On Thu, 2014-01-16 at 09:27 -0800, Michael Dalton wrote:
> Sorry, just realized - I think disabling NAPI is necessary but not
> sufficient. There is also the issue that refill_work() could be
> scheduled. If refill_work() executes, it will re-enable NAPI. We'd need
> to cancel the vi->refill delayed work to prevent this AFAICT, and also
> ensure that no other function re-schedules vi->refill or re-enables NAPI
> (virtnet_open/close, virtnet_set_queues, and virtnet_freeze/restore).
>
> How is the following sequence of operations:
> rtnl_lock();
> cancel_delayed_work_sync(&vi->refill);
> napi_disable(&rq->napi);
> read rq->mrg_avg_pkt_len
> virtnet_enable_napi();
> rtnl_unlock();
>
> Additionally, if we disable NAPI when reading this file, perhaps
> the permissions should be changed to 400 so that an unprivileged
> user cannot temporarily disable network RX processing by reading these
> sysfs files. Does that sound reasonable?
I think all this complexity makes no sense to me.
Who cares of sysfs reading a value that might be updated ?
This is purely a debugging utility.
As soon as you read the value, it might already have changed anyway.
Its a integer, just read it without special care.
atomic_read() has also same 'problem', and we do not care.
Make sure that a recompute (aka ewma_add()) does not store intermediate
wrong values, by using ACCESS_ONCE(), and thats enough. No need for the
seqcount overhead.
diff --git a/lib/average.c b/lib/average.c
index 99a67e662b3c..044e0b7f28a8 100644
--- a/lib/average.c
+++ b/lib/average.c
@@ -53,8 +53,10 @@ EXPORT_SYMBOL(ewma_init);
*/
struct ewma *ewma_add(struct ewma *avg, unsigned long val)
{
- avg->internal = avg->internal ?
- (((avg->internal << avg->weight) - avg->internal) +
+ unsigned long internal = ACCESS_ONCE(avg->internal);
+
+ ACCESS_ONCE(avg->internal) = internal ?
+ (((internal << avg->weight) - internal) +
(val << avg->factor)) >> avg->weight :
(val << avg->factor);
return avg;
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