Re: [PATCH v11 09/10] migration: Implement MigrateChannelList to hmp migration flow.

[Fabiano Rosas <farosas@suse.de>]

Het Gala <het.gala@nutanix.com> writes:

> On 10/4/2023 8:55 PM, Fabiano Rosas wrote:
>> Het Gala<het.gala@nutanix.com>  writes:
>>
>>> Integrate MigrateChannelList with all transport backends
>>> (socket, exec and rdma) for both src and dest migration
>>> endpoints for hmp migration.
>>>
>>> Suggested-by: Aravind Retnakaran<aravind.retnakaran@nutanix.com>
>>> Signed-off-by: Het Gala<het.gala@nutanix.com>
>>> Reviewed-by: Daniel P. Berrangé<berrange@redhat.com>
>>> ---
>>>   migration/migration-hmp-cmds.c | 15 +++++++++++++--
>>>   migration/migration.c          |  5 ++---
>>>   migration/migration.h          |  3 ++-
>>>   3 files changed, 17 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/migration/migration-hmp-cmds.c b/migration/migration-hmp-cmds.c
>>> index a2e6a5c51e..a1657f3d37 100644
>>> --- a/migration/migration-hmp-cmds.c
>>> +++ b/migration/migration-hmp-cmds.c
>>> @@ -441,9 +441,14 @@ void hmp_migrate_incoming(Monitor *mon, const QDict *qdict)
>>>   {
>>>       Error *err = NULL;
>>>       const char *uri = qdict_get_str(qdict, "uri");
>>> +    MigrationChannelList *caps = NULL;
>>> +    g_autoptr(MigrationChannel) channel = g_new0(MigrationChannel, 1);
>> Just the pointer here. If I remember correctly the g_autoptr here would
>> cause a double free when freeing the caps.
>
> Yes, we'll just have 'g_autoptr(MigrationChannel) channel = NULL'.
>
> Is it because inside QAPI_LIST_PREPEND, caps will be refrencing to the 
> same memory as 'channel', we don't need to free channel ?

Slightly different scenario here. Here the issue is that we will free
the caps with qapi_free_MigrationChannel() before returning. Then, at
function exit the g_autoptr will try to free 'channel', which has
already been freed along with 'caps'. That's a double free, I think it
hits an assert inside glib, if I remember correctly.

> I am still not 
> sure what is the right place to use g_steal_pointer(), is this a right 
> place to use (non-error paths) ?

It doesn't look like we need it here. As long as the qapi list has a
reference and we're freeing the caps, then channel should be freed by
that function already.

>>>   
>>> -    qmp_migrate_incoming(uri, false, NULL, &err);
>>> +    migrate_uri_parse(uri, &channel, &err);
>>> +    QAPI_LIST_PREPEND(caps, channel);
>>>   
>>> +    qmp_migrate_incoming(NULL, true, caps, &err);
>>> +    qapi_free_MigrationChannelList(caps);
>>>       hmp_handle_error(mon, err);
>>>   }
>>>   
>>> @@ -730,9 +735,15 @@ void hmp_migrate(Monitor *mon, const QDict *qdict)
>>>       bool resume = qdict_get_try_bool(qdict, "resume", false);
>>>       const char *uri = qdict_get_str(qdict, "uri");
>>>       Error *err = NULL;
>>> +    MigrationChannelList *caps = NULL;
>>> +    g_autoptr(MigrationChannel) channel = g_new0(MigrationChannel, 1);
>> Same here. We free the channel via caps and we attribute it below, no
>> need to allocate.
> Ack.
>>>   
>>> -    qmp_migrate(uri, false, NULL, !!blk, blk, !!inc, inc,
>>> +    migrate_uri_parse(uri, &channel, &err);
>>> +    QAPI_LIST_PREPEND(caps, channel);
>>> +
>>> +    qmp_migrate(NULL, true, caps, !!blk, blk, !!inc, inc,
>>>                    false, false, true, resume, &err);
>>> +    qapi_free_MigrationChannelList(caps);
>>>       if (hmp_handle_error(mon, err)) {
> Regards,
> Het Gala