Re: [dpdk-dev] [PATCH v2 1/2] lib/distributor: fix deadlock issue for aarch64

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

Hi Ruifeng,
	Typically, we have followed the convention of adding comments whenever C11 atomic APIs are used. Can you please add comments indicating why acquire or release semantics are used?

> -----Original Message-----
> From: Ruifeng Wang <ruifeng.wang@arm.com>
> Sent: Friday, October 11, 2019 9:44 PM
> To: david.hunt@intel.com
> Cc: dev@dpdk.org; hkalra@marvell.com; Gavin Hu (Arm Technology China)
> <Gavin.Hu@arm.com>; Honnappa Nagarahalli
> <Honnappa.Nagarahalli@arm.com>; nd <nd@arm.com>; Ruifeng Wang (Arm
> Technology China) <Ruifeng.Wang@arm.com>; stable@dpdk.org
> Subject: [PATCH v2 1/2] lib/distributor: fix deadlock issue for aarch64
> 
> Distributor and worker threads rely on data structs in cache line for
> synchronization. The shared data structs were not protected.
> This caused deadlock issue on weaker memory ordering platforms as aarch64.
> Fix this issue by adding memory barriers to ensure synchronization among
> cores.
> 
> Bugzilla ID: 342
> Fixes: 775003ad2f96 ("distributor: add new burst-capable library")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Ruifeng Wang <ruifeng.wang@arm.com>
> Reviewed-by: Gavin Hu <gavin.hu@arm.com>
> ---
>  lib/librte_distributor/meson.build           |  5 ++
>  lib/librte_distributor/rte_distributor.c     | 39 ++++++++++------
>  lib/librte_distributor/rte_distributor_v20.c | 49 +++++++++++++-------
>  3 files changed, 63 insertions(+), 30 deletions(-)
> 
> diff --git a/lib/librte_distributor/meson.build
> b/lib/librte_distributor/meson.build
> index dba7e3b2a..26577dbc1 100644
> --- a/lib/librte_distributor/meson.build
> +++ b/lib/librte_distributor/meson.build
> @@ -9,3 +9,8 @@ else
>  endif
>  headers = files('rte_distributor.h')
>  deps += ['mbuf']
> +
> +# for clang 32-bit compiles we need libatomic for 64-bit atomic ops if
> +cc.get_id() == 'clang' and dpdk_conf.get('RTE_ARCH_64') == false
> +	ext_deps += cc.find_library('atomic')
> +endif
> diff --git a/lib/librte_distributor/rte_distributor.c
> b/lib/librte_distributor/rte_distributor.c
> index 21eb1fb0a..b653146d0 100644
> --- a/lib/librte_distributor/rte_distributor.c
> +++ b/lib/librte_distributor/rte_distributor.c
> @@ -50,7 +50,8 @@ rte_distributor_request_pkt_v1705(struct
> rte_distributor *d,
> 
>  	retptr64 = &(buf->retptr64[0]);
>  	/* Spin while handshake bits are set (scheduler clears it) */
> -	while (unlikely(*retptr64 & RTE_DISTRIB_GET_BUF)) {
> +	while (unlikely(__atomic_load_n(retptr64, __ATOMIC_ACQUIRE)
> +			& RTE_DISTRIB_GET_BUF)) {
>  		rte_pause();
>  		uint64_t t = rte_rdtsc()+100;
> 
> @@ -76,7 +77,8 @@ rte_distributor_request_pkt_v1705(struct
> rte_distributor *d,
>  	 * Finally, set the GET_BUF  to signal to distributor that cache
>  	 * line is ready for processing
>  	 */
> -	*retptr64 |= RTE_DISTRIB_GET_BUF;
> +	__atomic_store_n(retptr64, *retptr64 | RTE_DISTRIB_GET_BUF,
> +			__ATOMIC_RELEASE);
>  }
>  BIND_DEFAULT_SYMBOL(rte_distributor_request_pkt, _v1705, 17.05);
> MAP_STATIC_SYMBOL(void rte_distributor_request_pkt(struct rte_distributor
> *d, @@ -99,7 +101,8 @@ rte_distributor_poll_pkt_v1705(struct
> rte_distributor *d,
>  	}
> 
>  	/* If bit is set, return */
> -	if (buf->bufptr64[0] & RTE_DISTRIB_GET_BUF)
> +	if (__atomic_load_n(&(buf->bufptr64[0]), __ATOMIC_ACQUIRE)
> +		& RTE_DISTRIB_GET_BUF)
>  		return -1;
> 
>  	/* since bufptr64 is signed, this should be an arithmetic shift */ @@ -
> 115,7 +118,8 @@ rte_distributor_poll_pkt_v1705(struct rte_distributor *d,
>  	 * mbuf pointers, so toggle the bit so scheduler can start working
>  	 * on the next cacheline while we're working.
