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To: Stephen Hemminger <stephen@networkplumber.org>,
=?UTF-8?Q?Mattias_R=c3=b6nnblom?= <mattias.ronnblom@ericsson.com>
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Errors-To: dev-bounces@dpdk.org
On 4/2/22 02:50, Stephen Hemminger wrote:
> On Wed, 30 Mar 2022 16:26:02 +0200
> Mattias Rönnblom <mattias.ronnblom@ericsson.com> wrote:
>
>> + __atomic_store_n(&seqlock->sn, sn, __ATOMIC_RELAXED);
>> +
>> + /* __ATOMIC_RELEASE to prevent stores after (in program
>> order)
>> + * from happening before the sn store.
>> + */
>> + rte_atomic_thread_fence(__ATOMIC_RELEASE);
> Couldn't atomic store with __ATOMIC_RELEASE do same thing?
No, store-release wouldn't prevent later stores from moving up. It only
ensures that earlier loads and stores have completed before
store-release completes. If later stores could move before a supposed
store-release(seqlock->sn), readers could see inconsistent (torn) data
with a valid sequence number.
>> +static inline void
>> +rte_seqlock_write_end(rte_seqlock_t *seqlock)
>> +{
>> + uint32_t sn;
>> +
>> + sn = seqlock->sn + 1;
>> +
>> + /* synchronizes-with the load acquire in rte_seqlock_begin()
>> */
>> + __atomic_store_n(&seqlock->sn, sn, __ATOMIC_RELEASE);
>> +
>> + rte_spinlock_unlock(&seqlock->lock);
> Atomic store is not necessary here, the atomic operation in
> spinlock_unlock wil assure theat the seqeuence number update is
> ordered correctly.
Load-acquire(seqlock->sn) in rte_seqlock_begin() must be paired with
store-release(seqlock->sn) in rte_seqlock_write_end() or there wouldn't
exist any synchronize-with relationship. Readers don't access the spin
lock so any writer-side updates to the spin lock don't mean anything to
readers.