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block: Mark drain related functions GRAPH_RDLOCK

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Draining recursively traverses the graph, therefore we need to make sure
that also such accesses to the graph are protected by the graph rdlock.

There are 3 different drain callers to consider:
1. drain in the main loop: no issue at all, rdlock is nop.
2. drain in an iothread: rdlock only works in main loop or coroutines,
   so disallow it.
3. drain in a coroutine (regardless of AioContext): the drain mechanism
   takes care of scheduling a BH in the bs->aio_context that will
   then take care of perform the actual draining. This is wrong,
   because as pointed in (2) if bs->aio_context is an iothread then
   rdlock won't work. Therefore change bdrv_co_yield_to_drain to
   schedule the BH in the main loop.

Caller (2) also implies that we need to modify test-bdrv-drain.c to
disallow draining in the iothreads.

For some places, we know that they will hold the lock, but we don't have
the GRAPH_RDLOCK annotations yet. In this case, add assume_graph_lock()
with a FIXME comment. These places will be removed once everything is
properly annotated.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20230929145157.45443-6-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Emanuele Giuseppe Esposito 2023-09-29 16:51:40 +02:00 • committed by Kevin Wolf
parent 2b3912f135
commit d05ab380db
5 changed files with 54 additions and 17 deletions
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6
include/block/block_int-common.h
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@ -963,15 +963,15 @@ struct BdrvChildClass {
* Note that this can be nested. If drained_begin() was called twice, new
* I/O is allowed only after drained_end() was called twice, too.
*/
void (*drained_begin)(BdrvChild *child);
void (*drained_end)(BdrvChild *child);
void GRAPH_RDLOCK_PTR (*drained_begin)(BdrvChild *child);
void GRAPH_RDLOCK_PTR (*drained_end)(BdrvChild *child);
/*
* Returns whether the parent has pending requests for the child. This
* callback is polled after .drained_begin() has been called until all
* activity on the child has stopped.
*/
bool (*drained_poll)(BdrvChild *child);
bool GRAPH_RDLOCK_PTR (*drained_poll)(BdrvChild *child);
/*
* Notifies the parent that the filename of its child has changed (e.g.