Am 31/03/2022 um 18:40 schrieb Paolo Bonzini:
> On 3/31/22 15:51, Emanuele Giuseppe Esposito wrote:
>>
>> bdrv_graph_list_wrlock <-> start_exclusive
>> bdrv_graph_list_wrunlock <-> end_exclusive
>> bdrv_graph_list_rdlock <-> cpu_exec_start
>> bdrv_graph_list_rdunlock <-> cpu_exec_end
>
> This wouldn't protect the list but the whole graph, i.e. the parents and
> children of all BDSes. So the functions would be:
>
> bdrv_graph_wrlock <-> start_exclusive
> bdrv_graph_wrunlock <-> end_exclusive
> bdrv_graph_rdlock <-> cpu_exec_start
> bdrv_graph_rdunlock <-> cpu_exec_end
>
>
> The list itself would be used internally to implement the write-side
> lock and unlock primitives, but it would not be protected by the above
> functions. So there would be a couple additional functions:
>
> bdrv_graph_list_lock <-> cpu_list_lock
> bdrv_graph_list_unlock <-> cpu_list_unlock
The list would be graph_bdrv_states, why do we need to protect it with a
lock? Currently it is protected by BQL, and theoretically only
bdrv_graph_wrlock iterates on it. And as we defined in the assertion
below, wrlock is always in the main loop too.
>
>> +void bdrv_graph_list_rdlock(BlockDriverState *bs);
>> +void bdrv_graph_list_rdunlock(BlockDriverState *bs);
>
> Apart from the naming change, these two would be coroutine_fn.
>
>> +#define BS_GRAPH_READER(bs) /* in main loop OR bs->reading_graph */
>> +#define BS_GRAPH_WRITER(bs) /* in main loop AND bs->bs_graph_pending_op
>
> bs_graph_pending_op is not part of bs->, it is a global variable
> (corresponding to pending_cpus in cpus-common.c). I would call it
> bs_graph_pending_reader since you have "has_writer" below.
Hmm maybe it is better to go with has_waiter and pending_op. This is
because as "pending" we always have 1 write and zero or more reads.
Rest makes sense.
Emanuele
>
> Also, this second #define does not need an argument, and is really the
> same as assert_bdrv_graph_writable(bs). So perhaps you can rename the
> first one to assert_bdrv_graph_readable(bs).
>
>>
>> + /*
>> + * If true, the main loop is modifying the graph.
>> + * bs cannot read the graph.
>> + * Protected by bs_graph_list_lock.
>> + */
>> + bool has_writer;
>
> Note that it's "has_waiter" in cpus-common.c. :) has_writer is fine too.
>
> Paolo
>
