Date:   Wed, 20 May 2020 08:09:07 +0100
From:   Cristian Marussi <cristian.marussi@....com>
To:     Lukasz Luba <lukasz.luba@....com>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Cc:     cristian.marussi@....com, sudeep.holla@....com
Subject: Re: [PATCH v4 07/13] firmware: arm_scmi: Add notification dispatch
 and delivery

On Mon, Mar 16, 2020 at 02:46:05PM +0000, Cristian Marussi wrote:
> On Thu, Mar 12, 2020 at 09:43:31PM +0000, Lukasz Luba wrote:
> > 
> > 

Hi Lukasz,

I went back looking deeper into the possible race issue you pointed out a
while ago understanding it a bit better down below.

> > On 3/12/20 6:34 PM, Cristian Marussi wrote:
> > > On 12/03/2020 13:51, Lukasz Luba wrote:
> > > > Hi Cristian,
> > > > 
> Hi Lukasz
> 
> > > > just one comment below...
> [snip]
> > > > > +	eh.timestamp = ts;
> > > > > +	eh.evt_id = evt_id;
> > > > > +	eh.payld_sz = len;
> > > > > +	kfifo_in(&r_evt->proto->equeue.kfifo, &eh, sizeof(eh));
> > > > > +	kfifo_in(&r_evt->proto->equeue.kfifo, buf, len);
> > > > > +	queue_work(r_evt->proto->equeue.wq,
> > > > > +		   &r_evt->proto->equeue.notify_work);
> > > > 
> > > > Is it safe to ignore the return value from the queue_work here?
> > > > 
> > > 
> > > In fact yes, we do not want to care: it returns true or false depending on the
> > > fact that the specific work was or not already queued, and we just rely on
> > > this behavior to keep kicking the worker only when needed but never kick
> > > more than one instance of it per-queue (so that there's only one reader
> > > wq and one writer here in the scmi_notify)...explaining better:
> > > 
> > > 1. we push an event (hdr+payld) to the protocol queue if we found that there was
> > > enough space on the queue
> > > 
> > > 2a. if at the time of the kfifo_in( ) the worker was already running
> > > (queue not empty) it will process our new event sooner or later and here
> > > the queue_work will return false, but we do not care in fact ... we
> > > tried to kick it just in case
> > > 
> > > 2b. if instead at the time of the kfifo_in() the queue was empty the worker would
> > > have probably already gone to the sleep and this queue_work() will return true and
> > > so this time it will effectively wake up the worker to process our items
> > > 
> > > The important thing here is that we are sure to wakeup the worker when needed
> > > but we are equally sure we are never causing the scheduling of more than one worker
> > > thread consuming from the same queue (because that would break the one reader/one writer
> > > assumption which let us use the fifo in a lockless manner): this is possible because
> > > queue_work checks if the required work item is already pending and in such a case backs
> > > out returning false and we have one work_item (notify_work) defined per-protocol and
> > > so per-queue.
> > 
> > I see. That's a good assumption: one work_item per protocol and simplify
> > the locking. What if there would be an edge case scenario when the
> > consumer (work_item) has handled the last item (there was NULL from
> > scmi_process_event_header()), while in meantime scmi_notify put into
> > the fifo new event but couldn't kick the queue_work. Would it stay there
> > till the next IRQ which triggers queue_work to consume two events (one
> > potentially a bit old)? Or we can ignore such race situation assuming
> > that cleaning of work item is instant and kfifo_in is slow?
> > 
> 
> In fact, this is a very good point, since between the moment the worker
> determines that the queue is empty and the moment in which the worker
> effectively exits (and it's marked as no more pending by the Kernel cmwq)
> there is a window of opportunity for a race in which the ISR could fill
> the queue with one more event and then fail to kick with queue_work() since
> the work is in fact still nominally marked as pending from the point of view
> of Kernel cmwq, as below:
> 
> ISR (core N)		|	WQ (core N+1)		cmwq flags	       	queued events
> ------------------------------------------------------------------------------------------------
> 			| if (queue_is_empty)		- WORK_PENDING		0 events queued
> 			+     ...			- WORK_PENDING		0 events queued
> 			+ } while (scmi_process_event_payload);
> 			+}// worker function exit 
> kfifo_in()		+     ...cmwq backing out	- WORK_PENDING		1 events queued
> kfifo_in()		+     ...cmwq backing out	- WORK_PENDING		1 events queued
> queue_work()		+     ...cmwq backing out	- WORK_PENDING		1 events queued
>   -> FALSE (pending)	+     ...cmwq backing out	- WORK_PENDING		1 events queued
> 			+     ...cmwq backing out	- WORK_PENDING		1 events queued
> 			+     ...cmwq backing out	- WORK_PENDING		1 events queued
> 			| ---- WORKER THREAD EXIT	- !WORK_PENDING		1 events queued
> 			| 		 		- !WORK_PENDING		1 events queued
> kfifo_in()		|     				- !WORK_PENDING		2 events queued
> kfifo_in()		|  				- !WORK_PENDING		2 events queued
> queue_work()		|     				- !WORK_PENDING		2 events queued
>    -> TRUE		| --- WORKER ENTER		- WORK_PENDING		2 events queued
> 			|  				- WORK_PENDING		2 events consumed
> 		
> where effectively the last event queued won't be consumed till the next
> iteration once another event is queued.
> 

