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
  61  * @cond: wait while this conditional expression is true
  62  *
  63  * Wait while a condition is true.  Use this to implement synchronous
  64  * operations that require event loop activity.
  65  *
  66  * The caller must be sure that something calls aio_wait_kick() when the value
  67  * of @cond might have changed.
  68  *
  69  * The caller's thread must be the IOThread that owns @ctx or the main loop
  70  * thread (with @ctx acquired exactly once).  This function cannot be used to
  71  * wait on conditions between two IOThreads since that could lead to deadlock,
  72  * go via the main loop instead.
  73  */
  74 #define AIO_WAIT_WHILE(wait, ctx, cond) ({                         \
  75     bool waited_ = false;                                          \
  76     bool busy_ = true;                                             \
  77     AioWait *wait_ = (wait);                                       \
  78     AioContext *ctx_ = (ctx);                                      \
  79     if (in_aio_context_home_thread(ctx_)) {                        \
  80         while ((cond) || busy_) {                                  \
  81             busy_ = aio_poll(ctx_, (cond));                        \
  82             waited_ |= !!(cond) | busy_;                           \
  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 (busy_) {                                            \
  90             if ((cond)) {                                          \
  91                 waited_ = busy_ = true;                            \
  92                 aio_context_release(ctx_);                         \
  93                 aio_poll(qemu_get_aio_context(), true);            \
  94                 aio_context_acquire(ctx_);                         \
  95             } else {                                               \
  96                 busy_ = aio_poll(ctx_, false);                     \
  97                 waited_ |= busy_;                                  \
  98             }                                                      \
  99         }                                                          \
 100         atomic_dec(&wait_->num_waiters);                           \
 101     }                                                              \
 102     waited_; })
 103
 104 /**
 105  * aio_wait_kick:
 106  * @wait: the aio wait object that should re-evaluate its condition
 107  *
 108  * Wake up the main thread if it is waiting on AIO_WAIT_WHILE().  During
 109  * synchronous operations performed in an IOThread, the main thread lets the
 110  * IOThread's event loop run, waiting for the operation to complete.  A
 111  * aio_wait_kick() call will wake up the main thread.
 112  */
 113 void aio_wait_kick(AioWait *wait);
 114
 115 /**
 116  * aio_wait_bh_oneshot:
 117  * @ctx: the aio context
 118  * @cb: the BH callback function
 119  * @opaque: user data for the BH callback function
 120  *
 121  * Run a BH in @ctx and wait for it to complete.
 122  *
 123  * Must be called from the main loop thread with @ctx acquired exactly once.
 124  * Note that main loop event processing may occur.
 125  */
 126 void aio_wait_bh_oneshot(AioContext *ctx, QEMUBHFunc *cb, void *opaque);
 127
 128 #endif /* QEMU_AIO_WAIT */