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Re: [Libguestfs] [PATCH libnbd v3 1/2] lib: Implement closure lifetimes.



On 7/25/19 10:43 AM, Eric Blake wrote:

>> +++ b/generator/states-reply-structured.c
>> @@ -298,7 +298,7 @@
>>           * current error rather than any earlier one. If the callback fails
>>           * without setting errno, then use the server's error below.
>>           */
>> -        if (cmd->cb.fn.read (cmd->cb.fn_user_data,
>> +        if (cmd->cb.fn.read (LIBNBD_CALLBACK_VALID, cmd->cb.fn_user_data,
>>                               cmd->data + (offset - cmd->offset),
>>                               0, offset, LIBNBD_READ_ERROR, &scratch) == -1)
>>            if (cmd->error == 0)
> 
> We could still optimize this file based on NBD_REPLY_FLAG_DONE, but that
> can be a followup.
> 
>> @@ -499,7 +499,7 @@
>>        /* Call the caller's extent function. */
>>        int error = cmd->error;
>>  
>> -      if (cmd->cb.fn.extent (cmd->cb.fn_user_data,
>> +      if (cmd->cb.fn.extent (LIBNBD_CALLBACK_VALID, cmd->cb.fn_user_data,
>>                               meta_context->name, cmd->offset,
>>                               &h->bs_entries[1], (length-4) / 4, &error) == -1)
>>          if (cmd->error == 0)
> 
> Hmm - no change to the FINISH state, which means you are relying on
> command retirement to free chunk/extent instead.  As long as that
> happens, we should be okay, though.

Another thought: if a user calls nbd_aio_pread_structured_callback (...,
chunk, &data, callback, &data, ...), where chunk ignores FREE but
callback(FREE) calls free(data), is there any problem with the fact that
we may end up with a call order of callback(VALID|FREE) before
chunk(FREE)?  I think we're okay - as long as chunk doesn't dereference
data except when VALID is set, then the fact that chunk(FREE) is called
with a stale pointer should not be a problem.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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