[2/2] lib/ethdev: fix memory ordering for call back functions

Message ID 20201002000711.41511-2-honnappa.nagarahalli@arm.com
State Superseded
Headers show
Series
  • [1/2] lib/ethdev: replace full barrier with relaxed barrier
Related show

Commit Message

Honnappa Nagarahalli Oct. 2, 2020, 12:07 a.m.
Call back functions are registered on the control plane. They
are accessed from the data plane. Hence, correct memory orderings
should be used to avoid race conditions.

Fixes: 4dc294158cac ("ethdev: support optional Rx and Tx callbacks")
Fixes: c8231c63ddcb ("ethdev: insert Rx callback as head of list")
Cc: bruce.richardson@intel.com
Cc: john.mcnamara@intel.com
Cc: reshma.pattan@intel.com
Cc: stable@dpdk.org

Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

Reviewed-by: Ola Liljedahl <ola.liljedahl@arm.com>

---
 lib/librte_ethdev/rte_ethdev.c | 28 +++++++++++++++++++++------
 lib/librte_ethdev/rte_ethdev.h | 35 ++++++++++++++++++++++++++--------
 2 files changed, 49 insertions(+), 14 deletions(-)

-- 
2.17.1

Comments

Phil Yang Oct. 9, 2020, 2:34 a.m. | #1
> -----Original Message-----

> From: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

> Sent: Friday, October 2, 2020 8:07 AM

> To: dev@dpdk.org; Honnappa Nagarahalli

> <Honnappa.Nagarahalli@arm.com>; Phil Yang <Phil.Yang@arm.com>;

> thomas@monjalon.net; arybchenko@solarflare.com; ferruh.yigit@intel.com

> Cc: abhinandan.gujjar@intel.com; nd <nd@arm.com>;

> bruce.richardson@intel.com; john.mcnamara@intel.com;

> reshma.pattan@intel.com; stable@dpdk.org

> Subject: [PATCH 2/2] lib/ethdev: fix memory ordering for call back functions

> 

> Call back functions are registered on the control plane. They

> are accessed from the data plane. Hence, correct memory orderings

> should be used to avoid race conditions.

> 

> Fixes: 4dc294158cac ("ethdev: support optional Rx and Tx callbacks")

> Fixes: c8231c63ddcb ("ethdev: insert Rx callback as head of list")

> Cc: bruce.richardson@intel.com

> Cc: john.mcnamara@intel.com

> Cc: reshma.pattan@intel.com

> Cc: stable@dpdk.org

> 

> Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

> Reviewed-by: Ola Liljedahl <ola.liljedahl@arm.com>



Reviewed-by: Phil Yang <phil.yang@arm.com>



> ---

>  lib/librte_ethdev/rte_ethdev.c | 28 +++++++++++++++++++++------

>  lib/librte_ethdev/rte_ethdev.h | 35 ++++++++++++++++++++++++++-------

> -

>  2 files changed, 49 insertions(+), 14 deletions(-)

> 

> diff --git a/lib/librte_ethdev/rte_ethdev.c b/lib/librte_ethdev/rte_ethdev.c

> index 59a41c07f..d89fcdc77 100644

> --- a/lib/librte_ethdev/rte_ethdev.c

> +++ b/lib/librte_ethdev/rte_ethdev.c

> @@ -4486,12 +4486,20 @@ rte_eth_add_rx_callback(uint16_t port_id,

> uint16_t queue_id,

>  		rte_eth_devices[port_id].post_rx_burst_cbs[queue_id];

> 

>  	if (!tail) {

> -		rte_eth_devices[port_id].post_rx_burst_cbs[queue_id] = cb;

> +		/* Stores to cb->fn and cb->param should complete before

> +		 * cb is visible to data plane.

> +		 */

> +		__atomic_store_n(

> +

> 	&rte_eth_devices[port_id].post_rx_burst_cbs[queue_id],

> +			cb, __ATOMIC_RELEASE);

> 

>  	} else {

>  		while (tail->next)

>  			tail = tail->next;

> -		tail->next = cb;

> +		/* Stores to cb->fn and cb->param should complete before

> +		 * cb is visible to data plane.

