Re: [Qemu-devel] [PATCH] linux-user: call cpu_copy under clone_lock

[Max Filippov]

Hi Alex,

On Tue, Apr 3, 2018 at 9:26 AM, Alex Bennée <address@hidden> wrote:
> Max Filippov <address@hidden> writes:
>
>> cpu_copy adds newly created CPU object to container/machine/unattached,
>> but does it w/o proper locking. As a result when multiple threads are
>> created rapidly QEMU may abort with the following message:
>>
>>   GLib-CRITICAL **: g_hash_table_iter_next: assertion
>>   'ri->version == ri->hash_table->version' failed
>>
>>   ERROR:qemu/qom/object.c:1663:object_get_canonical_path_component:
>>   code should not be reached
>>
>> Move cpu_copy invocation under clone_lock to fix that.
>
> So my main concern is are we duplicating something already (should be?)
> handled by fork_start/fork_end?

clone_lock already exists, it protects state in case of thread creation,
it just didn't protect enough of it.

The work done by fork_start/fork_end appears to be heavier than
what's needed for thread creation, because fork_start stops all
other CPUs (to make sure that child process won't get locks owned
by threads that no longer exist in the child process), which is not
required for thread creation, hence thread creation uses clone_lock.

> This serialises forks and ensures things like the cpu_list (which ~ a
> thread list for linux-user) are updated safely.

-- 
Thanks.
-- Max