In summary, looking better at Kernel cmwq code, my explanation above about
how the possible race could be exposed by a particular tricky limit condition
and the values assumed by the WORK_STRUCT_PENDING_BIT was ... bullshit :D

In fact there's no race at all because Kernel cmwq takes care to clear the above
PENDING flag BEFORE the user-provided worker-function starts to finally run:
such flag is active only when a work instance is queued pending for execution
but it is cleared just before execution effctively starts.

kernel/workqueue.c:process_one_work()

	set_work_pool_and_clear_pending(work, pool->id);
	....
	worker->current_func(work);

As a consequence in the racy scenario above where the ISR pushes events on the
queues after the worker has already determined the queue to be empty but while
the worker func is still being deactivated in terms of Kernel cmwq internal
handling, it is not a problem since the worker while running is already NO more
marked pending so the queue_work succeeds and a new work will simply be queued
and run once the current instance terminates fully and it is removed from pool.

On the other side in the normal non racy scenario, when the worker is processing
normally a non-empty queue, we'll end-up anyway queueing new items and a new work
from the ISR even if the currently executing one will in fact consume already
naturally the queued items: this will result (it's what I observe in fact) in a
final un-needed quick worker activation/deactivation processing zero items (empty
queue) which is in fact harmless.

Basically the racy condition is taken care by the Kernel cmwq itself, and in fact
there is an extensive explanation also of the barriers employed to properly
realize this in the comments around set_work_pool_and_clear_pending()

I'll add a comment in v8 just to note this behaviour.

