[dpdk-dev,v3] mbuf: use refcnt = 0 when debugging

  After commit 8f094a9ac5d7 ("mbuf: set mbuf fields while in pool") is it
much harder to detect a "double free".  If the developer makes a copy
of an mbuf pointer and frees it twice, this condition is never detected
and the mbuf gets returned to the pool twice.

Since this requires extra work to track, make this behavior conditional
on CONFIG_RTE_LIBRTE_MBUF_DEBUG.

Signed-off-by: Chas Williams <ciwillia@brocade.com>
---
 lib/librte_mbuf/rte_mbuf.c |  2 +-
 lib/librte_mbuf/rte_mbuf.h | 40 ++++++++++++++++++++++++++++++++++------
 2 files changed, 35 insertions(+), 7 deletions(-)
  

Hi Chas,

On Thu, Sep 07, 2017 at 05:21:51PM -0400, Charles (Chas) Williams wrote:
> After commit 8f094a9ac5d7 ("mbuf: set mbuf fields while in pool") is it
> much harder to detect a "double free".  If the developer makes a copy
> of an mbuf pointer and frees it twice, this condition is never detected
> and the mbuf gets returned to the pool twice.
> 
> Since this requires extra work to track, make this behavior conditional
> on CONFIG_RTE_LIBRTE_MBUF_DEBUG.
> 
> Signed-off-by: Chas Williams <ciwillia@brocade.com>
> ---
>  lib/librte_mbuf/rte_mbuf.c |  2 +-
>  lib/librte_mbuf/rte_mbuf.h | 40 ++++++++++++++++++++++++++++++++++------
>  2 files changed, 35 insertions(+), 7 deletions(-)
> 
> diff --git a/lib/librte_mbuf/rte_mbuf.c b/lib/librte_mbuf/rte_mbuf.c
> index 26a62b8..b0d222c 100644
> --- a/lib/librte_mbuf/rte_mbuf.c
> +++ b/lib/librte_mbuf/rte_mbuf.c
> @@ -145,7 +145,7 @@ rte_pktmbuf_init(struct rte_mempool *mp,
>  	m->pool = mp;
>  	m->nb_segs = 1;
>  	m->port = 0xff;
> -	rte_mbuf_refcnt_set(m, 1);
> +	rte_mbuf_refcnt_set(m, RTE_MBUF_UNUSED_CNT);
>  	m->next = NULL;
>  }
>  
> diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> index eaed7ee..1400b35 100644
> --- a/lib/librte_mbuf/rte_mbuf.h
> +++ b/lib/librte_mbuf/rte_mbuf.h
> @@ -671,11 +671,15 @@ struct rte_pktmbuf_pool_private {
>  
>  #ifdef RTE_LIBRTE_MBUF_DEBUG
>  
> +#define RTE_MBUF_UNUSED_CNT 0
> +
>  /**  check mbuf type in debug mode */
>  #define __rte_mbuf_sanity_check(m, is_h) rte_mbuf_sanity_check(m, is_h)
>  
>  #else /*  RTE_LIBRTE_MBUF_DEBUG */
>  
> +#define RTE_MBUF_UNUSED_CNT 1
> +
>  /**  check mbuf type in debug mode */
>  #define __rte_mbuf_sanity_check(m, is_h) do { } while (0)
>  
> @@ -721,6 +725,9 @@ rte_mbuf_refcnt_set(struct rte_mbuf *m, uint16_t new_value)
>  static inline uint16_t
>  rte_mbuf_refcnt_update(struct rte_mbuf *m, int16_t value)
>  {
> +#ifdef RTE_LIBRTE_MBUF_DEBUG
> +	RTE_ASSERT(rte_mbuf_refcnt_read(m) != 0);
> +#endif
>  	/*
>  	 * The atomic_add is an expensive operation, so we don't want to
>  	 * call it in the case where we know we are the uniq holder of
> @@ -791,10 +798,9 @@ void
>  rte_mbuf_sanity_check(const struct rte_mbuf *m, int is_header);
>  
>  #define MBUF_RAW_ALLOC_CHECK(m) do {				\
> -	RTE_ASSERT(rte_mbuf_refcnt_read(m) == 1);		\
> +	RTE_ASSERT(rte_mbuf_refcnt_read(m) == RTE_MBUF_UNUSED_CNT); \
>  	RTE_ASSERT((m)->next == NULL);				\
