block/graph-lock.c

   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
  21 #include "qemu/osdep.h"
  22 #include "qemu/main-loop.h"
  23 #include "block/graph-lock.h"
  24 #include "block/block.h"
  25 #include "block/block_int.h"
  26
  27 /* Dummy lock object to use for Thread Safety Analysis (TSA) */
  28 BdrvGraphLock graph_lock;
  29
  30 /* Protects the list of aiocontext and orphaned_reader_count */
  31 static QemuMutex aio_context_list_lock;
  32
  33 /* Written and read with atomic operations. */
  34 static int has_writer;
  35
  36 /*
  37  * A reader coroutine could move from an AioContext to another.
  38  * If this happens, there is no problem from the point of view of
  39  * counters. The problem is that the total count becomes
  40  * unbalanced if one of the two AioContexts gets deleted.
  41  * The count of readers must remain correct, so the AioContext's
  42  * balance is transferred to this glboal variable.
  43  * Protected by aio_context_list_lock.
  44  */
  45 static uint32_t orphaned_reader_count;
  46
  47 /* Queue of readers waiting for the writer to finish */
  48 static CoQueue reader_queue;
  49
  50 struct BdrvGraphRWlock {
  51     /* How many readers are currently reading the graph. */
  52     uint32_t reader_count;
  53
  54     /*
  55      * List of BdrvGraphRWlock kept in graph-lock.c
  56      * Protected by aio_context_list_lock
  57      */
  58     QTAILQ_ENTRY(BdrvGraphRWlock) next_aio;
  59 };
  60
  61 /*
  62  * List of BdrvGraphRWlock. This list ensures that each BdrvGraphRWlock
  63  * can safely modify only its own counter, avoid reading/writing
  64  * others and thus improving performances by avoiding cacheline bounces.
  65  */
  66 static QTAILQ_HEAD(, BdrvGraphRWlock) aio_context_list =
  67     QTAILQ_HEAD_INITIALIZER(aio_context_list);
  68
  69 static void __attribute__((__constructor__)) bdrv_init_graph_lock(void)
  70 {
  71     qemu_mutex_init(&aio_context_list_lock);
  72     qemu_co_queue_init(&reader_queue);
  73 }
  74
  75 void register_aiocontext(AioContext *ctx)
  76 {
  77     ctx->bdrv_graph = g_new0(BdrvGraphRWlock, 1);
  78     QEMU_LOCK_GUARD(&aio_context_list_lock);
  79     assert(ctx->bdrv_graph->reader_count == 0);
  80     QTAILQ_INSERT_TAIL(&aio_context_list, ctx->bdrv_graph, next_aio);
  81 }
  82
  83 void unregister_aiocontext(AioContext *ctx)
  84 {
  85     QEMU_LOCK_GUARD(&aio_context_list_lock);
  86     orphaned_reader_count += ctx->bdrv_graph->reader_count;
  87     QTAILQ_REMOVE(&aio_context_list, ctx->bdrv_graph, next_aio);
  88     g_free(ctx->bdrv_graph);
  89 }
  90
  91 static uint32_t reader_count(void)
  92 {
  93     BdrvGraphRWlock *brdv_graph;
  94     uint32_t rd;
  95
  96     QEMU_LOCK_GUARD(&aio_context_list_lock);
  97
  98     /* rd can temporarly be negative, but the total will *always* be >= 0 */
  99     rd = orphaned_reader_count;
 100     QTAILQ_FOREACH(brdv_graph, &aio_context_list, next_aio) {
 101         rd += qatomic_read(&brdv_graph->reader_count);
 102     }
 103
 104     /* shouldn't overflow unless there are 2^31 readers */
 105     assert((int32_t)rd >= 0);
 106     return rd;
 107 }
 108
 109 void bdrv_graph_wrlock(BlockDriverState *bs)
 110 {
 111     AioContext *ctx = NULL;
 112
 113     GLOBAL_STATE_CODE();
 114     assert(!qatomic_read(&has_writer));
 115
 116     /*
 117      * Release only non-mainloop AioContext. The mainloop often relies on the
 118      * BQL and doesn't lock the main AioContext before doing things.
 119      */
 120     if (bs) {
 121         ctx = bdrv_get_aio_context(bs);
 122         if (ctx != qemu_get_aio_context()) {
 123             aio_context_release(ctx);
 124         } else {
 125             ctx = NULL;
 126         }
 127     }
 128
 129     /* Make sure that constantly arriving new I/O doesn't cause starvation */
 130     bdrv_drain_all_begin_nopoll();
 131
 132     /*
 133      * reader_count == 0: this means writer will read has_reader as 1
 134      * reader_count >= 1: we don't know if writer read has_writer == 0 or 1,
 135      *                    but we need to wait.
 136      * Wait by allowing other coroutine (and possible readers) to continue.
 137      */
 138     do {
 139         /*
 140          * has_writer must be 0 while polling, otherwise we get a deadlock if
 141          * any callback involved during AIO_WAIT_WHILE() tries to acquire the
 142          * reader lock.
 143          */
 144         qatomic_set(&has_writer, 0);
 145         AIO_WAIT_WHILE_UNLOCKED(NULL, reader_count() >= 1);
 146         qatomic_set(&has_writer, 1);
 147
 148         /*
 149          * We want to only check reader_count() after has_writer = 1 is visible
 150          * to other threads. That way no more readers can sneak in after we've
 151          * determined reader_count() == 0.