>  	 */
> -	buf->bufptr64[0] |= RTE_DISTRIB_GET_BUF;
> +	__atomic_store_n(&(buf->bufptr64[0]),
> +		buf->bufptr64[0] | RTE_DISTRIB_GET_BUF,
> __ATOMIC_RELEASE);
> 
>  	return count;
>  }
> @@ -174,6 +178,7 @@ rte_distributor_return_pkt_v1705(struct
> rte_distributor *d,
>  			return -EINVAL;
>  	}
> 
> +	__atomic_thread_fence(__ATOMIC_ACQUIRE);
>  	for (i = 0; i < RTE_DIST_BURST_SIZE; i++)
>  		/* Switch off the return bit first */
>  		buf->retptr64[i] &= ~RTE_DISTRIB_RETURN_BUF; @@ -183,7
> +188,8 @@ rte_distributor_return_pkt_v1705(struct rte_distributor *d,
>  			RTE_DISTRIB_FLAG_BITS) |
> RTE_DISTRIB_RETURN_BUF;
> 
>  	/* set the GET_BUF but even if we got no returns */
> -	buf->retptr64[0] |= RTE_DISTRIB_GET_BUF;
> +	__atomic_store_n(&(buf->retptr64[0]),
> +		buf->retptr64[0] | RTE_DISTRIB_GET_BUF,
> __ATOMIC_RELEASE);
> 
>  	return 0;
>  }
> @@ -273,7 +279,8 @@ handle_returns(struct rte_distributor *d, unsigned
> int wkr)
>  	unsigned int count = 0;
>  	unsigned int i;
> 
> -	if (buf->retptr64[0] & RTE_DISTRIB_GET_BUF) {
> +	if (__atomic_load_n(&(buf->retptr64[0]), __ATOMIC_ACQUIRE)
> +		& RTE_DISTRIB_GET_BUF) {
>  		for (i = 0; i < RTE_DIST_BURST_SIZE; i++) {
>  			if (buf->retptr64[i] & RTE_DISTRIB_RETURN_BUF) {
>  				oldbuf = ((uintptr_t)(buf->retptr64[i] >> @@
> -287,7 +294,7 @@ handle_returns(struct rte_distributor *d, unsigned int wkr)
>  		d->returns.start = ret_start;
>  		d->returns.count = ret_count;
>  		/* Clear for the worker to populate with more returns */
> -		buf->retptr64[0] = 0;
> +		__atomic_store_n(&(buf->retptr64[0]), 0,
> __ATOMIC_RELEASE);
>  	}
>  	return count;
>  }
> @@ -307,7 +314,8 @@ release(struct rte_distributor *d, unsigned int wkr)
>  	struct rte_distributor_buffer *buf = &(d->bufs[wkr]);
>  	unsigned int i;
> 
> -	while (!(d->bufs[wkr].bufptr64[0] & RTE_DISTRIB_GET_BUF))
> +	while (!(__atomic_load_n(&(d->bufs[wkr].bufptr64[0]),
> __ATOMIC_ACQUIRE)
> +		& RTE_DISTRIB_GET_BUF))
>  		rte_pause();
> 
>  	handle_returns(d, wkr);
> @@ -328,7 +336,8 @@ release(struct rte_distributor *d, unsigned int wkr)
>  	d->backlog[wkr].count = 0;
> 
>  	/* Clear the GET bit */
> -	buf->bufptr64[0] &= ~RTE_DISTRIB_GET_BUF;
> +	__atomic_store_n(&(buf->bufptr64[0]),
> +		buf->bufptr64[0] & ~RTE_DISTRIB_GET_BUF,
> __ATOMIC_RELEASE);
>  	return  buf->count;
> 
>  }
> @@ -355,7 +364,8 @@ rte_distributor_process_v1705(struct rte_distributor
> *d,
>  	if (unlikely(num_mbufs == 0)) {
>  		/* Flush out all non-full cache-lines to workers. */
>  		for (wid = 0 ; wid < d->num_workers; wid++) {
> -			if (d->bufs[wid].bufptr64[0] & RTE_DISTRIB_GET_BUF)
> {
> +			if (__atomic_load_n(&(d->bufs[wid].bufptr64[0]),
> +				__ATOMIC_ACQUIRE) &
> RTE_DISTRIB_GET_BUF) {
>  				release(d, wid);
>  				handle_returns(d, wid);
>  			}
> @@ -367,7 +377,8 @@ rte_distributor_process_v1705(struct rte_distributor
> *d,
>  		uint16_t matches[RTE_DIST_BURST_SIZE];
>  		unsigned int pkts;
> 
> -		if (d->bufs[wkr].bufptr64[0] & RTE_DISTRIB_GET_BUF)
> +		if (__atomic_load_n(&(d->bufs[wkr].bufptr64[0]),
