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 temporarily 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 no_coroutine_fn bdrv_graph_wrlock(BlockDriverState *bs)
 110 {
 111     AioContext *ctx = NULL;
 112
 113     GLOBAL_STATE_CODE();
 114     assert(!qatomic_read(&has_writer));
 115     assert(!qemu_in_coroutine());
 116
 117     /*
 118      * Release only non-mainloop AioContext. The mainloop often relies on the
 119      * BQL and doesn't lock the main AioContext before doing things.
 120      */
 121     if (bs) {
 122         ctx = bdrv_get_aio_context(bs);
 123         if (ctx != qemu_get_aio_context()) {
 124             aio_context_release(ctx);
 125         } else {
 126             ctx = NULL;
 127         }
 128     }
 129
 130     /* Make sure that constantly arriving new I/O doesn't cause starvation */
 131     bdrv_drain_all_begin_nopoll();
 132
 133     /*
 134      * reader_count == 0: this means writer will read has_reader as 1
 135      * reader_count >= 1: we don't know if writer read has_writer == 0 or 1,
 136      *                    but we need to wait.
 137      * Wait by allowing other coroutine (and possible readers) to continue.
 138      */
 139     do {
 140         /*
 141          * has_writer must be 0 while polling, otherwise we get a deadlock if
 142          * any callback involved during AIO_WAIT_WHILE() tries to acquire the
 143          * reader lock.
 144          */
 145         qatomic_set(&has_writer, 0);
 146         AIO_WAIT_WHILE_UNLOCKED(NULL, reader_count() >= 1);
 147         qatomic_set(&has_writer, 1);
 148
 149         /*
 150          * We want to only check reader_count() after has_writer = 1 is visible
 151          * to other threads. That way no more readers can sneak in after we've
 152          * determined reader_count() == 0.
 153          */
 154         smp_mb();
 155     } while (reader_count() >= 1);
 156
 157     bdrv_drain_all_end();
 158
 159     if (ctx) {
 160         aio_context_acquire(bdrv_get_aio_context(bs));
 161     }
 162 }
 163
 164 void bdrv_graph_wrunlock(void)
 165 {
 166     GLOBAL_STATE_CODE();
 167     assert(qatomic_read(&has_writer));
 168
 169     WITH_QEMU_LOCK_GUARD(&aio_context_list_lock) {
 170         /*
 171          * No need for memory barriers, this works in pair with
 172          * the slow path of rdlock() and both take the lock.
 173          */
 174         qatomic_store_release(&has_writer, 0);
 175
 176         /* Wake up all coroutines that are waiting to read the graph */
 177         qemu_co_enter_all(&reader_queue, &aio_context_list_lock);
 178     }
 179
 180     /*
 181      * Run any BHs that were scheduled during the wrlock section and that
 182      * callers might expect to have finished (in particular, this is important
 183      * for bdrv_schedule_unref()).
 184      *
 185      * Do this only after restarting coroutines so that nested event loops in
 186      * BHs don't deadlock if their condition relies on the coroutine making
 187      * progress.
 188      */
 189     aio_bh_poll(qemu_get_aio_context());
 190 }
 191
 192 void coroutine_fn bdrv_graph_co_rdlock(void)
 193 {
 194     BdrvGraphRWlock *bdrv_graph;
 195     bdrv_graph = qemu_get_current_aio_context()->bdrv_graph;
 196
 197     for (;;) {
 198         qatomic_set(&bdrv_graph->reader_count,
 199                     bdrv_graph->reader_count + 1);
 200         /* make sure writer sees reader_count before we check has_writer */
 201         smp_mb();
 202
 203         /*
 204          * has_writer == 0: this means writer will read reader_count as >= 1
 205          * has_writer == 1: we don't know if writer read reader_count == 0
 206          *                  or > 0, but we need to wait anyways because
 207          *                  it will write.
 208          */
 209         if (!qatomic_read(&has_writer)) {
 210             break;
 211         }
 212
 213         /*
 214          * Synchronize access with reader_count() in bdrv_graph_wrlock().
 215          * Case 1:
 216          * If this critical section gets executed first, reader_count will
 217          * decrease and the reader will go to sleep.
 218          * Then the writer will read reader_count that does not take into
 219          * account this reader, and if there's no other reader it will
 220          * enter the write section.
 221          * Case 2:
 222          * If reader_count() critical section gets executed first,
 223          * then writer will read reader_count >= 1.
 224          * It will wait in AIO_WAIT_WHILE(), but once it releases the lock
 225          * we will enter this critical section and call aio_wait_kick().
 226          */
 227         WITH_QEMU_LOCK_GUARD(&aio_context_list_lock) {
 228             /*
 229              * Additional check when we use the above lock to synchronize
 230              * with bdrv_graph_wrunlock().
 231              * Case 1:
 232              * If this gets executed first, has_writer is still 1, so we reduce
 233              * reader_count and go to sleep.
 234              * Then the writer will set has_writer to 0 and wake up all readers,
 235              * us included.
 236              * Case 2:
 237              * If bdrv_graph_wrunlock() critical section gets executed first,
 238              * then it will set has_writer to 0 and wake up all other readers.
 239              * Then we execute this critical section, and therefore must check
 240              * again for has_writer, otherwise we sleep without any writer
 241              * actually running.
 242              */
 243             if (!qatomic_read(&has_writer)) {
 244                 return;
 245             }
 246
 247             /* slow path where reader sleeps */
 248             bdrv_graph->reader_count--;
 249             aio_wait_kick();
 250             qemu_co_queue_wait(&reader_queue, &aio_context_list_lock);
 251         }
 252     }
 253 }
 254
 255 void coroutine_fn bdrv_graph_co_rdunlock(void)
 256 {
 257     BdrvGraphRWlock *bdrv_graph;
 258     bdrv_graph = qemu_get_current_aio_context()->bdrv_graph;
 259
 260     qatomic_store_release(&bdrv_graph->reader_count,
 261                           bdrv_graph->reader_count - 1);
 262     /* make sure writer sees reader_count before we check has_writer */
 263     smp_mb();
 264
 265     /*
 266      * has_writer == 0: this means reader will read reader_count decreased
 267      * has_writer == 1: we don't know if writer read reader_count old or
 268      *                  new. Therefore, kick again so on next iteration
 269      *                  writer will for sure read the updated value.
 270      */
 271     if (qatomic_read(&has_writer)) {
 272         aio_wait_kick();
 273     }
 274 }
 275
 276 void bdrv_graph_rdlock_main_loop(void)
 277 {
 278     GLOBAL_STATE_CODE();
 279     assert(!qemu_in_coroutine());
 280 }
 281
 282 void bdrv_graph_rdunlock_main_loop(void)
 283 {
 284     GLOBAL_STATE_CODE();
 285     assert(!qemu_in_coroutine());
 286 }
 287
 288 void assert_bdrv_graph_readable(void)
 289 {
 290     /* reader_count() is slow due to aio_context_list_lock lock contention */
 291 #ifdef CONFIG_DEBUG_GRAPH_LOCK
 292     assert(qemu_in_main_thread() || reader_count());
 293 #endif
 294 }
 295
 296 void assert_bdrv_graph_writable(void)
 297 {
 298     assert(qemu_in_main_thread());
 299     assert(qatomic_read(&has_writer));
 300 }