include/block/graph-lock.h

   1 /*
   2  * Graph lock: rwlock to protect block layer graph manipulations (add/remove
   3  * edges and nodes)
   4  *
   5  *  Copyright (c) 2022 Red Hat
   6  *
   7  * This library is free software; you can redistribute it and/or
   8  * modify it under the terms of the GNU Lesser General Public
   9  * License as published by the Free Software Foundation; either
  10  * version 2.1 of the License, or (at your option) any later version.
  11  *
  12  * This library is distributed in the hope that it will be useful,
  13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  15  * Lesser General Public License for more details.
  16  *
  17  * You should have received a copy of the GNU Lesser General Public
  18  * License along with this library; if not, see <http://www.gnu.org/licenses/>.
  19  */
  20 #ifndef GRAPH_LOCK_H
  21 #define GRAPH_LOCK_H
  22
  23 #include "qemu/clang-tsa.h"
  24
  25 /**
  26  * Graph Lock API
  27  * This API provides a rwlock used to protect block layer
  28  * graph modifications like edge (BdrvChild) and node (BlockDriverState)
  29  * addition and removal.
  30  * Currently we have 1 writer only, the Main loop, and many
  31  * readers, mostly coroutines running in other AioContext thus other threads.
  32  *
  33  * We distinguish between writer (main loop, under BQL) that modifies the
  34  * graph, and readers (all other coroutines running in various AioContext),
  35  * that go through the graph edges, reading
  36  * BlockDriverState ->parents and->children.
  37  *
  38  * The writer (main loop)  has an "exclusive" access, so it first waits for
  39  * current read to finish, and then prevents incoming ones from
  40  * entering while it has the exclusive access.
  41  *
  42  * The readers (coroutines in multiple AioContext) are free to
  43  * access the graph as long the writer is not modifying the graph.
  44  * In case it is, they go in a CoQueue and sleep until the writer
  45  * is done.
  46  *
  47  * If a coroutine changes AioContext, the counter in the original and new
  48  * AioContext are left intact, since the writer does not care where is the
  49  * reader, but only if there is one.
  50  * As a result, some AioContexts might have a negative reader count, to
  51  * balance the positive count of the AioContext that took the lock.
  52  * This also means that when an AioContext is deleted it may have a nonzero
  53  * reader count. In that case we transfer the count to a global shared counter
  54  * so that the writer is always aware of all readers.
  55  */
  56 typedef struct BdrvGraphRWlock BdrvGraphRWlock;
  57
  58 /* Dummy lock object to use for Thread Safety Analysis (TSA) */
  59 typedef struct TSA_CAPABILITY("mutex") BdrvGraphLock {
  60 } BdrvGraphLock;
  61
  62 extern BdrvGraphLock graph_lock;
  63
  64 /*
  65  * clang doesn't check consistency in locking annotations between forward
  66  * declarations and the function definition. Having the annotation on the
  67  * definition, but not the declaration in a header file, may give the reader
  68  * a false sense of security because the condition actually remains unchecked
  69  * for callers in other source files.
  70  *
  71  * Therefore, as a convention, for public functions, GRAPH_RDLOCK and
  72  * GRAPH_WRLOCK annotations should be present only in the header file.
  73  */
  74 #define GRAPH_WRLOCK TSA_REQUIRES(graph_lock)
  75 #define GRAPH_RDLOCK TSA_REQUIRES_SHARED(graph_lock)
  76 #define GRAPH_UNLOCKED TSA_EXCLUDES(graph_lock)
  77
  78 /*
  79  * TSA annotations are not part of function types, so checks are defeated when
  80  * using a function pointer. As a workaround, annotate function pointers with
  81  * this macro that will require that the lock is at least taken while reading
  82  * the pointer. In most cases this is equivalent to actually protecting the
  83  * function call.
  84  */
  85 #define GRAPH_RDLOCK_PTR TSA_GUARDED_BY(graph_lock)
  86 #define GRAPH_WRLOCK_PTR TSA_GUARDED_BY(graph_lock)
  87 #define GRAPH_UNLOCKED_PTR
  88
  89 /*
  90  * register_aiocontext:
  91  * Add AioContext @ctx to the list of AioContext.
  92  * This list is used to obtain the total number of readers
  93  * currently running the graph.