Thanks

Cristian

> Given the fact that the ISR and the dedicated WQ on an SMP run effectively
> in parallel I do not think unfortunately that we can simply count on the fact
> the worker exit is faster than the kifos_in, enough to close the race window
> opportunity. (even if rare)
> 
> On the other side considering the impact of such scenario, I can imagine that
> it's not simply that we could only have a delayed delivery, but we must consider
> that if the delayed event is effectively the last one ever it would remain
> undelivered forever; this is particularly worrying in a scenario in which such
> last event is particularly important: imagine a system shutdown where a last
> system-power-off remains undelivered.
> 
> As a consequence I think this rare racy condition should be addressed somehow.
> 
> Looking at this scenario, it seems the classic situation in which you want to
> use some sort of completion to avoid missing out on events delivery, BUT in our
> usecase:
> 
> - placing the workers loaned from cmwq into an unbounded wait_for_completion()
>   once the queue is empty seems not the best to use resources (and probably
>   frowned upon)....using a few dedicated kernel threads to simply let them idle
>   waiting most of the time seems equally frowned upon (I could be wrong...))
> - the needed complete() in the ISR would introduce a spinlock_irqsave into the
>   interrupt path (there's already one inside queue_work in fact) so it is not
>   desirable, at least not if used on a regular base (for each event notified)
> 
> So I was thinking to try to reduce sensibly the above race window, more
> than eliminate it completely, by adding an early flag to be checked under
> specific conditions in order to retry the queue_work a few times when the race
> is hit, something like:
> 
> ISR (core N)		|	WQ (core N+1)
> -------------------------------------------------------------------------------
> 			| atomic_set(&exiting, 0);
> 			|
> 			| do {
> 			|	...
> 			| 	if (queue_is_empty)		- WORK_PENDING		0 events queued
> 			+          atomic_set(&exiting, 1)	- WORK_PENDING		0 events queued
> static int cnt=3	|          --> breakout of while	- WORK_PENDING		0 events queued
> kfifo_in()		|	....
> 			| } while (scmi_process_event_payload);
> kfifo_in()		|
> exiting = atomic_read()	|     ...cmwq backing out		- WORK_PENDING		1 events queued
> do {			|     ...cmwq backing out		- WORK_PENDING		1 events queued
>     ret = queue_work() 	|     ...cmwq backing out		- WORK_PENDING		1 events queued
>     if (ret || !exiting)|     ...cmwq backing out		- WORK_PENDING		1 events queued
> 	break;		|     ...cmwq backing out		- WORK_PENDING		1 events queued
>     mdelay(5);		|     ...cmwq backing out		- WORK_PENDING		1 events queued
>     exiting =		|     ...cmwq backing out		- WORK_PENDING		1 events queued
>       atomic_read;	|     ...cmwq backing out		- WORK_PENDING		1 events queued
> } while (--cnt);	|     ...cmwq backing out		- WORK_PENDING		1 events queued
> 			| ---- WORKER EXIT 			- !WORK_PENDING		0 events queued
> 
> like down below between the scissors.
> 
> Not tested or tried....I could be missing something...and the mdelay is horrible (and not
> the cleanest thing you've ever seen probably :D)...I'll have a chat with Sudeep too.
> 
> > > 
> > > Now probably I wrote too much of an explanation and confuse stuff more ... :D
> > 
> > No, thank you for the detailed explanation. I will continue my review.
> > 
> 
> Thanks
> 
> Regards
> 
> Cristian
> 
> 
> 
> -->8-----------------
> diff --git a/drivers/firmware/arm_scmi/notify.c b/drivers/firmware/arm_scmi/notify.c
> index 9eb6b8b71bac..8719e077358c 100644
> --- a/drivers/firmware/arm_scmi/notify.c
> +++ b/drivers/firmware/arm_scmi/notify.c
> @@ -223,6 +223,7 @@ struct scmi_notify_instance {
>   */
>  struct events_queue {
>  	size_t				sz;
> +	atomic_t			exiting;
>  	struct kfifo			kfifo;
>  	struct work_struct		notify_work;
>  	struct workqueue_struct		*wq;
> @@ -406,11 +407,16 @@ scmi_process_event_header(struct events_queue *eq,
>  
>  	outs = kfifo_out(&eq->kfifo, pd->eh,
>  			 sizeof(struct scmi_event_header));
> -	if (!outs)
> +	if (!outs) {
> +		atomic_set(&eq->exiting, 1);
> +		smp_mb__after_atomic();
>  		return NULL;
> +	}
>  	if (outs != sizeof(struct scmi_event_header)) {
>  		pr_err("SCMI Notifications: corrupted EVT header. Flush.\n");
>  		kfifo_reset_out(&eq->kfifo);
> +		atomic_set(&eq->exiting, 1);
> +		smp_mb__after_atomic();
>  		return NULL;
>  	}
>  
> @@ -446,6 +452,8 @@ scmi_process_event_payload(struct events_queue *eq,
>  	outs = kfifo_out(&eq->kfifo, pd->eh->payld, pd->eh->payld_sz);
>  	if (unlikely(!outs)) {
>  		pr_warn("--- EMPTY !!!!\n");
> +		atomic_set(&eq->exiting, 1);
> +		smp_mb__after_atomic();
>  		return false;
>  	}
>  
> @@ -455,6 +463,8 @@ scmi_process_event_payload(struct events_queue *eq,
>  	if (unlikely(outs != pd->eh->payld_sz)) {
>  		pr_err("SCMI Notifications: corrupted EVT Payload. Flush.\n");
>  		kfifo_reset_out(&eq->kfifo);
> +		atomic_set(&eq->exiting, 1);
> +		smp_mb__after_atomic();
>  		return false;
>  	}
>  
> @@ -526,6 +536,8 @@ static void scmi_events_dispatcher(struct work_struct *work)
>  	mdelay(200);
>  
>  	eq = container_of(work, struct events_queue, notify_work);
> +	atomic_set(&eq->exiting, 0);
> +	smp_mb__after_atomic();
>  	pd = container_of(eq, struct scmi_registered_protocol_events_desc,
>  			  equeue);
>  	/*
> @@ -579,6 +591,8 @@ static void scmi_events_dispatcher(struct work_struct *work)
>  int scmi_notify(const struct scmi_handle *handle, u8 proto_id, u8 evt_id,
>  		const void *buf, size_t len, u64 ts)
>  {
> +	bool exiting;
> +	static u8 cnt = 3;
>  	struct scmi_registered_event *r_evt;
>  	struct scmi_event_header eh;
>  	struct scmi_notify_instance *ni = handle->notify_priv;
> @@ -616,8 +630,20 @@ int scmi_notify(const struct scmi_handle *handle, u8 proto_id, u8 evt_id,
>  	kfifo_in(&r_evt->proto->equeue.kfifo, &eh, sizeof(eh));
>  	mdelay(30);
>  	kfifo_in(&r_evt->proto->equeue.kfifo, buf, len);
> -	queue_work(r_evt->proto->equeue.wq,
> -		   &r_evt->proto->equeue.notify_work);
> +
> +	smp_mb__before_atomic();
> +	exiting = atomic_read(&r_evt->proto->equeue.exiting);
> +	do {
> +		bool ret;
> +
> +		ret = queue_work(r_evt->proto->equeue.wq,
> +				 &r_evt->proto->equeue.notify_work);
> +		if (likely(ret || !exiting))
> +			break;
> +		mdelay(5);
> +		smp_mb__before_atomic();
> +		exiting = atomic_read(&r_evt->proto->equeue.exiting);
> +	} while (--cnt);
>  
>  	return 0;
>  }
> @@ -655,6 +681,7 @@ static int scmi_initialize_events_queue(struct scmi_notify_instance *ni,
>  				       &equeue->kfifo);
>  	if (ret)
>  		return ret;
> +	atomic_set(&equeue->exiting, 0);
>  
>  	INIT_WORK(&equeue->notify_work, scmi_events_dispatcher);
>  	equeue->wq = ni->notify_wq;
> --<8-----------------------

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