[PATCH v5] eal: add seqlock

[fengchengwen]

On 2022/5/1 22:03, Mattias Rönnblom wrote:
> A sequence lock (seqlock) is synchronization primitive which allows
> for data-race free, low-overhead, high-frequency reads, especially for
> data structures shared across many cores and which are updated
> relatively infrequently.
> 

...

> +}
> +
> +static void
> +reader_stop(struct reader *reader)
> +{
> +	__atomic_store_n(&reader->stop, 1, __ATOMIC_RELAXED);
> +}
> +
> +#define NUM_WRITERS (2) /* main lcore + one worker */
> +#define MIN_NUM_READERS (2)
> +#define MAX_READERS (RTE_MAX_LCORE - NUM_WRITERS - 1)

Why minus 1 ?
Suggest define MAX_READERS RTE_MAX_LCORE to avoid underflow with small size VM.

> +#define MIN_LCORE_COUNT (NUM_WRITERS + MIN_NUM_READERS)
> +
> +/* Only a compile-time test */
> +static rte_seqlock_t __rte_unused static_init_lock = RTE_SEQLOCK_INITIALIZER;
> +
> +static int
> +test_seqlock(void)
> +{
> +	struct reader readers[MAX_READERS];
> +	unsigned int num_readers;
> +	unsigned int num_lcores;
> +	unsigned int i;
> +	unsigned int lcore_id;
> +	unsigned int reader_lcore_ids[MAX_READERS];
> +	unsigned int worker_writer_lcore_id = 0;
> +	int rc = TEST_SUCCESS;
> +
> +	num_lcores = rte_lcore_count();
> +
> +	if (num_lcores < MIN_LCORE_COUNT) {
> +		printf("Too few cores to run test. Skipping.\n");
> +		return TEST_SKIPPED;
> +	}
> +
> +	num_readers = num_lcores - NUM_WRITERS;
> +
> +	struct data *data = rte_zmalloc(NULL, sizeof(struct data), 0);

Please check whether the value of data is NULL.

> +
> +	i = 0;
> +	RTE_LCORE_FOREACH_WORKER(lcore_id) {
> +		if (i == 0) {
> +			rte_eal_remote_launch(writer_run, data, lcore_id);
> +			worker_writer_lcore_id = lcore_id;
> +		} else {
> +			unsigned int reader_idx = i - 1;
> +			struct reader *reader = &readers[reader_idx];
> +
> +			reader->data = data;
> +			reader->stop = 0;
> +
> +			rte_eal_remote_launch(reader_run, reader, lcore_id);
> +			reader_lcore_ids[reader_idx] = lcore_id;
> +		}
> +		i++;
> +	}
> +
> +	if (writer_run(data) != 0 ||
> +	    rte_eal_wait_lcore(worker_writer_lcore_id) != 0)
> +		rc = TEST_FAILED;
> +
> +	for (i = 0; i < num_readers; i++) {
> +		reader_stop(&readers[i]);
> +		if (rte_eal_wait_lcore(reader_lcore_ids[i]) != 0)
> +			rc = TEST_FAILED;
> +	}
> +

Please free data memory.

> +	return rc;
> +}
> +
> +REGISTER_TEST_COMMAND(seqlock_autotest, test_seqlock);
> diff --git a/doc/api/doxy-api-index.md b/doc/api/doxy-api-index.md
> index 4245b9635c..f23e33ae30 100644
> --- a/doc/api/doxy-api-index.md
> +++ b/doc/api/doxy-api-index.md
> @@ -77,6 +77,7 @@ The public API headers are grouped by topics:
>    [rwlock]             (@ref rte_rwlock.h),
>    [spinlock]           (@ref rte_spinlock.h),
>    [ticketlock]         (@ref rte_ticketlock.h),
> +  [seqlock]            (@ref rte_seqlock.h),
>    [RCU]                (@ref rte_rcu_qsbr.h)
>  

...

