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/*
* 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/clang-tsa.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;
/* Dummy lock object to use for Thread Safety Analysis (TSA) */
typedef struct TSA_CAPABILITY("mutex") BdrvGraphLock {
} BdrvGraphLock;
extern BdrvGraphLock graph_lock;
/*
* clang doesn't check consistency in locking annotations between forward
* declarations and the function definition. Having the annotation on the
* definition, but not the declaration in a header file, may give the reader
* a false sense of security because the condition actually remains unchecked
* for callers in other source files.
*
* Therefore, as a convention, for public functions, GRAPH_RDLOCK and
* GRAPH_WRLOCK annotations should be present only in the header file.
*/
#define GRAPH_WRLOCK TSA_REQUIRES(graph_lock)
#define GRAPH_RDLOCK TSA_REQUIRES_SHARED(graph_lock)
#define GRAPH_UNLOCKED TSA_EXCLUDES(graph_lock)
/*
* TSA annotations are not part of function types, so checks are defeated when
* using a function pointer. As a workaround, annotate function pointers with
* this macro that will require that the lock is at least taken while reading
* the pointer. In most cases this is equivalent to actually protecting the
* function call.
*/
#define GRAPH_RDLOCK_PTR TSA_GUARDED_BY(graph_lock)
#define GRAPH_WRLOCK_PTR TSA_GUARDED_BY(graph_lock)
#define GRAPH_UNLOCKED_PTR
/*
* 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.
*
* If @bs is non-NULL, its AioContext is temporarily released.
*
* This function polls. Callers must not hold the lock of any AioContext other
* than the current one and the one of @bs.
*/
void bdrv_graph_wrlock(BlockDriverState *bs) TSA_ACQUIRE(graph_lock) TSA_NO_TSA;
/*
* bdrv_graph_wrunlock:
* Write finished, reset global has_writer to 0 and restart
* all readers that are waiting.
*/
void bdrv_graph_wrunlock(void) TSA_RELEASE(graph_lock) TSA_NO_TSA;
/*
* 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 TSA_ACQUIRE_SHARED(graph_lock) TSA_NO_TSA
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 TSA_RELEASE_SHARED(graph_lock) TSA_NO_TSA
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 TSA_ACQUIRE_SHARED(graph_lock) TSA_NO_TSA
bdrv_graph_rdlock_main_loop(void);
void TSA_RELEASE_SHARED(graph_lock) TSA_NO_TSA
bdrv_graph_rdunlock_main_loop(void);
/*
* assert_bdrv_graph_readable:
* Make sure that the reader is either the main loop,
* or there is at least a reader helding the rdlock.
* In this way an incoming writer is aware of the read and waits.
*/
void GRAPH_RDLOCK assert_bdrv_graph_readable(void);
/*
* assert_bdrv_graph_writable:
* Make sure that the writer is the main loop and has set @has_writer,
* so that incoming readers will pause.
*/
void GRAPH_WRLOCK assert_bdrv_graph_writable(void);
/*
* Calling this function tells TSA that we know that the lock is effectively
* taken even though we cannot prove it (yet) with GRAPH_RDLOCK. This can be
* useful in intermediate stages of a conversion to using the GRAPH_RDLOCK
* macro.
*/
static inline void TSA_ASSERT_SHARED(graph_lock) TSA_NO_TSA
assume_graph_lock(void)
{
}
typedef struct GraphLockable { } GraphLockable;
/*
* In C, compound literals have the lifetime of an automatic variable.
* In C++ it would be different, but then C++ wouldn't need QemuLockable
* either...
*/
#define GML_OBJ_() (&(GraphLockable) { })
/*
* This is not marked as TSA_ACQUIRE_SHARED() because TSA doesn't understand the
* cleanup attribute and would therefore complain that the graph is never
* unlocked. TSA_ASSERT_SHARED() makes sure that the following calls know that
* we hold the lock while unlocking is left unchecked.
*/
static inline GraphLockable * TSA_ASSERT_SHARED(graph_lock) TSA_NO_TSA coroutine_fn
graph_lockable_auto_lock(GraphLockable *x)
{
bdrv_graph_co_rdlock();
return x;
}
static inline void TSA_NO_TSA coroutine_fn
graph_lockable_auto_unlock(GraphLockable *x)
{
bdrv_graph_co_rdunlock();
}
G_DEFINE_AUTOPTR_CLEANUP_FUNC(GraphLockable, graph_lockable_auto_unlock)
#define WITH_GRAPH_RDLOCK_GUARD_(var) \
for (g_autoptr(GraphLockable) var = graph_lockable_auto_lock(GML_OBJ_()); \
var; \
graph_lockable_auto_unlock(var), var = NULL)
#define WITH_GRAPH_RDLOCK_GUARD() \
WITH_GRAPH_RDLOCK_GUARD_(glue(graph_lockable_auto, __COUNTER__))
#define GRAPH_RDLOCK_GUARD(x) \
g_autoptr(GraphLockable) \
glue(graph_lockable_auto, __COUNTER__) G_GNUC_UNUSED = \
graph_lockable_auto_lock(GML_OBJ_())
typedef struct GraphLockableMainloop { } GraphLockableMainloop;
/*
* In C, compound literals have the lifetime of an automatic variable.
* In C++ it would be different, but then C++ wouldn't need QemuLockable
* either...
*/
#define GMLML_OBJ_() (&(GraphLockableMainloop) { })
/*
* This is not marked as TSA_ACQUIRE_SHARED() because TSA doesn't understand the
* cleanup attribute and would therefore complain that the graph is never
* unlocked. TSA_ASSERT_SHARED() makes sure that the following calls know that
* we hold the lock while unlocking is left unchecked.
*/
static inline GraphLockableMainloop * TSA_ASSERT_SHARED(graph_lock) TSA_NO_TSA
graph_lockable_auto_lock_mainloop(GraphLockableMainloop *x)
{
bdrv_graph_rdlock_main_loop();
return x;
}
static inline void TSA_NO_TSA
graph_lockable_auto_unlock_mainloop(GraphLockableMainloop *x)
{
bdrv_graph_rdunlock_main_loop();
}
G_DEFINE_AUTOPTR_CLEANUP_FUNC(GraphLockableMainloop,
graph_lockable_auto_unlock_mainloop)
#define GRAPH_RDLOCK_GUARD_MAINLOOP(x) \
g_autoptr(GraphLockableMainloop) \
glue(graph_lockable_auto, __COUNTER__) G_GNUC_UNUSED = \
graph_lockable_auto_lock_mainloop(GMLML_OBJ_())
#endif /* GRAPH_LOCK_H */