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 * If @bs is non-NULL, its AioContext is temporarily released. 115 * 116 * This function polls. Callers must not hold the lock of any AioContext other 117 * than the current one and the one of @bs. 118 */ 119 void bdrv_graph_wrlock(BlockDriverState *bs) TSA_ACQUIRE(graph_lock) TSA_NO_TSA; 120 121 /* 122 * bdrv_graph_wrunlock: 123 * Write finished, reset global has_writer to 0 and restart 124 * all readers that are waiting. 125 */ 126 void bdrv_graph_wrunlock(void) TSA_RELEASE(graph_lock) TSA_NO_TSA; 127 128 /* 129 * bdrv_graph_co_rdlock: 130 * Read the bs graph. This usually means traversing all nodes in 131 * the graph, therefore it can't happen while another thread is 132 * modifying it. 133 * Increases the reader counter of the current aiocontext, 134 * and if has_writer is set, it means that the writer is modifying 135 * the graph, therefore wait in a coroutine queue. 136 * The writer will then wake this coroutine once it is done. 137 * 138 * This lock should be taken from Iothreads (IO_CODE() class of functions) 139 * because it signals the writer that there are some 140 * readers currently running, or waits until the current 141 * write is finished before continuing. 142 * Calling this function from the Main Loop with BQL held 143 * is not necessary, since the Main Loop itself is the only 144 * writer, thus won't be able to read and write at the same time. 145 * The only exception to that is when we can't take the lock in the 146 * function/coroutine itself, and need to delegate the caller (usually main 147 * loop) to take it and wait that the coroutine ends, so that 148 * we always signal that a reader is running. 149 */ 150 void coroutine_fn TSA_ACQUIRE_SHARED(graph_lock) TSA_NO_TSA 151 bdrv_graph_co_rdlock(void); 152 153 /* 154 * bdrv_graph_rdunlock: 155 * Read terminated, decrease the count of readers in the current aiocontext. 156 * If the writer is waiting for reads to finish (has_writer == 1), signal 157 * the writer that we are done via aio_wait_kick() to let it continue. 158 */ 159 void coroutine_fn TSA_RELEASE_SHARED(graph_lock) TSA_NO_TSA 160 bdrv_graph_co_rdunlock(void); 161 162 /* 163 * bdrv_graph_rd{un}lock_main_loop: 164 * Just a placeholder to mark where the graph rdlock should be taken 165 * in the main loop. It is just asserting that we are not 166 * in a coroutine and in GLOBAL_STATE_CODE. 167 */ 168 void TSA_ACQUIRE_SHARED(graph_lock) TSA_NO_TSA 169 bdrv_graph_rdlock_main_loop(void); 170 171 void TSA_RELEASE_SHARED(graph_lock) TSA_NO_TSA 172 bdrv_graph_rdunlock_main_loop(void); 173 174 /* 175 * assert_bdrv_graph_readable: 176 * Make sure that the reader is either the main loop, 177 * or there is at least a reader helding the rdlock. 178 * In this way an incoming writer is aware of the read and waits. 179 */ 180 void GRAPH_RDLOCK assert_bdrv_graph_readable(void); 181 182 /* 183 * assert_bdrv_graph_writable: 184 * Make sure that the writer is the main loop and has set @has_writer, 185 * so that incoming readers will pause. 186 */ 187 void GRAPH_WRLOCK assert_bdrv_graph_writable(void); 188 189 /* 190 * Calling this function tells TSA that we know that the lock is effectively 191 * taken even though we cannot prove it (yet) with GRAPH_RDLOCK. This can be 192 * useful in intermediate stages of a conversion to using the GRAPH_RDLOCK 193 * macro. 194 */ 195 static inline void TSA_ASSERT_SHARED(graph_lock) TSA_NO_TSA 196 assume_graph_lock(void) 197 { 198 } 199 200 typedef struct GraphLockable { } GraphLockable; 201 202 /* 203 * In C, compound literals have the lifetime of an automatic variable. 204 * In C++ it would be different, but then C++ wouldn't need QemuLockable 205 * either... 206 */ 207 #define GML_OBJ_() (&(GraphLockable) { }) 208 209 /* 210 * This is not marked as TSA_ACQUIRE_SHARED() because TSA doesn't understand the 211 * cleanup attribute and would therefore complain that the graph is never 212 * unlocked. TSA_ASSERT_SHARED() makes sure that the following calls know that 213 * we hold the lock while unlocking is left unchecked. 214 */ 215 static inline GraphLockable * TSA_ASSERT_SHARED(graph_lock) TSA_NO_TSA coroutine_fn 216 graph_lockable_auto_lock(GraphLockable *x) 217 { 218 bdrv_graph_co_rdlock(); 219 return x; 220 } 221 222 static inline void TSA_NO_TSA coroutine_fn 223 graph_lockable_auto_unlock(GraphLockable *x) 224 { 225 bdrv_graph_co_rdunlock(); 226 } 227 228 G_DEFINE_AUTOPTR_CLEANUP_FUNC(GraphLockable, graph_lockable_auto_unlock) 229 230 #define WITH_GRAPH_RDLOCK_GUARD_(var) \ 231 for (g_autoptr(GraphLockable) var = graph_lockable_auto_lock(GML_OBJ_()); \ 232 var; \ 233 graph_lockable_auto_unlock(var), var = NULL) 234 235 #define WITH_GRAPH_RDLOCK_GUARD() \ 236 WITH_GRAPH_RDLOCK_GUARD_(glue(graph_lockable_auto, __COUNTER__)) 237 238 #define GRAPH_RDLOCK_GUARD(x) \ 239 g_autoptr(GraphLockable) \ 240 glue(graph_lockable_auto, __COUNTER__) G_GNUC_UNUSED = \ 241 graph_lockable_auto_lock(GML_OBJ_()) 242 243 244 typedef struct GraphLockableMainloop { } GraphLockableMainloop; 245 246 /* 247 * In C, compound literals have the lifetime of an automatic variable. 248 * In C++ it would be different, but then C++ wouldn't need QemuLockable 249 * either... 250 */ 251 #define GMLML_OBJ_() (&(GraphLockableMainloop) { }) 252 253 /* 254 * This is not marked as TSA_ACQUIRE_SHARED() because TSA doesn't understand the 255 * cleanup attribute and would therefore complain that the graph is never 256 * unlocked. TSA_ASSERT_SHARED() makes sure that the following calls know that 257 * we hold the lock while unlocking is left unchecked. 258 */ 259 static inline GraphLockableMainloop * TSA_ASSERT_SHARED(graph_lock) TSA_NO_TSA 260 graph_lockable_auto_lock_mainloop(GraphLockableMainloop *x) 261 { 262 bdrv_graph_rdlock_main_loop(); 263 return x; 264 } 265 266 static inline void TSA_NO_TSA 267 graph_lockable_auto_unlock_mainloop(GraphLockableMainloop *x) 268 { 269 bdrv_graph_rdunlock_main_loop(); 270 } 271 272 G_DEFINE_AUTOPTR_CLEANUP_FUNC(GraphLockableMainloop, 273 graph_lockable_auto_unlock_mainloop) 274 275 #define GRAPH_RDLOCK_GUARD_MAINLOOP(x) \ 276 g_autoptr(GraphLockableMainloop) \ 277 glue(graph_lockable_auto, __COUNTER__) G_GNUC_UNUSED = \ 278 graph_lockable_auto_lock_mainloop(GMLML_OBJ_()) 279 280 #endif /* GRAPH_LOCK_H */ 281 282