  94  */
  95 void register_aiocontext(AioContext *ctx);
  96
  97 /*
  98  * unregister_aiocontext:
  99  * Removes AioContext @ctx to the list of AioContext.
 100  */
 101 void unregister_aiocontext(AioContext *ctx);
 102
 103 /*
 104  * bdrv_graph_wrlock:
 105  * Start an exclusive write operation to modify the graph. This means we are
 106  * adding or removing an edge or a node in the block layer graph. Nobody else
 107  * is allowed to access the graph.
 108  *
 109  * Must only be called from outside bdrv_graph_co_rdlock.
 110  *
 111  * The wrlock can only be taken from the main loop, with BQL held, as only the
 112  * main loop is allowed to modify the graph.
 113  *
 114  * This function polls. Callers must not hold the lock of any AioContext other
 115  * than the current one.
 116  */
 117 void bdrv_graph_wrlock(void) TSA_ACQUIRE(graph_lock) TSA_NO_TSA;
 118
 119 /*
 120  * bdrv_graph_wrunlock:
 121  * Write finished, reset global has_writer to 0 and restart
 122  * all readers that are waiting.
 123  */
 124 void bdrv_graph_wrunlock(void) TSA_RELEASE(graph_lock) TSA_NO_TSA;
 125
 126 /*
 127  * bdrv_graph_co_rdlock:
 128  * Read the bs graph. This usually means traversing all nodes in
 129  * the graph, therefore it can't happen while another thread is
 130  * modifying it.
 131  * Increases the reader counter of the current aiocontext,
 132  * and if has_writer is set, it means that the writer is modifying
 133  * the graph, therefore wait in a coroutine queue.
 134  * The writer will then wake this coroutine once it is done.
 135  *
 136  * This lock should be taken from Iothreads (IO_CODE() class of functions)
 137  * because it signals the writer that there are some
 138  * readers currently running, or waits until the current
 139  * write is finished before continuing.
 140  * Calling this function from the Main Loop with BQL held
 141  * is not necessary, since the Main Loop itself is the only
 142  * writer, thus won't be able to read and write at the same time.
 143  * The only exception to that is when we can't take the lock in the
 144  * function/coroutine itself, and need to delegate the caller (usually main
 145  * loop) to take it and wait that the coroutine ends, so that
 146  * we always signal that a reader is running.
 147  */
 148 void coroutine_fn TSA_ACQUIRE_SHARED(graph_lock) TSA_NO_TSA
 149 bdrv_graph_co_rdlock(void);
 150
 151 /*
 152  * bdrv_graph_rdunlock:
 153  * Read terminated, decrease the count of readers in the current aiocontext.
 154  * If the writer is waiting for reads to finish (has_writer == 1), signal
 155  * the writer that we are done via aio_wait_kick() to let it continue.
 156  */
 157 void coroutine_fn TSA_RELEASE_SHARED(graph_lock) TSA_NO_TSA
 158 bdrv_graph_co_rdunlock(void);
 159
 160 /*
 161  * bdrv_graph_rd{un}lock_main_loop:
 162  * Just a placeholder to mark where the graph rdlock should be taken
 163  * in the main loop. It is just asserting that we are not
 164  * in a coroutine and in GLOBAL_STATE_CODE.
 165  */
 166 void TSA_ACQUIRE_SHARED(graph_lock) TSA_NO_TSA
 167 bdrv_graph_rdlock_main_loop(void);
 168
 169 void TSA_RELEASE_SHARED(graph_lock) TSA_NO_TSA
 170 bdrv_graph_rdunlock_main_loop(void);
 171
 172 /*
 173  * assert_bdrv_graph_readable:
 174  * Make sure that the reader is either the main loop,
 175  * or there is at least a reader helding the rdlock.
 176  * In this way an incoming writer is aware of the read and waits.
 177  */
 178 void GRAPH_RDLOCK assert_bdrv_graph_readable(void);
 179
 180 /*
 181  * assert_bdrv_graph_writable:
 182  * Make sure that the writer is the main loop and has set @has_writer,
 183  * so that incoming readers will pause.
 184  */
 185 void GRAPH_WRLOCK assert_bdrv_graph_writable(void);
 186
 187 /*
 188  * Calling this function tells TSA that we know that the lock is effectively
 189  * taken even though we cannot prove it (yet) with GRAPH_RDLOCK. This can be
 190  * useful in intermediate stages of a conversion to using the GRAPH_RDLOCK
 191  * macro.