> + */
> +__rte_experimental
> +static inline bool
> +rte_seqlock_read_retry(const rte_seqlock_t *seqlock, uint32_t begin_sn)
> +{
> +	uint32_t end_sn;
> +
> +	/* An odd sequence number means the protected data was being
> +	 * modified already at the point of the rte_seqlock_read_begin()
> +	 * call.
> +	 */
> +	if (unlikely(begin_sn & 1))
> +		return true;
> +
> +	/* make sure the data loads happens before the sn load */
> +	rte_atomic_thread_fence(__ATOMIC_ACQUIRE);

In ARMv8, the rte_atomic_thread_fence(__ATOMIC_ACQUIRE) and rte_smp_rmb() both output 'dma ishld'
Suggest use rte_smp_rmb(), please see below comment.

> +
> +	end_sn = __atomic_load_n(&seqlock->sn, __ATOMIC_RELAXED);
> +
> +	/* A writer incremented the sequence number during this read
> +	 * critical section.
> +	 */
> +	if (unlikely(begin_sn != end_sn))
> +		return true;
> +
> +	return false;
> +}
> +
> +/**
> + * @warning
> + * @b EXPERIMENTAL: this API may change without prior notice.
> + *
> + * Begin a write-side critical section.
> + *
> + * A call to this function acquires the write lock associated @p
> + * seqlock, and marks the beginning of a write-side critical section.
> + *
> + * After having called this function, the caller may go on to modify
> + * (both read and write) the protected data, in an atomic or
> + * non-atomic manner.
> + *
> + * After the necessary updates have been performed, the application
> + * calls rte_seqlock_write_unlock().
> + *
> + * This function is not preemption-safe in the sense that preemption
> + * of the calling thread may block reader progress until the writer
> + * thread is rescheduled.
> + *
> + * Unlike rte_seqlock_read_begin(), each call made to
> + * rte_seqlock_write_lock() must be matched with an unlock call.
> + *
> + * @param seqlock
> + *   A pointer to the seqlock.
> + *
> + * @see rte_seqlock_write_unlock()
> + */
> +__rte_experimental
> +static inline void
> +rte_seqlock_write_lock(rte_seqlock_t *seqlock)
> +{
> +	uint32_t sn;
> +
> +	/* to synchronize with other writers */
> +	rte_spinlock_lock(&seqlock->lock);
> +
> +	sn = seqlock->sn + 1;
> +
> +	__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);

In ARMv8, rte_atomic_thread_fence(__ATOMIC_RELEASE) will output 'dmb ish', and
rte_smp_wmb() will output 'dma ishst'.
Suggest use rte_smp_wmb(). I think here only need to use store mb here.

> +}
> +
> +/**
> + * @warning
> + * @b EXPERIMENTAL: this API may change without prior notice.
> + *
> + * End a write-side critical section.
> + *
> + * A call to this function marks the end of the write-side critical
> + * section, for @p seqlock. After this call has been made, the protected
> + * data may no longer be modified.
> + *
> + * @param seqlock
> + *   A pointer to the seqlock.
> + *
> + * @see rte_seqlock_write_lock()
> + */
> +__rte_experimental
> +static inline void
> +rte_seqlock_write_unlock(rte_seqlock_t *seqlock)
> +{
> +	uint32_t sn;
> +
> +	sn = seqlock->sn + 1;
> +
> +	/* synchronizes-with the load acquire in rte_seqlock_read_begin() */
> +	__atomic_store_n(&seqlock->sn, sn, __ATOMIC_RELEASE);
> +
> +	rte_spinlock_unlock(&seqlock->lock);
> +}
> +
> +#ifdef __cplusplus
> +}
> +#endif
> +
> +#endif  /* _RTE_SEQLOCK_H_ */
> diff --git a/lib/eal/version.map b/lib/eal/version.map
> index b53eeb30d7..4a9d0ed899 100644
> --- a/lib/eal/version.map
> +++ b/lib/eal/version.map
> @@ -420,6 +420,9 @@ EXPERIMENTAL {
>  	rte_intr_instance_free;
>  	rte_intr_type_get;
>  	rte_intr_type_set;
> +
> +	# added in 22.07
> +	rte_seqlock_init;
>  };
>  
>  INTERNAL {
> 

Reviewed-by: Chengwen Feng <fengchengwen at huawei.com>