>  	RTE_ASSERT((m)->nb_segs == 1);				\
> -	__rte_mbuf_sanity_check(m, 0);				\
>  } while (0)
>  
>  /**
> @@ -825,6 +831,10 @@ static inline struct rte_mbuf *rte_mbuf_raw_alloc(struct rte_mempool *mp)
>  		return NULL;
>  	m = (struct rte_mbuf *)mb;
>  	MBUF_RAW_ALLOC_CHECK(m);
> +#ifdef RTE_LIBRTE_MBUF_DEBUG
> +	rte_mbuf_refcnt_set(m, 1);
> +#endif
> +	__rte_mbuf_sanity_check(m, 0);
>  	return m;
>  }
>  
> @@ -846,10 +856,9 @@ static __rte_always_inline void
>  rte_mbuf_raw_free(struct rte_mbuf *m)
>  {
>  	RTE_ASSERT(RTE_MBUF_DIRECT(m));
> -	RTE_ASSERT(rte_mbuf_refcnt_read(m) == 1);
> +	RTE_ASSERT(rte_mbuf_refcnt_read(m) == RTE_MBUF_UNUSED_CNT);
>  	RTE_ASSERT(m->next == NULL);
>  	RTE_ASSERT(m->nb_segs == 1);
> -	__rte_mbuf_sanity_check(m, 0);
>  	rte_mempool_put(m->pool, m);
>  }
>  
> @@ -1159,21 +1168,37 @@ static inline int rte_pktmbuf_alloc_bulk(struct rte_mempool *pool,
>  	case 0:
>  		while (idx != count) {
>  			MBUF_RAW_ALLOC_CHECK(mbufs[idx]);
> +#ifdef RTE_LIBRTE_MBUF_DEBUG
> +			rte_mbuf_refcnt_set(mbufs[idx], 1);
> +#endif
> +			__rte_mbuf_sanity_check(mbufs[idx], 0);
>  			rte_pktmbuf_reset(mbufs[idx]);
>  			idx++;
>  			/* fall-through */
>  	case 3:
>  			MBUF_RAW_ALLOC_CHECK(mbufs[idx]);
> +#ifdef RTE_LIBRTE_MBUF_DEBUG
> +			rte_mbuf_refcnt_set(mbufs[idx], 1);
> +#endif
> +			__rte_mbuf_sanity_check(mbufs[idx], 0);
>  			rte_pktmbuf_reset(mbufs[idx]);
>  			idx++;
>  			/* fall-through */
>  	case 2:
>  			MBUF_RAW_ALLOC_CHECK(mbufs[idx]);
> +#ifdef RTE_LIBRTE_MBUF_DEBUG
> +			rte_mbuf_refcnt_set(mbufs[idx], 1);
> +#endif
> +			__rte_mbuf_sanity_check(mbufs[idx], 0);
>  			rte_pktmbuf_reset(mbufs[idx]);
>  			idx++;
>  			/* fall-through */
>  	case 1:
>  			MBUF_RAW_ALLOC_CHECK(mbufs[idx]);
> +#ifdef RTE_LIBRTE_MBUF_DEBUG
> +			rte_mbuf_refcnt_set(mbufs[idx], 1);
> +#endif
> +			__rte_mbuf_sanity_check(mbufs[idx], 0);
>  			rte_pktmbuf_reset(mbufs[idx]);
>  			idx++;
>  			/* fall-through */
> @@ -1271,7 +1296,7 @@ static inline void rte_pktmbuf_detach(struct rte_mbuf *m)
>  	if (rte_mbuf_refcnt_update(md, -1) == 0) {
>  		md->next = NULL;
>  		md->nb_segs = 1;
> -		rte_mbuf_refcnt_set(md, 1);
> +		rte_mbuf_refcnt_set(md, RTE_MBUF_UNUSED_CNT);
>  		rte_mbuf_raw_free(md);
>  	}
>  }
> @@ -1304,6 +1329,9 @@ rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
>  			m->next = NULL;
>  			m->nb_segs = 1;
>  		}
> +#ifdef RTE_LIBRTE_MBUF_DEBUG
> +		rte_mbuf_refcnt_set(m, RTE_MBUF_UNUSED_CNT);
> +#endif
>  
>  		return m;
>  
> @@ -1317,7 +1345,7 @@ rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
>  			m->next = NULL;
>  			m->nb_segs = 1;
>  		}
> -		rte_mbuf_refcnt_set(m, 1);
> +		rte_mbuf_refcnt_set(m, RTE_MBUF_UNUSED_CNT);
>  
>  		return m;
>  	}
> -- 
> 2.1.4
> 

I'm afraid this won't work for drivers that use rte_mempool_get()
instead of rte_mbuf_raw_alloc(), because they will expect that
refcount is 1.

I didn't try, but an alternative to detect these double-frees
could be to enable RTE_LIBRTE_MEMPOOL_DEBUG. What do you think?

Olivier