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graph-lock: Honour read locks even in the main thread

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There are some conditions under which we don't actually need to do
anything for taking a reader lock: Writing the graph is only possible
from the main context while holding the BQL. So if a reader is running
in the main context under the BQL and knows that it won't be interrupted
until the next writer runs, we don't actually need to do anything.

This is the case if the reader code neither has a nested event loop
(this is forbidden anyway while you hold the lock) nor is a coroutine
(because a writer could run when the coroutine has yielded).

These conditions are exactly what bdrv_graph_rdlock_main_loop() asserts.
They are not fulfilled in bdrv_graph_co_rdlock(), which always runs in a
coroutine.

This deletes the shortcuts in bdrv_graph_co_rdlock() that skip taking
the reader lock in the main thread.

Reported-by: Fiona Ebner <f.ebner@proxmox.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230510203601.418015-9-kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Kevin Wolf 2023-05-10 22:36:01 +02:00
parent 018e5987b5
commit 71438d8dac
1 changed files with 0 additions and 10 deletions
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10
block/graph-lock.c
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@ -162,11 +162,6 @@ void coroutine_fn bdrv_graph_co_rdlock(void)
BdrvGraphRWlock *bdrv_graph; BdrvGraphRWlock *bdrv_graph;
bdrv_graph = qemu_get_current_aio_context()->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 (;;) { for (;;) {
qatomic_set(&bdrv_graph->reader_count, qatomic_set(&bdrv_graph->reader_count,
bdrv_graph->reader_count + 1); bdrv_graph->reader_count + 1);
@ -230,11 +225,6 @@ void coroutine_fn bdrv_graph_co_rdunlock(void)
BdrvGraphRWlock *bdrv_graph; BdrvGraphRWlock *bdrv_graph;
bdrv_graph = qemu_get_current_aio_context()->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, qatomic_store_release(&bdrv_graph->reader_count,
bdrv_graph->reader_count - 1); bdrv_graph->reader_count - 1);
/* make sure writer sees reader_count before we check has_writer */ /* make sure writer sees reader_count before we check has_writer */