[PATCH] ethdev: fix strict aliasing lead to link cannot be up

[fengchengwen]

Hi Morten,

On 2024/4/11 14:58, Morten Brørup wrote:
>> From: Chengwen Feng [mailto:fengchengwen at huawei.com]
>> Sent: Thursday, 11 April 2024 05.08
>>
>> 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: stable at dpdk.org
>>
>> Signed-off-by: Chengwen Feng <fengchengwen at huawei.com>
>> Signed-off-by: Dengdui Huang <huangdengdui at huawei.com>
>> ---
> 
> The patch mixes atomic and non-atomic access.
> This is not new for DPDK, which used to rely on compiler built-in atomics.
> 
> I'm not sure it needs to be changed, but my suggestion is inline below.
> I don't think it makes any practical different for 64 bit arch, but it might for 32 bit arch.
> 
>>  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,
> 
> old_link.val64 should be written using:
> rte_atomic_store_explicit(&old_link.val64, ..., rte_memory_order_seq_cst)

I'm afraid I don't agree this, the &dev->data->dev_link.val64 should use atomic not the stack variable old_link.

> 
>> +						      new_link->val64,
> 
> new_link->val64 should be read using:
> rte_atomic_load_explicit(&new_link->val64, rte_memory_order_seq_cst)

The same reason with above.

> 
>> +						      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,
> 
> link->val64 should be written using:
> rte_atomic_store_explicit(&link->val64, ..., rte_memory_order_seq_cst)

The same reason with above, the &dev->data->dev_link.val64 should use atomic not the stack variable link.

> 
>> +					       rte_memory_order_seq_cst);
>>  }
>>
>>  /**
>> diff --git a/lib/ethdev/rte_ethdev.h b/lib/ethdev/rte_ethdev.h
>> index 147257d6a2..0b5d3d2318 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 {
>> +		uint64_t val64; /**< used for atomic64 read/write */
> 
> The type of val64 should be:
> RTE_ATOMIC(uint64_t)

ack

Plus: yes, this patch mixes atomic and non-atomic access, but the main reason
is that we want to simplify the implementation. If we want to separate it clearly,
maybe we should defined as this:
    struct rte_eth_link {
        union {
            RTE_ATOMIC(uint64_t) atomic64; /**< used for atomic64 read/write */
            struct {
                uint64_t val64;
            };
            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] */
            };
        };
    };

Thanks

> 
>> +		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
> 
> .
>