
 337  * unless you really need dynamic memory allocation.
 338  *
 339  * Returns: a pointer to the new timer
 340  */
 341 static inline QEMUTimer *aio_timer_new(AioContext *ctx, QEMUClockType type,
 342                                        int scale,
 343                                        QEMUTimerCB *cb, void *opaque)
 344 {
 345     return timer_new_tl(ctx->tlg.tl[type], scale, cb, opaque);
 346 }
 347
 348 /**
 349  * aio_timer_init:
 350  * @ctx: the aio context
 351  * @ts: the timer
 352  * @type: the clock type
 353  * @scale: the scale
 354  * @cb: the callback to call on timer expiry
 355  * @opaque: the opaque pointer to pass to the callback
 356  *
 357  * Initialise a new timer attached to the context @ctx.
 358  * The caller is responsible for memory allocation.
 359  */
 360 static inline void aio_timer_init(AioContext *ctx,
 361                                   QEMUTimer *ts, QEMUClockType type,
 362                                   int scale,
 363                                   QEMUTimerCB *cb, void *opaque)
 364 {
 365     timer_init_tl(ts, ctx->tlg.tl[type], scale, cb, opaque);
 366 }
 367
 368 /**
 369  * aio_compute_timeout:
 370  * @ctx: the aio context
 371  *
 372  * Compute the timeout that a blocking aio_poll should use.
 373  */
 374 int64_t aio_compute_timeout(AioContext *ctx);
 375
 376 #endif