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

[Olivier MATZ <olivier.matz@6wind.com>]

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