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 #include "qemu/main-loop.h"
  30
  31 /**
  32  * AioWait:
  33  *
  34  * An object that facilitates synchronous waiting on a condition. A single
  35  * global AioWait object (global_aio_wait) is used internally.
  36  *
  37  * The main loop can wait on an operation running in an IOThread as follows:
  38  *
  39  *   AioContext *ctx = ...;
  40  *   MyWork work = { .done = false };
  41  *   schedule_my_work_in_iothread(ctx, &work);
  42  *   AIO_WAIT_WHILE(ctx, !work.done);
  43  *
  44  * The IOThread must call aio_wait_kick() to notify the main loop when
  45  * work.done changes:
  46  *
  47  *   static void do_work(...)
  48  *   {
  49  *       ...
  50  *       work.done = true;
  51  *       aio_wait_kick();
  52  *   }
  53  */
  54 typedef struct {
  55     /* Number of waiting AIO_WAIT_WHILE() callers. Accessed with atomic ops. */
  56     unsigned num_waiters;
  57 } AioWait;
  58
  59 extern AioWait global_aio_wait;
  60
  61 /**
  62  * AIO_WAIT_WHILE:
  63  * @ctx: the aio context, or NULL if multiple aio contexts (for which the
  64  *       caller does not hold a lock) are involved in the polling condition.
  65  * @cond: wait while this conditional expression is true
  66  *
  67  * Wait while a condition is true.  Use this to implement synchronous
  68  * operations that require event loop activity.
  69  *
  70  * The caller must be sure that something calls aio_wait_kick() when the value
  71  * of @cond might have changed.
  72  *
  73  * The caller's thread must be the IOThread that owns @ctx or the main loop
  74  * thread (with @ctx acquired exactly once).  This function cannot be used to
  75  * wait on conditions between two IOThreads since that could lead to deadlock,
  76  * go via the main loop instead.
  77  */
  78 #define AIO_WAIT_WHILE(ctx, cond) ({                               \
  79     bool waited_ = false;                                          \
  80     AioWait *wait_ = &global_aio_wait;                             \
  81     AioContext *ctx_ = (ctx);                                      \
  82     /* Increment wait_->num_waiters before evaluating cond. */     \
  83     qatomic_inc(&wait_->num_waiters);                              \
  84     /* Paired with smp_mb in aio_wait_kick(). */                   \
  85     smp_mb();                                                      \
  86     if (ctx_ && in_aio_context_home_thread(ctx_)) {                \
  87         while ((cond)) {                                           \
  88             aio_poll(ctx_, true);                                  \
  89             waited_ = true;                                        \
  90         }                                                          \
  91     } else {                                                       \
  92         assert(qemu_get_current_aio_context() ==                   \
  93                qemu_get_aio_context());                            \
  94         while ((cond)) {                                           \
  95             if (ctx_) {                                            \
  96                 aio_context_release(ctx_);                         \
  97             }                                                      \
  98             aio_poll(qemu_get_aio_context(), true);                \
  99             if (ctx_) {                                            \
 100                 aio_context_acquire(ctx_);                         \
 101             }                                                      \
 102             waited_ = true;                                        \
 103         }                                                          \
 104     }                                                              \
 105     qatomic_dec(&wait_->num_waiters);                              \
 106     waited_; })
 107
 108 /**
 109  * aio_wait_kick:
 110  * Wake up the main thread if it is waiting on AIO_WAIT_WHILE().  During
 111  * synchronous operations performed in an IOThread, the main thread lets the
 112  * IOThread's event loop run, waiting for the operation to complete.  A
 113  * aio_wait_kick() call will wake up the main thread.
 114  */
 115 void aio_wait_kick(void);
 116
 117 /**
 118  * aio_wait_bh_oneshot:
 119  * @ctx: the aio context
 120  * @cb: the BH callback function
 121  * @opaque: user data for the BH callback function
 122  *
 123  * Run a BH in @ctx and wait for it to complete.
 124  *
 125  * Must be called from the main loop thread with @ctx acquired exactly once.
 126  * Note that main loop event processing may occur.
 127  */
 128 void aio_wait_bh_oneshot(AioContext *ctx, QEMUBHFunc *cb, void *opaque);
 129
 130 /**
 131  * in_aio_context_home_thread:
 132  * @ctx: the aio context
 133  *
 134  * Return whether we are running in the thread that normally runs @ctx.  Note
 135  * that acquiring/releasing ctx does not affect the outcome, each AioContext
 136  * still only has one home thread that is responsible for running it.
 137  */
 138 static inline bool in_aio_context_home_thread(AioContext *ctx)
 139 {
 140     if (ctx == qemu_get_current_aio_context()) {
 141         return true;
 142     }
 143
 144     if (ctx == qemu_get_aio_context()) {
 145         return qemu_mutex_iothread_locked();
 146     } else {
 147         return false;
 148     }
 149 }
 150
 151 #endif /* QEMU_AIO_WAIT_H */