> +			__ATOMIC_ACQUIRE) & RTE_DISTRIB_GET_BUF)
>  			d->bufs[wkr].count = 0;
> 
>  		if ((num_mbufs - next_idx) < RTE_DIST_BURST_SIZE) @@ -
> 465,7 +476,8 @@ rte_distributor_process_v1705(struct rte_distributor *d,
> 
>  	/* Flush out all non-full cache-lines to workers. */
>  	for (wid = 0 ; wid < d->num_workers; wid++)
> -		if ((d->bufs[wid].bufptr64[0] & RTE_DISTRIB_GET_BUF))
> +		if ((__atomic_load_n(&(d->bufs[wid].bufptr64[0]),
> +			__ATOMIC_ACQUIRE) & RTE_DISTRIB_GET_BUF))
>  			release(d, wid);
> 
>  	return num_mbufs;
> @@ -574,7 +586,8 @@ rte_distributor_clear_returns_v1705(struct
> rte_distributor *d)
> 
>  	/* throw away returns, so workers can exit */
>  	for (wkr = 0; wkr < d->num_workers; wkr++)
> -		d->bufs[wkr].retptr64[0] = 0;
> +		__atomic_store_n(&(d->bufs[wkr].retptr64[0]), 0,
> +				__ATOMIC_RELEASE);
>  }
>  BIND_DEFAULT_SYMBOL(rte_distributor_clear_returns, _v1705, 17.05);
> MAP_STATIC_SYMBOL(void rte_distributor_clear_returns(struct
> rte_distributor *d), diff --git a/lib/librte_distributor/rte_distributor_v20.c
> b/lib/librte_distributor/rte_distributor_v20.c
> index cdc0969a8..41411e3c1 100644
> --- a/lib/librte_distributor/rte_distributor_v20.c
> +++ b/lib/librte_distributor/rte_distributor_v20.c
> @@ -34,9 +34,10 @@ rte_distributor_request_pkt_v20(struct
> rte_distributor_v20 *d,
>  	union rte_distributor_buffer_v20 *buf = &d->bufs[worker_id];
>  	int64_t req = (((int64_t)(uintptr_t)oldpkt) << RTE_DISTRIB_FLAG_BITS)
>  			| RTE_DISTRIB_GET_BUF;
> -	while (unlikely(buf->bufptr64 & RTE_DISTRIB_FLAGS_MASK))
> +	while (unlikely(__atomic_load_n(&(buf->bufptr64),
> __ATOMIC_ACQUIRE)
> +		& RTE_DISTRIB_FLAGS_MASK))
>  		rte_pause();
> -	buf->bufptr64 = req;
> +	__atomic_store_n(&(buf->bufptr64), req, __ATOMIC_RELEASE);
>  }
>  VERSION_SYMBOL(rte_distributor_request_pkt, _v20, 2.0);
> 
> @@ -45,7 +46,8 @@ rte_distributor_poll_pkt_v20(struct rte_distributor_v20
> *d,
>  		unsigned worker_id)
>  {
>  	union rte_distributor_buffer_v20 *buf = &d->bufs[worker_id];
> -	if (buf->bufptr64 & RTE_DISTRIB_GET_BUF)
> +	if (__atomic_load_n(&(buf->bufptr64), __ATOMIC_ACQUIRE)
> +		& RTE_DISTRIB_GET_BUF)
>  		return NULL;
> 
>  	/* since bufptr64 is signed, this should be an arithmetic shift */ @@ -
> 73,7 +75,7 @@ rte_distributor_return_pkt_v20(struct rte_distributor_v20 *d,
>  	union rte_distributor_buffer_v20 *buf = &d->bufs[worker_id];
>  	uint64_t req = (((int64_t)(uintptr_t)oldpkt) <<
> RTE_DISTRIB_FLAG_BITS)
>  			| RTE_DISTRIB_RETURN_BUF;
> -	buf->bufptr64 = req;
> +	__atomic_store_n(&(buf->bufptr64), req, __ATOMIC_RELEASE);
>  	return 0;
>  }
>  VERSION_SYMBOL(rte_distributor_return_pkt, _v20, 2.0); @@ -117,7 +119,7
> @@ handle_worker_shutdown(struct rte_distributor_v20 *d, unsigned int
> wkr)  {
>  	d->in_flight_tags[wkr] = 0;
>  	d->in_flight_bitmask &= ~(1UL << wkr);
> -	d->bufs[wkr].bufptr64 = 0;
> +	__atomic_store_n(&(d->bufs[wkr].bufptr64), 0, __ATOMIC_RELEASE);
>  	if (unlikely(d->backlog[wkr].count != 0)) {
>  		/* On return of a packet, we need to move the
>  		 * queued packets for this core elsewhere.