> +		 */

> +		__atomic_store_n(&tail->next, cb, __ATOMIC_RELEASE);

>  	}

>  	rte_spinlock_unlock(&rte_eth_rx_cb_lock);

> 

> @@ -4576,12 +4584,20 @@ rte_eth_add_tx_callback(uint16_t port_id,

> uint16_t queue_id,

>  		rte_eth_devices[port_id].pre_tx_burst_cbs[queue_id];

> 

>  	if (!tail) {

> -		rte_eth_devices[port_id].pre_tx_burst_cbs[queue_id] = cb;

> +		/* Stores to cb->fn and cb->param should complete before

> +		 * cb is visible to data plane.

> +		 */

> +		__atomic_store_n(

> +

> 	&rte_eth_devices[port_id].pre_tx_burst_cbs[queue_id],

> +			cb, __ATOMIC_RELEASE);

> 

>  	} else {

>  		while (tail->next)

>  			tail = tail->next;

> -		tail->next = cb;

> +		/* Stores to cb->fn and cb->param should complete before

> +		 * cb is visible to data plane.

> +		 */

> +		__atomic_store_n(&tail->next, cb, __ATOMIC_RELEASE);

>  	}

>  	rte_spinlock_unlock(&rte_eth_tx_cb_lock);

> 

> @@ -4612,7 +4628,7 @@ rte_eth_remove_rx_callback(uint16_t port_id,

> uint16_t queue_id,

>  		cb = *prev_cb;

>  		if (cb == user_cb) {

>  			/* Remove the user cb from the callback list. */

> -			*prev_cb = cb->next;

> +			__atomic_store_n(prev_cb, cb->next,

> __ATOMIC_RELAXED);

>  			ret = 0;

>  			break;

>  		}

> @@ -4646,7 +4662,7 @@ rte_eth_remove_tx_callback(uint16_t port_id,

> uint16_t queue_id,

>  		cb = *prev_cb;

>  		if (cb == user_cb) {

>  			/* Remove the user cb from the callback list. */

> -			*prev_cb = cb->next;

> +			__atomic_store_n(prev_cb, cb->next,

> __ATOMIC_RELAXED);

>  			ret = 0;

>  			break;

>  		}

> diff --git a/lib/librte_ethdev/rte_ethdev.h b/lib/librte_ethdev/rte_ethdev.h

> index 70295d7ab..d810e3e38 100644

> --- a/lib/librte_ethdev/rte_ethdev.h

> +++ b/lib/librte_ethdev/rte_ethdev.h

> @@ -3734,7 +3734,8 @@ struct rte_eth_rxtx_callback;

>   *   The callback function

>   * @param user_param

>   *   A generic pointer parameter which will be passed to each invocation of

> the

> - *   callback function on this port and queue.

> + *   callback function on this port and queue. Inter-thread synchronization

> + *   of any user data changes is the responsibility of the user.

>   *

>   * @return

>   *   NULL on error.

> @@ -3763,7 +3764,8 @@ rte_eth_add_rx_callback(uint16_t port_id, uint16_t

> queue_id,

>   *   The callback function

>   * @param user_param

>   *   A generic pointer parameter which will be passed to each invocation of

> the

> - *   callback function on this port and queue.

> + *   callback function on this port and queue. Inter-thread synchronization

> + *   of any user data changes is the responsibility of the user.

>   *

>   * @return

>   *   NULL on error.

> @@ -3791,7 +3793,8 @@ rte_eth_add_first_rx_callback(uint16_t port_id,

> uint16_t queue_id,

>   *   The callback function

>   * @param user_param

>   *   A generic pointer parameter which will be passed to each invocation of

> the

> - *   callback function on this port and queue.

> + *   callback function on this port and queue. Inter-thread synchronization

> + *   of any user data changes is the responsibility of the user.

>   *

>   * @return

>   *   NULL on error.

> @@ -3816,7 +3819,9 @@ rte_eth_add_tx_callback(uint16_t port_id, uint16_t

> queue_id,

>   *   on that queue.

>   *

>   * - After a short delay - where the delay is sufficient to allow any

> - *   in-flight callbacks to complete.

> + *   in-flight callbacks to complete. Alternately, the RCU mechanism can be

> + *   used to detect when data plane threads have ceased referencing the

> + *   callback memory.

