Hi Morten,
On 2024/4/11 20:44, Morten Brørup wrote:
>> From: Chengwen Feng [mailto:fengcheng...@huawei.com]
>> Sent: Thursday, 11 April 2024 14.04
>>
>> Fix a problem introduced by a compiler upgrade (from gcc10 to gcc12.3),
>> which will lead the hns3 NIC can't link up. The root cause is strict
>> aliasing violation in rte_eth_linkstatus_set() with hns3 driver, see
>> [1] for more details.
>>
>> This commit use union to avoid such aliasing violation.
>>
>> [1] Strict aliasing problem with rte_eth_linkstatus_set()
>> https://marc.info/?l=dpdk-dev&m=171274148514777&w=3
>>
>> Cc: sta...@dpdk.org
>>
>> Signed-off-by: Chengwen Feng <fengcheng...@huawei.com>
>> Signed-off-by: Dengdui Huang <huangdeng...@huawei.com>
>>
>> ---
>> v3: fix checkpatch warning "missing --in-reply-to".
>> v2: add RTE_ATOMIC(uint64_t) wrap which address Morten's comment.
>>
>> ---
>> lib/ethdev/ethdev_driver.h | 23 +++++++----------------
>> lib/ethdev/rte_ethdev.h | 16 ++++++++++------
>> 2 files changed, 17 insertions(+), 22 deletions(-)
>>
>> diff --git a/lib/ethdev/ethdev_driver.h b/lib/ethdev/ethdev_driver.h
>> index 0dbf2dd6a2..9d831d5c84 100644
>> --- a/lib/ethdev/ethdev_driver.h
>> +++ b/lib/ethdev/ethdev_driver.h
>> @@ -1674,18 +1674,13 @@ static inline int
>> rte_eth_linkstatus_set(struct rte_eth_dev *dev,
>> const struct rte_eth_link *new_link)
>> {
>> - RTE_ATOMIC(uint64_t) *dev_link = (uint64_t __rte_atomic *)&(dev-
>>> data->dev_link);
>> - union {
>> - uint64_t val64;
>> - struct rte_eth_link link;
>> - } orig;
>> -
>> - RTE_BUILD_BUG_ON(sizeof(*new_link) != sizeof(uint64_t));
>> + struct rte_eth_link old_link;
>>
>> - orig.val64 = rte_atomic_exchange_explicit(dev_link, *(const
>> uint64_t *)new_link,
>> - rte_memory_order_seq_cst);
>> + old_link.val64 = rte_atomic_exchange_explicit(&dev->data-
>>> dev_link.val64,
>
> You are right; old_link has local scope and is on the stack, so atomic store
> is not required.
>
> And since rte_eth_linkstatus_set() is an internal function called from the
> driver only, it is probably safe to assume that *new_link is on the caller's
> stack and doesn't change while being accessed by this function.
> I guess that new_link is passed by reference for performance and
> future-proofing reasons; it could have been passed by value instead. If it
> was passed by value, atomic access would certainly not be required.
> In other words: You are right here too; new_link does not require atomic load.
>
>> + new_link->val64,
>> + rte_memory_order_seq_cst);
>>
>> - return (orig.link.link_status == new_link->link_status) ? -1 : 0;
>> + return (old_link.link_status == new_link->link_status) ? -1 : 0;
>> }
>>
>> /**
>> @@ -1701,12 +1696,8 @@ static inline void
>> rte_eth_linkstatus_get(const struct rte_eth_dev *dev,
>> struct rte_eth_link *link)
>> {
>> - RTE_ATOMIC(uint64_t) *src = (uint64_t __rte_atomic *)&(dev->data-
>>> dev_link);
>> - uint64_t *dst = (uint64_t *)link;
>> -
>> - RTE_BUILD_BUG_ON(sizeof(*link) != sizeof(uint64_t));
>> -
>> - *dst = rte_atomic_load_explicit(src, rte_memory_order_seq_cst);
>> + link->val64 = rte_atomic_load_explicit(&dev->data->dev_link.val64,
>> + rte_memory_order_seq_cst);
>
> It is not safe to assume that the link pointer points to local memory on the
> caller's stack.
> The link pointer might point to a shared memory area, used by multiple
> threads/processes, so it needs to be stored atomically using
> rte_atomic_store_explicit(&link->val64, ..., rte_memory_order_seq_cst).
I checked every call of rte_eth_linkstatus_get in DPDK, and all the link
parameters are local variables.
The dev->data->dev_link is placed in shared memory which could access from
different threads/processes, it seems no need maintain another link struct
which act the same role.
So I think we should keep current impl, and not using
rte_atomic_store_explicit(&link->val64,...
Thanks
>
>> }
>>
>> /**
>> diff --git a/lib/ethdev/rte_ethdev.h b/lib/ethdev/rte_ethdev.h
>> index 147257d6a2..ccf43e468a 100644
>> --- a/lib/ethdev/rte_ethdev.h
>> +++ b/lib/ethdev/rte_ethdev.h
>> @@ -332,12 +332,16 @@ struct rte_eth_stats {
>> /**
>> * A structure used to retrieve link-level information of an Ethernet
>> port.
>> */
>> -__extension__
>> -struct __rte_aligned(8) rte_eth_link { /**< aligned for atomic64
>> read/write */
>> - uint32_t link_speed; /**< RTE_ETH_SPEED_NUM_ */
>> - uint16_t link_duplex : 1; /**< RTE_ETH_LINK_[HALF/FULL]_DUPLEX
>> */
>> - uint16_t link_autoneg : 1; /**< RTE_ETH_LINK_[AUTONEG/FIXED] */
>> - uint16_t link_status : 1; /**< RTE_ETH_LINK_[DOWN/UP] */
>> +struct rte_eth_link {
>> + union {
>> + RTE_ATOMIC(uint64_t) val64; /**< used for atomic64
>> read/write */
>> + struct {
>> + uint32_t link_speed; /**< RTE_ETH_SPEED_NUM_
>> */
>> + uint16_t link_duplex : 1; /**<
>> RTE_ETH_LINK_[HALF/FULL]_DUPLEX */
>> + uint16_t link_autoneg : 1; /**<
>> RTE_ETH_LINK_[AUTONEG/FIXED] */
>> + uint16_t link_status : 1; /**<
>> RTE_ETH_LINK_[DOWN/UP] */
>> + };
>> + };
>> };
>>
>> /**@{@name Link negotiation
>> --
>> 2.17.1
>
> .
>
