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_INTERNAL:
  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_INTERNAL(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__after_rmw();                                           \
  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             aio_poll(qemu_get_aio_context(), true);                \
  96             waited_ = true;                                        \
  97         }                                                          \
  98     }                                                              \
  99     qatomic_dec(&wait_->num_waiters);                              \
 100     waited_; })
 101
 102 #define AIO_WAIT_WHILE(ctx, cond)                                  \
 103     AIO_WAIT_WHILE_INTERNAL(ctx, cond)
 104
 105 /* TODO replace this with AIO_WAIT_WHILE() in a future patch */
 106 #define AIO_WAIT_WHILE_UNLOCKED(ctx, cond)                         \
 107     AIO_WAIT_WHILE_INTERNAL(ctx, cond)
 108
 109 /**
 110  * aio_wait_kick:
 111  * Wake up the main thread if it is waiting on AIO_WAIT_WHILE().  During
 112  * synchronous operations performed in an IOThread, the main thread lets the
 113  * IOThread's event loop run, waiting for the operation to complete.  A
 114  * aio_wait_kick() call will wake up the main thread.
 115  */
 116 void aio_wait_kick(void);
 117
 118 /**
 119  * aio_wait_bh_oneshot:
 120  * @ctx: the aio context
 121  * @cb: the BH callback function
 122  * @opaque: user data for the BH callback function
 123  *
 124  * Run a BH in @ctx and wait for it to complete.
 125  *
 126  * Must be called from the main loop thread without @ctx acquired.
 127  * Note that main loop event processing may occur.
 128  */
 129 void aio_wait_bh_oneshot(AioContext *ctx, QEMUBHFunc *cb, void *opaque);
 130
 131 /**
 132  * in_aio_context_home_thread:
 133  * @ctx: the aio context
 134  *
 135  * Return whether we are running in the thread that normally runs @ctx.  Note
 136  * that acquiring/releasing ctx does not affect the outcome, each AioContext
 137  * still only has one home thread that is responsible for running it.
 138  */
 139 static inline bool in_aio_context_home_thread(AioContext *ctx)
 140 {
 141     if (ctx == qemu_get_current_aio_context()) {
 142         return true;
 143     }
 144
 145     if (ctx == qemu_get_aio_context()) {
 146         return bql_locked();
 147     } else {
 148         return false;
 149     }
 150 }
 151
 152 #endif /* QEMU_AIO_WAIT_H */