>   *

>   * @param port_id

>   *   The port identifier of the Ethernet device.

> @@ -3849,7 +3854,9 @@ int rte_eth_remove_rx_callback(uint16_t port_id,

> uint16_t queue_id,

>   *   on that queue.

>   *

>   * - After a short delay - where the delay is sufficient to allow any

> - *   in-flight callbacks to complete.

> + *   in-flight callbacks to complete. Alternately, the RCU mechanism can be

> + *   used to detect when data plane threads have ceased referencing the

> + *   callback memory.

>   *

>   * @param port_id

>   *   The port identifier of the Ethernet device.

> @@ -4510,10 +4517,16 @@ rte_eth_rx_burst(uint16_t port_id, uint16_t

> queue_id,

>  				     rx_pkts, nb_pkts);

> 

>  #ifdef RTE_ETHDEV_RXTX_CALLBACKS

> -	if (unlikely(dev->post_rx_burst_cbs[queue_id] != NULL)) {

> -		struct rte_eth_rxtx_callback *cb =

> -				dev->post_rx_burst_cbs[queue_id];

> +	/* __ATOMIC_RELEASE memory order was used when the

> +	 * call back was inserted into the list.

> +	 * Since there is a clear dependency between loading

> +	 * cb and cb->fn/cb->next, __ATOMIC_ACQUIRE memory order is

> +	 * not required.

> +	 */

> +	struct rte_eth_rxtx_callback *cb =

> +			dev->post_rx_burst_cbs[queue_id];

> 

> +	if (unlikely(cb != NULL)) {

>  		do {

>  			nb_rx = cb->fn.rx(port_id, queue_id, rx_pkts, nb_rx,

>  						nb_pkts, cb->param);

> @@ -4775,6 +4788,12 @@ rte_eth_tx_burst(uint16_t port_id, uint16_t

> queue_id,

>  #endif

> 

>  #ifdef RTE_ETHDEV_RXTX_CALLBACKS

> +	/* __ATOMIC_RELEASE memory order was used when the

> +	 * call back was inserted into the list.

> +	 * Since there is a clear dependency between loading

> +	 * cb and cb->fn/cb->next, __ATOMIC_ACQUIRE memory order is

> +	 * not required.

> +	 */

>  	struct rte_eth_rxtx_callback *cb = dev-

> >pre_tx_burst_cbs[queue_id];

> 

>  	if (unlikely(cb != NULL)) {

> --

> 2.17.1
Ferruh Yigit Oct. 13, 2020, 10:32 a.m. | #2
On 10/2/2020 1:07 AM, Honnappa Nagarahalli wrote:
> Call back functions are registered on the control plane. They

> are accessed from the data plane. Hence, correct memory orderings

> should be used to avoid race conditions.

> 

> Fixes: 4dc294158cac ("ethdev: support optional Rx and Tx callbacks")

> Fixes: c8231c63ddcb ("ethdev: insert Rx callback as head of list")

> Cc: bruce.richardson@intel.com

> Cc: john.mcnamara@intel.com

> Cc: reshma.pattan@intel.com

> Cc: stable@dpdk.org

> 

> Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

> Reviewed-by: Ola Liljedahl <ola.liljedahl@arm.com>


+Konstantin & Jerin,

Can you please help reviewing this one too?

@Honnappa,

Is this patch supersedes the following one in the queue by ZY Qiu:
https://patches.dpdk.org/patch/66301/
Ananyev, Konstantin Oct. 13, 2020, 2:25 p.m. | #3
> 

> Call back functions are registered on the control plane. They

> are accessed from the data plane. Hence, correct memory orderings

> should be used to avoid race conditions.

