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>

1 files changed, 139 insertions, 0 deletions

diff --git a/include/block/graph-lock.h b/include/block/graph-lock.h
new file mode 100644
index 0000000000..82edb62cfa
--- /dev/null
+++ b/include/block/graph-lock.h
@@ -0,0 +1,139 @@
+/*
+ * 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/>.
+ */
+#ifndef GRAPH_LOCK_H
+#define GRAPH_LOCK_H
+
+#include "qemu/osdep.h"
+
+#include "qemu/coroutine.h"
+
+/**
+ * Graph Lock API
+ * This API provides a rwlock used to protect block layer
+ * graph modifications like edge (BdrvChild) and node (BlockDriverState)
+ * addition and removal.
+ * Currently we have 1 writer only, the Main loop, and many
+ * readers, mostly coroutines running in other AioContext thus other threads.
+ *
+ * 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
+ * BlockDriverState ->parents and->children.
+ *
+ * The writer (main loop)  has an "exclusive" access, so it first waits for
+ * 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 is the
+ * reader, 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.
+ */
+typedef struct BdrvGraphRWlock BdrvGraphRWlock;
+
+/*
+ * register_aiocontext:
+ * Add AioContext @ctx to the list of AioContext.
+ * This list is used to obtain the total number of readers
+ * currently running the graph.
+ */
+void register_aiocontext(AioContext *ctx);
+
+/*
+ * unregister_aiocontext:
+ * Removes AioContext @ctx to the list of AioContext.
+ */
+void unregister_aiocontext(AioContext *ctx);
+
+/*
+ * bdrv_graph_wrlock:
+ * Start an exclusive write operation to modify the graph. This means we are
+ * adding or removing an edge or a node in the block layer graph. Nobody else
+ * is allowed to access the graph.
+ *
+ * Must only be called from outside bdrv_graph_co_rdlock.
+ *
+ * The wrlock can only be taken from the main loop, with BQL held, as only the
+ * main loop is allowed to modify the graph.
+ *
+ * This function polls. Callers must not hold the lock of any AioContext other
+ * than the current one.
+ */
+void bdrv_graph_wrlock(void);
+
+/*
+ * bdrv_graph_wrunlock:
+ * Write finished, reset global has_writer to 0 and restart
+ * all readers that are waiting.
+ */
+void bdrv_graph_wrunlock(void);
+
+/*
+ * bdrv_graph_co_rdlock:
+ * Read the bs graph. This usually means traversing all nodes in
+ * the graph, therefore it can't happen while another thread is
+ * modifying it.
+ * Increases the reader counter of the current aiocontext,
+ * and if has_writer is set, it means that the writer is modifying
+ * the graph, therefore wait in a coroutine queue.
+ * The writer will then wake this coroutine once it is done.
+ *
+ * This lock should be taken from Iothreads (IO_CODE() class of functions)
+ * because it signals the writer that there are some
+ * readers currently running, or waits until the current
+ * write is finished before continuing.
+ * Calling this function from the Main Loop with BQL held
+ * is not necessary, since the Main Loop itself is the only
+ * writer, thus won't be able to read and write at the same time.
+ * The only exception to that is when we can't take the lock in the
+ * function/coroutine itself, and need to delegate the caller (usually main
+ * loop) to take it and wait that the coroutine ends, so that
+ * we always signal that a reader is running.
+ */
+void coroutine_fn bdrv_graph_co_rdlock(void);
+
+/*
+ * bdrv_graph_rdunlock:
+ * Read terminated, decrease the count of readers in the current aiocontext.
+ * If the writer is waiting for reads to finish (has_writer == 1), signal
+ * the writer that we are done via aio_wait_kick() to let it continue.
+ */
+void coroutine_fn bdrv_graph_co_rdunlock(void);
+
+/*
+ * bdrv_graph_rd{un}lock_main_loop:
+ * Just a placeholder to mark where the graph rdlock should be taken
+ * in the main loop. It is just asserting that we are not
+ * in a coroutine and in GLOBAL_STATE_CODE.
+ */
+void bdrv_graph_rdlock_main_loop(void);
+void bdrv_graph_rdunlock_main_loop(void);
+
+#endif /* GRAPH_LOCK_H */
+