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

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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>
This commit is contained in:
Paolo Bonzini 2022-12-07 14:18:22 +01:00 • committed by Kevin Wolf
parent da0bd74434
commit aead9dc9d1
5 changed files with 411 additions and 0 deletions
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9
include/block/aio.h
View file

@ -22,6 +22,7 @@
#include "qemu/event_notifier.h"
#include "qemu/thread.h"
#include "qemu/timer.h"
#include "block/graph-lock.h"
typedef struct BlockAIOCB BlockAIOCB;
typedef void BlockCompletionFunc(void *opaque, int ret);
@ -127,6 +128,14 @@ struct AioContext {
/* Used by AioContext users to protect from multi-threaded access. */
QemuRecMutex lock;
/*
* Keep track of readers and writers of the block layer graph.
* This is essential to avoid performing additions and removal
* of nodes and edges from block graph while some
* other thread is traversing it.
*/
BdrvGraphRWlock *bdrv_graph;
/* The list of registered AIO handlers. Protected by ctx->list_lock. */
AioHandlerList aio_handlers;