include/block/aio-wait.h

/*
 * AioContext wait support
 *
 * Copyright (C) 2018 Red Hat, Inc.
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

#ifndef QEMU_AIO_WAIT_H
#define QEMU_AIO_WAIT_H

#include "block/aio.h"

/**
 * AioWait:
 *
 * An object that facilitates synchronous waiting on a condition.  The main
 * loop can wait on an operation running in an IOThread as follows:
 *
 *   AioWait *wait = ...;
 *   AioContext *ctx = ...;
 *   MyWork work = { .done = false };
 *   schedule_my_work_in_iothread(ctx, &work);
 *   AIO_WAIT_WHILE(wait, ctx, !work.done);
 *
 * The IOThread must call aio_wait_kick() to notify the main loop when
 * work.done changes:
 *
 *   static void do_work(...)
 *   {
 *       ...
 *       work.done = true;
 *       aio_wait_kick(wait);
 *   }
 */
typedef struct {
    /* Is the main loop waiting for a kick?  Accessed with atomic ops. */
    bool need_kick;
} AioWait;

/**
 * AIO_WAIT_WHILE:
 * @wait: the aio wait object
 * @ctx: the aio context
 * @cond: wait while this conditional expression is true
 *
 * Wait while a condition is true.  Use this to implement synchronous
 * operations that require event loop activity.
 *
 * The caller must be sure that something calls aio_wait_kick() when the value
 * of @cond might have changed.
 *
 * The caller's thread must be the IOThread that owns @ctx or the main loop
 * thread (with @ctx acquired exactly once).  This function cannot be used to
 * wait on conditions between two IOThreads since that could lead to deadlock,
 * go via the main loop instead.
 */
#define AIO_WAIT_WHILE(wait, ctx, cond) ({                  \
    bool waited_ = false;                                   \
    bool busy_ = true;                                      \
    AioWait *wait_ = (wait);                                \
    AioContext *ctx_ = (ctx);                               \
    if (in_aio_context_home_thread(ctx_)) {                 \
        while ((cond) || busy_) {                           \
            busy_ = aio_poll(ctx_, (cond));                 \
            waited_ |= !!(cond) | busy_;                    \
        }                                                   \
    } else {                                                \
        assert(qemu_get_current_aio_context() ==            \
               qemu_get_aio_context());                     \
        assert(!wait_->need_kick);                          \
        /* Set wait_->need_kick before evaluating cond.  */ \
        atomic_mb_set(&wait_->need_kick, true);             \
        while (busy_) {                                     \
            if ((cond)) {                                   \
                waited_ = busy_ = true;                     \
                aio_context_release(ctx_);                  \
                aio_poll(qemu_get_aio_context(), true);     \
                aio_context_acquire(ctx_);                  \
            } else {                                        \
                busy_ = aio_poll(ctx_, false);              \
                waited_ |= busy_;                           \
            }                                               \
        }                                                   \
        atomic_set(&wait_->need_kick, false);               \
    }                                                       \
    waited_; })

/**
 * aio_wait_kick:
 * @wait: the aio wait object that should re-evaluate its condition
 *
 * Wake up the main thread if it is waiting on AIO_WAIT_WHILE().  During
 * synchronous operations performed in an IOThread, the main thread lets the
 * IOThread's event loop run, waiting for the operation to complete.  A
 * aio_wait_kick() call will wake up the main thread.
 */
void aio_wait_kick(AioWait *wait);

#endif /* QEMU_AIO_WAIT */