> 

> Fixes: 4dc294158cac ("ethdev: support optional Rx and Tx callbacks")

> Fixes: c8231c63ddcb ("ethdev: insert Rx callback as head of list")

> Cc: bruce.richardson@intel.com

> Cc: john.mcnamara@intel.com

> Cc: reshma.pattan@intel.com

> Cc: stable@dpdk.org

> 

> Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

> Reviewed-by: Ola Liljedahl <ola.liljedahl@arm.com>

> ---

>  lib/librte_ethdev/rte_ethdev.c | 28 +++++++++++++++++++++------

>  lib/librte_ethdev/rte_ethdev.h | 35 ++++++++++++++++++++++++++--------

>  2 files changed, 49 insertions(+), 14 deletions(-)

> 

> diff --git a/lib/librte_ethdev/rte_ethdev.c b/lib/librte_ethdev/rte_ethdev.c

> index 59a41c07f..d89fcdc77 100644

> --- a/lib/librte_ethdev/rte_ethdev.c

> +++ b/lib/librte_ethdev/rte_ethdev.c

> @@ -4486,12 +4486,20 @@ rte_eth_add_rx_callback(uint16_t port_id, uint16_t queue_id,

>  		rte_eth_devices[port_id].post_rx_burst_cbs[queue_id];

> 

>  	if (!tail) {

> -		rte_eth_devices[port_id].post_rx_burst_cbs[queue_id] = cb;

> +		/* Stores to cb->fn and cb->param should complete before

> +		 * cb is visible to data plane.

> +		 */

> +		__atomic_store_n(

> +			&rte_eth_devices[port_id].post_rx_burst_cbs[queue_id],

> +			cb, __ATOMIC_RELEASE);

> 

>  	} else {

>  		while (tail->next)

>  			tail = tail->next;

> -		tail->next = cb;

> +		/* Stores to cb->fn and cb->param should complete before

> +		 * cb is visible to data plane.

> +		 */

> +		__atomic_store_n(&tail->next, cb, __ATOMIC_RELEASE);

>  	}

>  	rte_spinlock_unlock(&rte_eth_rx_cb_lock);

> 

> @@ -4576,12 +4584,20 @@ rte_eth_add_tx_callback(uint16_t port_id, uint16_t queue_id,

>  		rte_eth_devices[port_id].pre_tx_burst_cbs[queue_id];

> 

>  	if (!tail) {

> -		rte_eth_devices[port_id].pre_tx_burst_cbs[queue_id] = cb;

> +		/* Stores to cb->fn and cb->param should complete before

> +		 * cb is visible to data plane.

> +		 */

> +		__atomic_store_n(

> +			&rte_eth_devices[port_id].pre_tx_burst_cbs[queue_id],

> +			cb, __ATOMIC_RELEASE);

> 

>  	} else {

>  		while (tail->next)

>  			tail = tail->next;

> -		tail->next = cb;

> +		/* Stores to cb->fn and cb->param should complete before

> +		 * cb is visible to data plane.

> +		 */

> +		__atomic_store_n(&tail->next, cb, __ATOMIC_RELEASE);

>  	}

>  	rte_spinlock_unlock(&rte_eth_tx_cb_lock);

> 

> @@ -4612,7 +4628,7 @@ rte_eth_remove_rx_callback(uint16_t port_id, uint16_t queue_id,

>  		cb = *prev_cb;

>  		if (cb == user_cb) {

>  			/* Remove the user cb from the callback list. */

> -			*prev_cb = cb->next;

> +			__atomic_store_n(prev_cb, cb->next, __ATOMIC_RELAXED);

>  			ret = 0;

>  			break;

>  		}

> @@ -4646,7 +4662,7 @@ rte_eth_remove_tx_callback(uint16_t port_id, uint16_t queue_id,

>  		cb = *prev_cb;

>  		if (cb == user_cb) {

>  			/* Remove the user cb from the callback list. */

> -			*prev_cb = cb->next;

> +			__atomic_store_n(prev_cb, cb->next, __ATOMIC_RELAXED);

>  			ret = 0;

>  			break;

>  		}

> diff --git a/lib/librte_ethdev/rte_ethdev.h b/lib/librte_ethdev/rte_ethdev.h

> index 70295d7ab..d810e3e38 100644

> --- a/lib/librte_ethdev/rte_ethdev.h

> +++ b/lib/librte_ethdev/rte_ethdev.h

> @@ -3734,7 +3734,8 @@ struct rte_eth_rxtx_callback;

>   *   The callback function

>   * @param user_param

>   *   A generic pointer parameter which will be passed to each invocation of the

> - *   callback function on this port and queue.

> + *   callback function on this port and queue. Inter-thread synchronization

> + *   of any user data changes is the responsibility of the user.

