Re: [dpdk-dev] [PATCH] hash: document breakage with multi-writer thread

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Subject: [PATCH] hash: document breakage with multi-writer
thread

The code in rte_cuckoo_hash multi-writer support is broken if
write operations are called from a non-EAL thread.

rte_lcore_id() wil return LCORE_ID_ANY (UINT32_MAX) for non EAL
thread and that leads to using wrong local cache.

Add error checks and document the restriction.

Having multiple non-EAL writer threads is a valid use case. Should
we fix the

issue instead?

Discovered this the hard way...

Fixing is non-trivial. Basically, the local cache has to be take out
and that leads to having to do real locking or atomic operations.

Looking at rte_hash_create function:

        if (params->extra_flag &
RTE_HASH_EXTRA_FLAGS_MULTI_WRITER_ADD) {
                use_local_cache = 1;
                writer_takes_lock = 1;
        }

The writer locks are in place already. The code to handle the case
when local cache is taken out is also there.
What we need is another input flag that says 'multi writer + non-eal

threads'

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which would set 'use_local_cache = 0' and 'writer_takes_lock = 1'.
Not sure, it would be valuable addition. But looks like this is what
you were expecting when you had enabled
'RTE_HASH_EXTRA_FLAGS_MULTI_WRITER_ADD'. Many other APIs in DPDK

do

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not provide this kind of MT safety.

[Wang, Yipeng]
If possible, we can try to not add new flags, because there are already a lot of
flag options.
How about in the code, we check if the writer is a non-eal or not by checking
the rte_lcore_id, and operate on the global queue?
Could this work?
If(h->use_local_cache) {
	lcore_id = rte_lcore_id();
	if(lcore_id == LCORE_ID_ANY) {   // this is non-eal threads
		<call rte_ring_mp/mc_* to directly operate on global queue>
	}
	Else {
		<original path>
	}
}

The other thing I wanted to do was saving on the memory allocated for the local cache when the writers are non-eal threads. Without knowing the kind of threads upfront, we might have to create the local cache when a writer adds the entry first time.