Re: [Xen-devel] [PATCH 3/3] xen: optimize xenbus driver for multiple concurrent xenstore accesses

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

>>> +static int process_msg(void)
>>> +{
>>> +   static struct xs_thread_state_read state;
>>> +   struct xb_req_data *req;
>>> +   int err;
>>> +   unsigned int len;
>>> +
>>> +   if (!state.in_msg) {
>>> +           state.in_msg = true;
>>> +           state.in_hdr = true;
>>> +           state.used = 0;
>>> +
>>> +           /*
>>> +            * We must disallow save/restore while reading a message.
>>> +            * A partial read across s/r leaves us out of sync with
>>> +            * xenstored.
>>> +            */
>>> +           mutex_lock(&xs_response_mutex);
>>> +
>>> +           if (!xb_data_to_read()) {
>>> +                   /* We raced with save/restore: pending data 'gone'. */
>>> +                   mutex_unlock(&xs_response_mutex);
>>> +                   state.in_msg = false;

Just noticed: should in_hdr be set to false here as well?

>>> +                   return 0;
>>> +           }

Or set it to true here.

>>> +   }
>>> +
>>> +   if (state.in_hdr) {
>>> +           if (state.used != sizeof(state.msg)) {
>>> +                   err = xb_read((void *)&state.msg + state.used,
>>> +                                 sizeof(state.msg) - state.used);
>>> +                   if (err < 0)
>>> +                           goto out;
>>> +                   state.used += err;
>>> +                   if (state.used != sizeof(state.msg))
>>> +                           return 0;
>> Would it be possible to do locking at the caller? I understand that you
>> are trying to hold the lock across multiple invocations of this function
>> but it feels somewhat counter-intuitive and bug-prone.
> I think that would be difficult.
>
>> If it's not possible then at least please add a comment explaining
>> locking algorithm.
> Okay. Something like:
>
> /*
>  * xs_response_mutex is locked as long as we are processing one
>  * message. state.in_msg will be true as long as we are holding the
>  * lock in process_msg().


Then in_msg is the same as mutex_is_locked(&xs_response_mutex). And if
so, do we really need it?


-boris


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