>   *

>   * @return

>   *   NULL on error.

> @@ -3763,7 +3764,8 @@ rte_eth_add_rx_callback(uint16_t port_id, uint16_t queue_id,

>   *   The callback function

>   * @param user_param

>   *   A generic pointer parameter which will be passed to each invocation of the

> - *   callback function on this port and queue.

> + *   callback function on this port and queue. Inter-thread synchronization

> + *   of any user data changes is the responsibility of the user.

>   *

>   * @return

>   *   NULL on error.

> @@ -3791,7 +3793,8 @@ rte_eth_add_first_rx_callback(uint16_t port_id, uint16_t queue_id,

>   *   The callback function

>   * @param user_param

>   *   A generic pointer parameter which will be passed to each invocation of the

> - *   callback function on this port and queue.

> + *   callback function on this port and queue. Inter-thread synchronization

> + *   of any user data changes is the responsibility of the user.

>   *

>   * @return

>   *   NULL on error.

> @@ -3816,7 +3819,9 @@ rte_eth_add_tx_callback(uint16_t port_id, uint16_t queue_id,

>   *   on that queue.

>   *

>   * - After a short delay - where the delay is sufficient to allow any

> - *   in-flight callbacks to complete.

> + *   in-flight callbacks to complete. Alternately, the RCU mechanism can be

> + *   used to detect when data plane threads have ceased referencing the

> + *   callback memory.

>   *

>   * @param port_id

>   *   The port identifier of the Ethernet device.

> @@ -3849,7 +3854,9 @@ int rte_eth_remove_rx_callback(uint16_t port_id, uint16_t queue_id,

>   *   on that queue.

>   *

>   * - After a short delay - where the delay is sufficient to allow any

> - *   in-flight callbacks to complete.

> + *   in-flight callbacks to complete. Alternately, the RCU mechanism can be

> + *   used to detect when data plane threads have ceased referencing the

> + *   callback memory.

>   *

>   * @param port_id

>   *   The port identifier of the Ethernet device.

> @@ -4510,10 +4517,16 @@ rte_eth_rx_burst(uint16_t port_id, uint16_t queue_id,

>  				     rx_pkts, nb_pkts);

> 

>  #ifdef RTE_ETHDEV_RXTX_CALLBACKS

> -	if (unlikely(dev->post_rx_burst_cbs[queue_id] != NULL)) {

> -		struct rte_eth_rxtx_callback *cb =

> -				dev->post_rx_burst_cbs[queue_id];

> +	/* __ATOMIC_RELEASE memory order was used when the

> +	 * call back was inserted into the list.

> +	 * Since there is a clear dependency between loading

> +	 * cb and cb->fn/cb->next, __ATOMIC_ACQUIRE memory order is

> +	 * not required.

> +	 */

> +	struct rte_eth_rxtx_callback *cb =

> +			dev->post_rx_burst_cbs[queue_id];

> 

> +	if (unlikely(cb != NULL)) {

>  		do {

>  			nb_rx = cb->fn.rx(port_id, queue_id, rx_pkts, nb_rx,

>  						nb_pkts, cb->param);

> @@ -4775,6 +4788,12 @@ rte_eth_tx_burst(uint16_t port_id, uint16_t queue_id,

>  #endif

> 

>  #ifdef RTE_ETHDEV_RXTX_CALLBACKS

> +	/* __ATOMIC_RELEASE memory order was used when the

> +	 * call back was inserted into the list.

> +	 * Since there is a clear dependency between loading

> +	 * cb and cb->fn/cb->next, __ATOMIC_ACQUIRE memory order is

> +	 * not required.

> +	 */

>  	struct rte_eth_rxtx_callback *cb = dev->pre_tx_burst_cbs[queue_id];

> 

>  	if (unlikely(cb != NULL)) {

> --


Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>


> 2.17.1
Honnappa Nagarahalli Oct. 13, 2020, 3:25 p.m. | #4
<snip>

> Subject: Re: [PATCH 2/2] lib/ethdev: fix memory ordering for call back

> functions

> 

> On 10/2/2020 1:07 AM, Honnappa Nagarahalli wrote:

> > Call back functions are registered on the control plane. They are

> > accessed from the data plane. Hence, correct memory orderings should

> > be used to avoid race conditions.