> @@ -165,18 +167,23 @@ process_returns(struct rte_distributor_v20 *d)
>  		const int64_t data = d->bufs[wkr].bufptr64;
>  		uintptr_t oldbuf = 0;
> 
> -		if (data & RTE_DISTRIB_GET_BUF) {
> +		if (__atomic_load_n(&data, __ATOMIC_ACQUIRE)
> +			& RTE_DISTRIB_GET_BUF) {
>  			flushed++;
>  			if (d->backlog[wkr].count)
> -				d->bufs[wkr].bufptr64 =
> -						backlog_pop(&d-
> >backlog[wkr]);
> +				__atomic_store_n(&(d->bufs[wkr].bufptr64),
> +					backlog_pop(&d->backlog[wkr]),
> +					__ATOMIC_RELEASE);
>  			else {
> -				d->bufs[wkr].bufptr64 =
> RTE_DISTRIB_GET_BUF;
> +				__atomic_store_n(&(d->bufs[wkr].bufptr64),
> +					RTE_DISTRIB_GET_BUF,
> +					__ATOMIC_RELEASE);
>  				d->in_flight_tags[wkr] = 0;
>  				d->in_flight_bitmask &= ~(1UL << wkr);
>  			}
>  			oldbuf = data >> RTE_DISTRIB_FLAG_BITS;
> -		} else if (data & RTE_DISTRIB_RETURN_BUF) {
> +		} else if (__atomic_load_n(&data, __ATOMIC_ACQUIRE)
> +			& RTE_DISTRIB_RETURN_BUF) {
>  			handle_worker_shutdown(d, wkr);
>  			oldbuf = data >> RTE_DISTRIB_FLAG_BITS;
>  		}
> @@ -251,21 +258,26 @@ rte_distributor_process_v20(struct
> rte_distributor_v20 *d,
>  			}
>  		}
> 
> -		if ((data & RTE_DISTRIB_GET_BUF) &&
> +		if ((__atomic_load_n(&data, __ATOMIC_ACQUIRE)
> +			& RTE_DISTRIB_GET_BUF) &&
>  				(d->backlog[wkr].count || next_mb)) {
> 
>  			if (d->backlog[wkr].count)
> -				d->bufs[wkr].bufptr64 =
> -						backlog_pop(&d-
> >backlog[wkr]);
> +				__atomic_store_n(&(d->bufs[wkr].bufptr64),
> +						backlog_pop(&d-
> >backlog[wkr]),
> +						__ATOMIC_RELEASE);
> 
>  			else {
> -				d->bufs[wkr].bufptr64 = next_value;
> +				__atomic_store_n(&(d->bufs[wkr].bufptr64),
> +						next_value,
> +						__ATOMIC_RELEASE);
>  				d->in_flight_tags[wkr] = new_tag;
>  				d->in_flight_bitmask |= (1UL << wkr);
>  				next_mb = NULL;
>  			}
>  			oldbuf = data >> RTE_DISTRIB_FLAG_BITS;
> -		} else if (data & RTE_DISTRIB_RETURN_BUF) {
> +		} else if (__atomic_load_n(&data, __ATOMIC_ACQUIRE)
> +			& RTE_DISTRIB_RETURN_BUF) {
>  			handle_worker_shutdown(d, wkr);
>  			oldbuf = data >> RTE_DISTRIB_FLAG_BITS;
>  		}
> @@ -280,13 +292,16 @@ rte_distributor_process_v20(struct
> rte_distributor_v20 *d,
>  	 * if they are ready */
>  	for (wkr = 0; wkr < d->num_workers; wkr++)
>  		if (d->backlog[wkr].count &&
> -				(d->bufs[wkr].bufptr64 &
> RTE_DISTRIB_GET_BUF)) {
> +				(__atomic_load_n(&(d->bufs[wkr].bufptr64),
> +				__ATOMIC_ACQUIRE) &
> RTE_DISTRIB_GET_BUF)) {
> 
>  			int64_t oldbuf = d->bufs[wkr].bufptr64 >>
>  					RTE_DISTRIB_FLAG_BITS;
>  			store_return(oldbuf, d, &ret_start, &ret_count);
> 
> -			d->bufs[wkr].bufptr64 = backlog_pop(&d-
> >backlog[wkr]);
> +			__atomic_store_n(&(d->bufs[wkr].bufptr64),
> +				backlog_pop(&d->backlog[wkr]),
> +				__ATOMIC_RELEASE);
>  		}
> 
>  	d->returns.start = ret_start;
> --
> 2.17.1