1 files changed, 275 insertions, 0 deletions
diff --git a/block/graph-lock.c b/block/graph-lock.c
new file mode 100644
index 0000000000..454c31e691
--- /dev/null
+++ b/block/graph-lock.c
@@ -0,0 +1,275 @@
+/*
+ * Graph lock: rwlock to protect block layer graph manipulations (add/remove
+ * edges and nodes)
+ *
+ *  Copyright (c) 2022 Red Hat
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "qemu/osdep.h"
+#include "qemu/main-loop.h"
+#include "block/graph-lock.h"
+#include "block/block.h"
+#include "block/block_int.h"
+
+/* Dummy lock object to use for Thread Safety Analysis (TSA) */
+BdrvGraphLock graph_lock;
+
+/* Protects the list of aiocontext and orphaned_reader_count */
+static QemuMutex aio_context_list_lock;
+
+/* Written and read with atomic operations. */
+static int has_writer;
+
+/*
+ * A reader coroutine could move from an AioContext to another.
+ * If this happens, there is no problem from the point of view of
+ * counters. The problem is that the total count becomes
+ * unbalanced if one of the two AioContexts gets deleted.
+ * The count of readers must remain correct, so the AioContext's
+ * balance is transferred to this glboal variable.
+ * Protected by aio_context_list_lock.
+ */
+static uint32_t orphaned_reader_count;
+
+/* Queue of readers waiting for the writer to finish */
+static CoQueue reader_queue;
+
+struct BdrvGraphRWlock {
+    /* How many readers are currently reading the graph. */
+    uint32_t reader_count;
+
+    /*
+     * List of BdrvGraphRWlock kept in graph-lock.c
+     * Protected by aio_context_list_lock
+     */
+    QTAILQ_ENTRY(BdrvGraphRWlock) next_aio;
+};
+
+/*
+ * List of BdrvGraphRWlock. This list ensures that each BdrvGraphRWlock
+ * can safely modify only its own counter, avoid reading/writing
+ * others and thus improving performances by avoiding cacheline bounces.
+ */
+static QTAILQ_HEAD(, BdrvGraphRWlock) aio_context_list =
+    QTAILQ_HEAD_INITIALIZER(aio_context_list);
+
+static void __attribute__((__constructor__)) bdrv_init_graph_lock(void)
+{
+    qemu_mutex_init(&aio_context_list_lock);
+    qemu_co_queue_init(&reader_queue);
+}
+
+void register_aiocontext(AioContext *ctx)
+{
+    ctx->bdrv_graph = g_new0(BdrvGraphRWlock, 1);
+    QEMU_LOCK_GUARD(&aio_context_list_lock);
+    assert(ctx->bdrv_graph->reader_count == 0);
+    QTAILQ_INSERT_TAIL(&aio_context_list, ctx->bdrv_graph, next_aio);
+}
+
+void unregister_aiocontext(AioContext *ctx)
+{
+    QEMU_LOCK_GUARD(&aio_context_list_lock);
+    orphaned_reader_count += ctx->bdrv_graph->reader_count;
+    QTAILQ_REMOVE(&aio_context_list, ctx->bdrv_graph, next_aio);
+    g_free(ctx->bdrv_graph);
+}
+
+static uint32_t reader_count(void)
+{
+    BdrvGraphRWlock *brdv_graph;
+    uint32_t rd;
+
+    QEMU_LOCK_GUARD(&aio_context_list_lock);
+
+    /* rd can temporarly be negative, but the total will *always* be >= 0 */
+    rd = orphaned_reader_count;
+    QTAILQ_FOREACH(brdv_graph, &aio_context_list, next_aio) {
+        rd += qatomic_read(&brdv_graph->reader_count);
+    }
+
+    /* shouldn't overflow unless there are 2^31 readers */
+    assert((int32_t)rd >= 0);
+    return rd;
+}
+
+void bdrv_graph_wrlock(void)
+{
+    GLOBAL_STATE_CODE();
+    assert(!qatomic_read(&has_writer));
+
+    /* Make sure that constantly arriving new I/O doesn't cause starvation */
+    bdrv_drain_all_begin_nopoll();
+
+    /*
+     * reader_count == 0: this means writer will read has_reader as 1
+     * reader_count >= 1: we don't know if writer read has_writer == 0 or 1,
+     *                    but we need to wait.
+     * Wait by allowing other coroutine (and possible readers) to continue.
+     */
+    do {
+        /*
+         * has_writer must be 0 while polling, otherwise we get a deadlock if
+         * any callback involved during AIO_WAIT_WHILE() tries to acquire the
+         * reader lock.
+         */
+        qatomic_set(&has_writer, 0);
+        AIO_WAIT_WHILE(qemu_get_aio_context(), reader_count() >= 1);
+        qatomic_set(&has_writer, 1);
+
+        /*
+         * We want to only check reader_count() after has_writer = 1 is visible
+         * to other threads. That way no more readers can sneak in after we've
+         * determined reader_count() == 0.