> >

> > Fixes: 4dc294158cac ("ethdev: support optional Rx and Tx callbacks")

> > Fixes: c8231c63ddcb ("ethdev: insert Rx callback as head of list")

> > Cc: bruce.richardson@intel.com

> > Cc: john.mcnamara@intel.com

> > Cc: reshma.pattan@intel.com

> > Cc: stable@dpdk.org

> >

> > Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

> > Reviewed-by: Ola Liljedahl <ola.liljedahl@arm.com>

> 

> +Konstantin & Jerin,

> 

> Can you please help reviewing this one too?

> 

> @Honnappa,

> 

> Is this patch supersedes the following one in the queue by ZY Qiu:

> https://patches.dpdk.org/patch/66301/

Yes, my patch should supersede. I will create a V2 to address the issue ZY Qiu's patch is trying to address.

Patch

diff --git a/lib/librte_ethdev/rte_ethdev.c b/lib/librte_ethdev/rte_ethdev.c
index 59a41c07f..d89fcdc77 100644
--- a/lib/librte_ethdev/rte_ethdev.c
+++ b/lib/librte_ethdev/rte_ethdev.c
@@ -4486,12 +4486,20 @@  rte_eth_add_rx_callback(uint16_t port_id, uint16_t queue_id,
 		rte_eth_devices[port_id].post_rx_burst_cbs[queue_id];
 
 	if (!tail) {
-		rte_eth_devices[port_id].post_rx_burst_cbs[queue_id] = cb;
+		/* Stores to cb->fn and cb->param should complete before
+		 * cb is visible to data plane.
+		 */
+		__atomic_store_n(
+			&rte_eth_devices[port_id].post_rx_burst_cbs[queue_id],
+			cb, __ATOMIC_RELEASE);
 
 	} else {
 		while (tail->next)
 			tail = tail->next;
-		tail->next = cb;
+		/* Stores to cb->fn and cb->param should complete before
+		 * cb is visible to data plane.
+		 */
+		__atomic_store_n(&tail->next, cb, __ATOMIC_RELEASE);
 	}
 	rte_spinlock_unlock(&rte_eth_rx_cb_lock);
 
@@ -4576,12 +4584,20 @@  rte_eth_add_tx_callback(uint16_t port_id, uint16_t queue_id,
 		rte_eth_devices[port_id].pre_tx_burst_cbs[queue_id];
 
 	if (!tail) {
-		rte_eth_devices[port_id].pre_tx_burst_cbs[queue_id] = cb;
+		/* Stores to cb->fn and cb->param should complete before
+		 * cb is visible to data plane.
+		 */
+		__atomic_store_n(
+			&rte_eth_devices[port_id].pre_tx_burst_cbs[queue_id],
+			cb, __ATOMIC_RELEASE);
 
 	} else {
 		while (tail->next)
 			tail = tail->next;
-		tail->next = cb;
+		/* Stores to cb->fn and cb->param should complete before
+		 * cb is visible to data plane.
+		 */
+		__atomic_store_n(&tail->next, cb, __ATOMIC_RELEASE);
 	}
 	rte_spinlock_unlock(&rte_eth_tx_cb_lock);
 
