include/block/aio-wait.h

   1 /*
   2  * AioContext wait support
   3  *
   4  * Copyright (C) 2018 Red Hat, Inc.
   5  *
   6  * Permission is hereby granted, free of charge, to any person obtaining a copy
   7  * of this software and associated documentation files (the "Software"), to deal
   8  * in the Software without restriction, including without limitation the rights
   9  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  10  * copies of the Software, and to permit persons to whom the Software is
  11  * furnished to do so, subject to the following conditions:
  12  *
  13  * The above copyright notice and this permission notice shall be included in
  14  * all copies or substantial portions of the Software.
  15  *
  16  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  17  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  18  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
  19  * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  20  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  21  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  22  * THE SOFTWARE.
  23  */
  24
  25 #ifndef QEMU_AIO_WAIT_H
  26 #define QEMU_AIO_WAIT_H
  27
  28 #include "block/aio.h"
  29
  30 /**
  31  * AioWait:
  32  *
  33  * An object that facilitates synchronous waiting on a condition.  The main
  34  * loop can wait on an operation running in an IOThread as follows:
  35  *
  36  *   AioWait *wait = ...;
  37  *   AioContext *ctx = ...;
  38  *   MyWork work = { .done = false };
  39  *   schedule_my_work_in_iothread(ctx, &work);
  40  *   AIO_WAIT_WHILE(wait, ctx, !work.done);
  41  *
  42  * The IOThread must call aio_wait_kick() to notify the main loop when
  43  * work.done changes:
  44  *
  45  *   static void do_work(...)
  46  *   {
  47  *       ...
  48  *       work.done = true;
  49  *       aio_wait_kick(wait);
  50  *   }
  51  */
  52 typedef struct {
  53     /* Number of waiting AIO_WAIT_WHILE() callers. Accessed with atomic ops. */
  54     unsigned num_waiters;
  55 } AioWait;
  56
  57 /**
  58  * AIO_WAIT_WHILE:
  59  * @wait: the aio wait object
  60  * @ctx: the aio context, or NULL if multiple aio contexts (for which the
  61  *       caller does not hold a lock) are involved in the polling condition.
  62  * @cond: wait while this conditional expression is true
  63  *
  64  * Wait while a condition is true.  Use this to implement synchronous
  65  * operations that require event loop activity.
  66  *
  67  * The caller must be sure that something calls aio_wait_kick() when the value
  68  * of @cond might have changed.
  69  *
  70  * The caller's thread must be the IOThread that owns @ctx or the main loop
  71  * thread (with @ctx acquired exactly once).  This function cannot be used to
  72  * wait on conditions between two IOThreads since that could lead to deadlock,
  73  * go via the main loop instead.
  74  */
  75 #define AIO_WAIT_WHILE(wait, ctx, cond) ({                         \
  76     bool waited_ = false;                                          \
  77     AioWait *wait_ = (wait);                                       \
  78     AioContext *ctx_ = (ctx);                                      \
  79     if (ctx_ && in_aio_context_home_thread(ctx_)) {                \
  80         while ((cond)) {                                           \
  81             aio_poll(ctx_, true);                                  \
  82             waited_ = true;                                        \
  83         }                                                          \
  84     } else {                                                       \
  85         assert(qemu_get_current_aio_context() ==                   \
  86                qemu_get_aio_context());                            \
  87         /* Increment wait_->num_waiters before evaluating cond. */ \
  88         atomic_inc(&wait_->num_waiters);                           \
  89         while ((cond)) {                                           \
  90             if (ctx_) {                                            \
  91                 aio_context_release(ctx_);                         \
  92             }                                                      \
  93             aio_poll(qemu_get_aio_context(), true);                \
  94             if (ctx_) {                                            \
  95                 aio_context_acquire(ctx_);                         \
  96             }                                                      \
  97             waited_ = true;                                        \
  98         }                                                          \
  99         atomic_dec(&wait_->num_waiters);                           \
 100     }                                                              \
 101     waited_; })
 102
 103 /**
 104  * aio_wait_kick:
 105  * @wait: the aio wait object that should re-evaluate its condition
 106  *
 107  * Wake up the main thread if it is waiting on AIO_WAIT_WHILE().  During
 108  * synchronous operations performed in an IOThread, the main thread lets the
 109  * IOThread's event loop run, waiting for the operation to complete.  A
 110  * aio_wait_kick() call will wake up the main thread.
 111  */
 112 void aio_wait_kick(AioWait *wait);
 113
 114 /**
 115  * aio_wait_bh_oneshot:
 116  * @ctx: the aio context
 117  * @cb: the BH callback function
 118  * @opaque: user data for the BH callback function
 119  *
 120  * Run a BH in @ctx and wait for it to complete.
 121  *
 122  * Must be called from the main loop thread with @ctx acquired exactly once.
 123  * Note that main loop event processing may occur.
 124  */
 125 void aio_wait_bh_oneshot(AioContext *ctx, QEMUBHFunc *cb, void *opaque);
 126
 127 #endif /* QEMU_AIO_WAIT */