+         */
+        smp_mb();
+    } while (reader_count() >= 1);
+
+    bdrv_drain_all_end();
+}
+
+void bdrv_graph_wrunlock(void)
+{
+    GLOBAL_STATE_CODE();
+    QEMU_LOCK_GUARD(&aio_context_list_lock);
+    assert(qatomic_read(&has_writer));
+
+    /*
+     * No need for memory barriers, this works in pair with
+     * the slow path of rdlock() and both take the lock.
+     */
+    qatomic_store_release(&has_writer, 0);
+
+    /* Wake up all coroutine that are waiting to read the graph */
+    qemu_co_enter_all(&reader_queue, &aio_context_list_lock);
+}
+
+void coroutine_fn bdrv_graph_co_rdlock(void)
+{
+    BdrvGraphRWlock *bdrv_graph;
+    bdrv_graph = qemu_get_current_aio_context()->bdrv_graph;
+
+    /* Do not lock if in main thread */
+    if (qemu_in_main_thread()) {
+        return;
+    }
+
+    for (;;) {
+        qatomic_set(&bdrv_graph->reader_count,
+                    bdrv_graph->reader_count + 1);
+        /* make sure writer sees reader_count before we check has_writer */
+        smp_mb();
+
+        /*
+         * has_writer == 0: this means writer will read reader_count as >= 1
+         * has_writer == 1: we don't know if writer read reader_count == 0
+         *                  or > 0, but we need to wait anyways because
+         *                  it will write.
+         */
+        if (!qatomic_read(&has_writer)) {
+            break;
+        }
+
+        /*
+         * Synchronize access with reader_count() in bdrv_graph_wrlock().
+         * Case 1:
+         * If this critical section gets executed first, reader_count will
+         * decrease and the reader will go to sleep.
+         * Then the writer will read reader_count that does not take into
+         * account this reader, and if there's no other reader it will
+         * enter the write section.
+         * Case 2:
+         * If reader_count() critical section gets executed first,
+         * then writer will read reader_count >= 1.
+         * It will wait in AIO_WAIT_WHILE(), but once it releases the lock
+         * we will enter this critical section and call aio_wait_kick().
+         */
+        WITH_QEMU_LOCK_GUARD(&aio_context_list_lock) {
+            /*
+             * Additional check when we use the above lock to synchronize
+             * with bdrv_graph_wrunlock().
+             * Case 1:
+             * If this gets executed first, has_writer is still 1, so we reduce
+             * reader_count and go to sleep.
+             * Then the writer will set has_writer to 0 and wake up all readers,
+             * us included.
+             * Case 2:
+             * If bdrv_graph_wrunlock() critical section gets executed first,
+             * then it will set has_writer to 0 and wake up all other readers.
+             * Then we execute this critical section, and therefore must check
+             * again for has_writer, otherwise we sleep without any writer
+             * actually running.
+             */
+            if (!qatomic_read(&has_writer)) {
+                return;
+            }
+
+            /* slow path where reader sleeps */
+            bdrv_graph->reader_count--;
+            aio_wait_kick();
+            qemu_co_queue_wait(&reader_queue, &aio_context_list_lock);
+        }
+    }
+}
+
+void coroutine_fn bdrv_graph_co_rdunlock(void)
+{
+    BdrvGraphRWlock *bdrv_graph;
+    bdrv_graph = qemu_get_current_aio_context()->bdrv_graph;
+
+    /* Do not lock if in main thread */
+    if (qemu_in_main_thread()) {
+        return;
+    }
+
+    qatomic_store_release(&bdrv_graph->reader_count,
+                          bdrv_graph->reader_count - 1);
+    /* make sure writer sees reader_count before we check has_writer */
+    smp_mb();
+
+    /*
+     * has_writer == 0: this means reader will read reader_count decreased
+     * has_writer == 1: we don't know if writer read reader_count old or
+     *                  new. Therefore, kick again so on next iteration
+     *                  writer will for sure read the updated value.
+     */
+    if (qatomic_read(&has_writer)) {
+        aio_wait_kick();
+    }
+}
+
+void bdrv_graph_rdlock_main_loop(void)
+{
+    GLOBAL_STATE_CODE();
+    assert(!qemu_in_coroutine());
+}
+
+void bdrv_graph_rdunlock_main_loop(void)
+{
+    GLOBAL_STATE_CODE();
+    assert(!qemu_in_coroutine());
+}
+
+void assert_bdrv_graph_readable(void)
+{
+    assert(qemu_in_main_thread() || reader_count());
+}
+
+void assert_bdrv_graph_writable(void)
+{
+    assert(qemu_in_main_thread());
+    assert(qatomic_read(&has_writer));
+}