 152          */
 153         smp_mb();
 154     } while (reader_count() >= 1);
 155
 156     bdrv_drain_all_end();
 157
 158     if (ctx) {
 159         aio_context_acquire(bdrv_get_aio_context(bs));
 160     }
 161 }
 162
 163 void bdrv_graph_wrunlock(void)
 164 {
 165     GLOBAL_STATE_CODE();
 166     QEMU_LOCK_GUARD(&aio_context_list_lock);
 167     assert(qatomic_read(&has_writer));
 168
 169     /*
 170      * No need for memory barriers, this works in pair with
 171      * the slow path of rdlock() and both take the lock.
 172      */
 173     qatomic_store_release(&has_writer, 0);
 174
 175     /* Wake up all coroutine that are waiting to read the graph */
 176     qemu_co_enter_all(&reader_queue, &aio_context_list_lock);
 177 }
 178
 179 void coroutine_fn bdrv_graph_co_rdlock(void)
 180 {
 181     BdrvGraphRWlock *bdrv_graph;
 182     bdrv_graph = qemu_get_current_aio_context()->bdrv_graph;
 183
 184     for (;;) {
 185         qatomic_set(&bdrv_graph->reader_count,
 186                     bdrv_graph->reader_count + 1);
 187         /* make sure writer sees reader_count before we check has_writer */
 188         smp_mb();
 189
 190         /*
 191          * has_writer == 0: this means writer will read reader_count as >= 1
 192          * has_writer == 1: we don't know if writer read reader_count == 0
 193          *                  or > 0, but we need to wait anyways because
 194          *                  it will write.
 195          */
 196         if (!qatomic_read(&has_writer)) {
 197             break;
 198         }
 199
 200         /*
 201          * Synchronize access with reader_count() in bdrv_graph_wrlock().
 202          * Case 1:
 203          * If this critical section gets executed first, reader_count will
 204          * decrease and the reader will go to sleep.
 205          * Then the writer will read reader_count that does not take into
 206          * account this reader, and if there's no other reader it will
 207          * enter the write section.
 208          * Case 2:
 209          * If reader_count() critical section gets executed first,
 210          * then writer will read reader_count >= 1.
 211          * It will wait in AIO_WAIT_WHILE(), but once it releases the lock
 212          * we will enter this critical section and call aio_wait_kick().
 213          */
 214         WITH_QEMU_LOCK_GUARD(&aio_context_list_lock) {
 215             /*
 216              * Additional check when we use the above lock to synchronize
 217              * with bdrv_graph_wrunlock().
 218              * Case 1:
 219              * If this gets executed first, has_writer is still 1, so we reduce
 220              * reader_count and go to sleep.
 221              * Then the writer will set has_writer to 0 and wake up all readers,
 222              * us included.
 223              * Case 2:
 224              * If bdrv_graph_wrunlock() critical section gets executed first,
 225              * then it will set has_writer to 0 and wake up all other readers.
 226              * Then we execute this critical section, and therefore must check
 227              * again for has_writer, otherwise we sleep without any writer
 228              * actually running.
 229              */
 230             if (!qatomic_read(&has_writer)) {
 231                 return;
 232             }
 233
 234             /* slow path where reader sleeps */
 235             bdrv_graph->reader_count--;
 236             aio_wait_kick();
 237             qemu_co_queue_wait(&reader_queue, &aio_context_list_lock);
 238         }
 239     }
 240 }
 241
 242 void coroutine_fn bdrv_graph_co_rdunlock(void)
 243 {
 244     BdrvGraphRWlock *bdrv_graph;
 245     bdrv_graph = qemu_get_current_aio_context()->bdrv_graph;
 246
 247     qatomic_store_release(&bdrv_graph->reader_count,
 248                           bdrv_graph->reader_count - 1);
 249     /* make sure writer sees reader_count before we check has_writer */
 250     smp_mb();
 251
 252     /*
 253      * has_writer == 0: this means reader will read reader_count decreased
 254      * has_writer == 1: we don't know if writer read reader_count old or
 255      *                  new. Therefore, kick again so on next iteration
 256      *                  writer will for sure read the updated value.
 257      */
 258     if (qatomic_read(&has_writer)) {
 259         aio_wait_kick();
 260     }
 261 }
 262
 263 void bdrv_graph_rdlock_main_loop(void)
 264 {
 265     GLOBAL_STATE_CODE();
 266     assert(!qemu_in_coroutine());
 267 }
 268
 269 void bdrv_graph_rdunlock_main_loop(void)
 270 {
 271     GLOBAL_STATE_CODE();
 272     assert(!qemu_in_coroutine());
 273 }
 274
 275 void assert_bdrv_graph_readable(void)
 276 {
 277     /* reader_count() is slow due to aio_context_list_lock lock contention */
 278 #ifdef CONFIG_DEBUG_GRAPH_LOCK
 279     assert(qemu_in_main_thread() || reader_count());
 280 #endif
 281 }
 282
 283 void assert_bdrv_graph_writable(void)
 284 {
 285     assert(qemu_in_main_thread());
 286     assert(qatomic_read(&has_writer));
 287 }