Re: [dpdk-dev] [PATCH 1/2] librte_ethdev: Introduce a function to release HW rings

[Renata Saiakhova <renata.saiakhova@ekinops.com>]

Hi Anatoly,

thanks! The fact that memzone is found and matched only based on the name, could it create potential problem like the one I described besides HW rings?

Kind regards,
Renata
________________________________
From: Burakov, Anatoly <anatoly.burakov@intel.com>
Sent: Tuesday, May 5, 2020 12:45 PM
To: Renata Saiakhova <renata.saiakhova@ekinops.com>; dev@dpdk.org <dev@dpdk.org>
Subject: Re: [dpdk-dev] [PATCH 1/2] librte_ethdev: Introduce a function to release HW rings

On 05-May-20 11:25 AM, Burakov, Anatoly wrote:
> On 03-May-20 5:26 PM, Renata Saiakhova wrote:
>> Free previously allocated memzone for HW rings
>>
>> Signed-off-by: Renata Saiakhova <Renata.Saiakhova@ekinops.com>
>> ---
>>   lib/librte_ethdev/rte_ethdev.c           | 23 +++++++++++++++++++++++
>>   lib/librte_ethdev/rte_ethdev_driver.h    | 14 ++++++++++++++
>>   lib/librte_ethdev/rte_ethdev_version.map |  1 +
>>   3 files changed, 38 insertions(+)
>>
>> diff --git a/lib/librte_ethdev/rte_ethdev.c
>> b/lib/librte_ethdev/rte_ethdev.c
>> index 72aed59a5..c6d27e1aa 100644
>> --- a/lib/librte_ethdev/rte_ethdev.c
>> +++ b/lib/librte_ethdev/rte_ethdev.c
>> @@ -4206,6 +4206,29 @@ rte_eth_dma_zone_reserve(const struct
>> rte_eth_dev *dev, const char *ring_name,
>>               RTE_MEMZONE_IOVA_CONTIG, align);
>>   }
>> +int
>> +rte_eth_dma_zone_free(const struct rte_eth_dev *dev, const char
>> *ring_name,
>> +        uint16_t queue_id)
>> +{
>> +    char z_name[RTE_MEMZONE_NAMESIZE];
>> +    const struct rte_memzone *mz;
>> +    int rc = 0;
>> +
>> +    snprintf(z_name, sizeof(z_name), "eth_p%d_q%d_%s",
>> +            dev->data->port_id, queue_id, ring_name);
>> +    if (rc >= RTE_MEMZONE_NAMESIZE) {
>> +        RTE_ETHDEV_LOG(ERR, "ring name too long\n");
>> +        rte_errno = ENAMETOOLONG;
>> +        return NULL;
>> +    }
>> +
>> +    mz = rte_memzone_lookup(z_name);
>> +    if (mz)
>> +        rc = rte_memzone_free(mz);
>
> This is racy. Please just use rte_memzone_free() unconditionally. It'll
> return 0 if memzone existed, or will set rte_errno to EINVAL if it
> didn't. (this is suboptimal, it should be ENOENT, but changing this
> would be an API break... I'll submit a patch for future release to fix
> this)
>

My apologies, just using rte_memzone_free will not solve the problem
because you don't have memzone pointer. Now that i think of it, the
rte_eth_dma_zone_reserve() suffers from this issue too, and the problem
is lack of atomic "find or create" memzone API.

This patch is OK for now, as it follows similar code in
rte_eth_dma_zone_reserve(), but ideally, this should be fixed at the
memzone API level. I'll see if i can cobble together a quick patchset
adding atomic "find or reserve" and "find and free" operations.

--
Thanks,
Anatoly