[Differential] [Commented On] D1810: Leave HYP mode upon startup

ian (Ian Lepore) phabric-noreply at FreeBSD.org
Sat Feb 14 18:57:09 UTC 2015


ian added a comment.

Replies from Wojciech Macek (who doesn't have direct write access here)

1. The write barrier.

Yep, it might be not necessary.  I was testing it on the revision which did not have br_prod_tail atomic.  I've just removed it and run the test again, but it will take about 24h, cause the reproduction rate is really low.  

2. Read barrier

I wouldn't say that it prevents reordering here.  It rather guarantees observability of the valid data.  Im my scenario I'm running drdb_dequeue_sc/buf_ring_dequeue_sc few times in the loop.  It's inline, so the compiler puts them inside the loop's body.  I suspect, that the issue is possible only on cortex-a15, due to its long pipeline and advanced branch predictors.  In normal case, where the ring is empty, the dequeue_sc returns NULL.  But the core wants to optimize memmory accessess, so there is a chance that it preloads its readbuffers with a data that can be used in further code execution.  The cons_head does not change within the function, so there is no reason why the core shouldn't early-load the br->br_ring[cons_head] pointer, which might be outdated.  The core just has no knowledge that the data is valid only when prod_tail changes.  Nevertheless, without rmb the buffer did not work properly - dequeue function returned some "old" mbuf which left there from the previous pas
 s.  This is basically how I found this bug.

REVISION DETAIL
  https://reviews.freebsd.org/D1810

To: zbb, andrew, ian
Cc: imp, freebsd-arm


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