@@ -4612,7 +4628,7 @@  rte_eth_remove_rx_callback(uint16_t port_id, uint16_t queue_id,
 		cb = *prev_cb;
 		if (cb == user_cb) {
 			/* Remove the user cb from the callback list. */
-			*prev_cb = cb->next;
+			__atomic_store_n(prev_cb, cb->next, __ATOMIC_RELAXED);
 			ret = 0;
 			break;
 		}
@@ -4646,7 +4662,7 @@  rte_eth_remove_tx_callback(uint16_t port_id, uint16_t queue_id,
 		cb = *prev_cb;
 		if (cb == user_cb) {
 			/* Remove the user cb from the callback list. */
-			*prev_cb = cb->next;
+			__atomic_store_n(prev_cb, cb->next, __ATOMIC_RELAXED);
 			ret = 0;
 			break;
 		}
diff --git a/lib/librte_ethdev/rte_ethdev.h b/lib/librte_ethdev/rte_ethdev.h
index 70295d7ab..d810e3e38 100644
--- a/lib/librte_ethdev/rte_ethdev.h
+++ b/lib/librte_ethdev/rte_ethdev.h
@@ -3734,7 +3734,8 @@  struct rte_eth_rxtx_callback;
  *   The callback function
  * @param user_param
  *   A generic pointer parameter which will be passed to each invocation of the
- *   callback function on this port and queue.
+ *   callback function on this port and queue. Inter-thread synchronization
+ *   of any user data changes is the responsibility of the user.
  *
  * @return
  *   NULL on error.
@@ -3763,7 +3764,8 @@  rte_eth_add_rx_callback(uint16_t port_id, uint16_t queue_id,
  *   The callback function
  * @param user_param
  *   A generic pointer parameter which will be passed to each invocation of the
- *   callback function on this port and queue.
+ *   callback function on this port and queue. Inter-thread synchronization
+ *   of any user data changes is the responsibility of the user.
  *
  * @return
  *   NULL on error.
@@ -3791,7 +3793,8 @@  rte_eth_add_first_rx_callback(uint16_t port_id, uint16_t queue_id,
  *   The callback function
  * @param user_param
  *   A generic pointer parameter which will be passed to each invocation of the
- *   callback function on this port and queue.
+ *   callback function on this port and queue. Inter-thread synchronization
+ *   of any user data changes is the responsibility of the user.
  *
  * @return
  *   NULL on error.
@@ -3816,7 +3819,9 @@  rte_eth_add_tx_callback(uint16_t port_id, uint16_t queue_id,
  *   on that queue.
  *
  * - After a short delay - where the delay is sufficient to allow any
- *   in-flight callbacks to complete.
+ *   in-flight callbacks to complete. Alternately, the RCU mechanism can be
+ *   used to detect when data plane threads have ceased referencing the
+ *   callback memory.
  *
  * @param port_id
  *   The port identifier of the Ethernet device.
@@ -3849,7 +3854,9 @@  int rte_eth_remove_rx_callback(uint16_t port_id, uint16_t queue_id,
  *   on that queue.
  *
  * - After a short delay - where the delay is sufficient to allow any
- *   in-flight callbacks to complete.
+ *   in-flight callbacks to complete. Alternately, the RCU mechanism can be
+ *   used to detect when data plane threads have ceased referencing the
+ *   callback memory.
  *
  * @param port_id
  *   The port identifier of the Ethernet device.
@@ -4510,10 +4517,16 @@  rte_eth_rx_burst(uint16_t port_id, uint16_t queue_id,
 				     rx_pkts, nb_pkts);
 
 #ifdef RTE_ETHDEV_RXTX_CALLBACKS
-	if (unlikely(dev->post_rx_burst_cbs[queue_id] != NULL)) {
-		struct rte_eth_rxtx_callback *cb =
-				dev->post_rx_burst_cbs[queue_id];
+	/* __ATOMIC_RELEASE memory order was used when the
+	 * call back was inserted into the list.
+	 * Since there is a clear dependency between loading
+	 * cb and cb->fn/cb->next, __ATOMIC_ACQUIRE memory order is
+	 * not required.
+	 */
+	struct rte_eth_rxtx_callback *cb =
+			dev->post_rx_burst_cbs[queue_id];
 
+	if (unlikely(cb != NULL)) {
 		do {
 			nb_rx = cb->fn.rx(port_id, queue_id, rx_pkts, nb_rx,
 						nb_pkts, cb->param);
@@ -4775,6 +4788,12 @@  rte_eth_tx_burst(uint16_t port_id, uint16_t queue_id,
 #endif
 
 #ifdef RTE_ETHDEV_RXTX_CALLBACKS
+	/* __ATOMIC_RELEASE memory order was used when the
+	 * call back was inserted into the list.
+	 * Since there is a clear dependency between loading
+	 * cb and cb->fn/cb->next, __ATOMIC_ACQUIRE memory order is
+	 * not required.
+	 */
 	struct rte_eth_rxtx_callback *cb = dev->pre_tx_burst_cbs[queue_id];
 
 	if (unlikely(cb != NULL)) {