graph-lock: Introduce a lock to protect block graph operations

Block layer graph operations are always run under BQL in the main loop.
This is proved by the assertion qemu_in_main_thread() and its wrapper
macro GLOBAL_STATE_CODE.

However, there are also concurrent coroutines running in other iothreads
that always try to traverse the graph. Currently this is protected
(among various other things) by the AioContext lock, but once this is
removed, we need to make sure that reads do not happen while modifying
the graph.

We distinguish between writer (main loop, under BQL) that modifies the
graph, and readers (all other coroutines running in various AioContext),
that go through the graph edges, reading ->parents and->children.

The writer (main loop) has "exclusive" access, so it first waits for any
current read to finish, and then prevents incoming ones from entering
while it has the exclusive access.

The readers (coroutines in multiple AioContext) are free to access the
graph as long the writer is not modifying the graph. In case it is, they
go in a CoQueue and sleep until the writer is done.

If a coroutine changes AioContext, the counter in the original and new
AioContext are left intact, since the writer does not care where the
reader is, but only if there is one.

As a result, some AioContexts might have a negative reader count, to
balance the positive count of the AioContext that took the lock.  This
also means that when an AioContext is deleted it may have a nonzero
reader count. In that case we transfer the count to a global shared
counter so that the writer is always aware of all readers.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20221207131838.239125-3-kwolf@redhat.com>
Reviewed-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

2 files changed, 262 insertions, 0 deletions

diff --git a/block/graph-lock.c b/block/graph-lock.c
new file mode 100644
index 0000000000..e033c6d9ac
--- /dev/null
+++ b/block/graph-lock.c
@@ -0,0 +1,261 @@
+/*
+ * 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"
+
+/* 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());
+}
diff --git a/block/meson.build b/block/meson.build
index c48a59571e..90011a2805 100644
--- a/block/meson.build
+++ b/block/meson.build
@@ -10,6 +10,7 @@ block_ss.add(files(
   'blkverify.c',
   'block-backend.c',
   'block-copy.c',
+  'graph-lock.c',
   'commit.c',
   'copy-on-read.c',
   'preallocate.c',