Re: [PATCH net-next] ptr_ring: make __ptr_ring_empty() checking more reliable
From: Jason Wang <hidden>
Date: 2021-05-27 04:57:45
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在 2021/5/26 下午8:29, Yunsheng Lin 写道:
Currently r->queue[] is cleared after r->consumer_head is moved forward, which makes the __ptr_ring_empty() checking called in page_pool_refill_alloc_cache() unreliable if the checking is done after the r->queue clearing and before the consumer_head moving forward. Move the r->queue[] clearing after consumer_head moving forward to make __ptr_ring_empty() checking more reliable.
If I understand this correctly, this can only happens if you run __ptr_ring_empty() in parallel with ptr_ring_discard_one(). I think those two needs to be serialized. Or did I miss anything? Thanks
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Signed-off-by: Yunsheng Lin <redacted> --- include/linux/ptr_ring.h | 26 +++++++++++++++++--------- 1 file changed, 17 insertions(+), 9 deletions(-)diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h index 808f9d3..f32f052 100644 --- a/include/linux/ptr_ring.h +++ b/include/linux/ptr_ring.h@@ -261,8 +261,7 @@ static inline void __ptr_ring_discard_one(struct ptr_ring *r) /* Note: we must keep consumer_head valid at all times for __ptr_ring_empty * to work correctly. */ - int consumer_head = r->consumer_head; - int head = consumer_head++; + int consumer_head = r->consumer_head + 1; /* Once we have processed enough entries invalidate them in * the ring all at once so producer can reuse their space in the ring.@@ -271,19 +270,28 @@ static inline void __ptr_ring_discard_one(struct ptr_ring *r) */ if (unlikely(consumer_head - r->consumer_tail >= r->batch || consumer_head >= r->size)) { + int tail = r->consumer_tail; + int head = consumer_head; + + if (unlikely(consumer_head >= r->size)) { + r->consumer_tail = 0; + WRITE_ONCE(r->consumer_head, 0); + } else { + r->consumer_tail = consumer_head; + WRITE_ONCE(r->consumer_head, consumer_head); + } + /* Zero out entries in the reverse order: this way we touch the * cache line that producer might currently be reading the last; * producer won't make progress and touch other cache lines * besides the first one until we write out all entries. */ - while (likely(head >= r->consumer_tail)) - r->queue[head--] = NULL; - r->consumer_tail = consumer_head; - } - if (unlikely(consumer_head >= r->size)) { - consumer_head = 0; - r->consumer_tail = 0; + while (likely(--head >= tail)) + r->queue[head] = NULL; + + return; } + /* matching READ_ONCE in __ptr_ring_empty for lockless tests */ WRITE_ONCE(r->consumer_head, consumer_head); }