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

[Het Gala <het.gala@nutanix.com>]

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On 10/9/2023 8:05 PM, Fabiano Rosas wrote:
> 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.
Ack. Yes, with the discussion in earlier patches, I also don't think we 
need g_autoptr too here. Normal pointer is enough as we are freeing the 
memory after the function is returned.
>>>>    
>>>> -    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
Regards,
Het Gala

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