 192  */
 193 static inline void TSA_ASSERT_SHARED(graph_lock) TSA_NO_TSA
 194 assume_graph_lock(void)
 195 {
 196 }
 197
 198 typedef struct GraphLockable { } GraphLockable;
 199
 200 /*
 201  * In C, compound literals have the lifetime of an automatic variable.
 202  * In C++ it would be different, but then C++ wouldn't need QemuLockable
 203  * either...
 204  */
 205 #define GML_OBJ_() (&(GraphLockable) { })
 206
 207 /*
 208  * This is not marked as TSA_ACQUIRE_SHARED() because TSA doesn't understand the
 209  * cleanup attribute and would therefore complain that the graph is never
 210  * unlocked. TSA_ASSERT_SHARED() makes sure that the following calls know that
 211  * we hold the lock while unlocking is left unchecked.
 212  */
 213 static inline GraphLockable * TSA_ASSERT_SHARED(graph_lock) TSA_NO_TSA coroutine_fn
 214 graph_lockable_auto_lock(GraphLockable *x)
 215 {
 216     bdrv_graph_co_rdlock();
 217     return x;
 218 }
 219
 220 static inline void TSA_NO_TSA coroutine_fn
 221 graph_lockable_auto_unlock(GraphLockable *x)
 222 {
 223     bdrv_graph_co_rdunlock();
 224 }
 225
 226 G_DEFINE_AUTOPTR_CLEANUP_FUNC(GraphLockable, graph_lockable_auto_unlock)
 227
 228 #define WITH_GRAPH_RDLOCK_GUARD_(var)                                         \
 229     for (g_autoptr(GraphLockable) var = graph_lockable_auto_lock(GML_OBJ_()); \
 230          var;                                                                 \
 231          graph_lockable_auto_unlock(var), var = NULL)
 232
 233 #define WITH_GRAPH_RDLOCK_GUARD() \
 234     WITH_GRAPH_RDLOCK_GUARD_(glue(graph_lockable_auto, __COUNTER__))
 235
 236 #define GRAPH_RDLOCK_GUARD(x)                                       \
 237     g_autoptr(GraphLockable)                                        \
 238     glue(graph_lockable_auto, __COUNTER__) G_GNUC_UNUSED =          \
 239             graph_lockable_auto_lock(GML_OBJ_())
 240
 241
 242 typedef struct GraphLockableMainloop { } GraphLockableMainloop;
 243
 244 /*
 245  * In C, compound literals have the lifetime of an automatic variable.
 246  * In C++ it would be different, but then C++ wouldn't need QemuLockable
 247  * either...
 248  */
 249 #define GMLML_OBJ_() (&(GraphLockableMainloop) { })
 250
 251 /*
 252  * This is not marked as TSA_ACQUIRE_SHARED() because TSA doesn't understand the
 253  * cleanup attribute and would therefore complain that the graph is never
 254  * unlocked. TSA_ASSERT_SHARED() makes sure that the following calls know that
 255  * we hold the lock while unlocking is left unchecked.
 256  */
 257 static inline GraphLockableMainloop * TSA_ASSERT_SHARED(graph_lock) TSA_NO_TSA
 258 graph_lockable_auto_lock_mainloop(GraphLockableMainloop *x)
 259 {
 260     bdrv_graph_rdlock_main_loop();
 261     return x;
 262 }
 263
 264 static inline void TSA_NO_TSA
 265 graph_lockable_auto_unlock_mainloop(GraphLockableMainloop *x)
 266 {
 267     bdrv_graph_rdunlock_main_loop();
 268 }
 269
 270 G_DEFINE_AUTOPTR_CLEANUP_FUNC(GraphLockableMainloop,
 271                               graph_lockable_auto_unlock_mainloop)
 272
 273 #define GRAPH_RDLOCK_GUARD_MAINLOOP(x)                              \
 274     g_autoptr(GraphLockableMainloop)                                \
 275     glue(graph_lockable_auto, __COUNTER__) G_GNUC_UNUSED =          \
 276             graph_lockable_auto_lock_mainloop(GMLML_OBJ_())
 277
 278 #endif /* GRAPH_LOCK_H */
 279