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

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Date:   Wed, 11 Jan 2017 10:29:01 -0500
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org
Subject: Re: [PATCH 3/3] xen: optimize xenbus driver for multiple concurrent
 xenstore accesses


>>> +
>>> +
>>> +static bool test_reply(struct xb_req_data *req)
>>> +{
>>> +	if (req->state == xb_req_state_got_reply || !xenbus_ok())
>>> +		return true;
>>> +
>>> +	/* Make sure to reread req->state each time. */
>>> +	cpu_relax();
>> I don't think I understand why this is needed.
> I need a compiler barrier. Otherwise the compiler read req->state only
> once outside the while loop.


Then barrier() looks the right primitive to use here. cpu_relax(), while
doing what you want, is intended for other purposes.


>
>>> +
>>> +	return false;
>>> +}
>>> +
>>
>>> +static void xs_send(struct xb_req_data *req, struct xsd_sockmsg *msg)
>>>  {
>>> -	mutex_lock(&xs_state.transaction_mutex);
>>> -	atomic_inc(&xs_state.transaction_count);
>>> -	mutex_unlock(&xs_state.transaction_mutex);
>>> -}
>>> +	bool notify;
>>>  
>>> -static void transaction_end(void)
>>> -{
>>> -	if (atomic_dec_and_test(&xs_state.transaction_count))
>>> -		wake_up(&xs_state.transaction_wq);
>>> -}
>>> +	req->msg = *msg;
>>> +	req->err = 0;
>>> +	req->state = xb_req_state_queued;
>>> +	init_waitqueue_head(&req->wq);
>>>  
>>> -static void transaction_suspend(void)
>>> -{
>>> -	mutex_lock(&xs_state.transaction_mutex);
>>> -	wait_event(xs_state.transaction_wq,
>>> -		   atomic_read(&xs_state.transaction_count) == 0);
>>> -}
>>> +	xs_request_enter(req);
>>>  
>>> -static void transaction_resume(void)
>>> -{
>>> -	mutex_unlock(&xs_state.transaction_mutex);
>>> +	req->msg.req_id = xs_request_id++;
>> Is it safe to do this without a lock?
> You are right: I should move this to xs_request_enter() inside the
> lock. I think I'll let return xs_request_enter() the request id.


Then please move xs_request_id's declaration close to xs_state_lock's
declaration (just like you are going to move the two other state variables)


>
>>> +static int xs_reboot_notify(struct notifier_block *nb,
>>> +			    unsigned long code, void *unused)
>>>  {
>>> -	struct xs_stored_msg *msg;
>>
>>
>>> +	struct xb_req_data *req;
>>> +
>>> +	mutex_lock(&xb_write_mutex);
>>> +	list_for_each_entry(req, &xs_reply_list, list)
>>> +		wake_up(&req->wq);
>>> +	list_for_each_entry(req, &xb_write_list, list)
>>> +		wake_up(&req->wq);
>> We are waking up waiters here but there is not guarantee that waiting
>> threads will have a chance to run, is there?
> You are right. But this isn't the point. We want to avoid blocking a
> reboot due to some needed thread waiting for xenstore. And this task
> is being accomplished here.


I think it's worth adding a comment mentioning this.

-boris