[PATCH] doc/eal: add caveat about spinlocks from non-pinned threads

[Mattias Rönnblom]

On 2022-06-10 17:28, Stephen Hemminger wrote:
> Need to warn users of DPDK spinlocks from non-pinned threads.
> This is similar wording to Linux documentation in pthread_spin_init.
> 
> Signed-off-by: Stephen Hemminger <stephen at networkplumber.org>
> ---
>   doc/guides/prog_guide/env_abstraction_layer.rst | 10 ++++++++++
>   1 file changed, 10 insertions(+)
> 
> diff --git a/doc/guides/prog_guide/env_abstraction_layer.rst b/doc/guides/prog_guide/env_abstraction_layer.rst
> index 5f0748fba1c0..45d3de8d84f6 100644
> --- a/doc/guides/prog_guide/env_abstraction_layer.rst
> +++ b/doc/guides/prog_guide/env_abstraction_layer.rst
> @@ -797,6 +797,16 @@ Known Issues
>   
>     The debug statistics of rte_ring, rte_mempool and rte_timer are not supported in an unregistered non-EAL pthread.
>   
> ++ locking
> +

Isn't this problem more general than locks? The use of any 
non-preemption safe data structures potentially causes such delays. 
Regular DPDK rings for sure. The lock-less stack? The hash library?

Both actual and open-coded spinlocks internal to the APIs are also very 
common.

> +  If a pthread, that is not pinned to an lcore acquires a lock such as a
> +  DPDK based lock (rte_spinlock, rte_rwlock, rte_ticketlock, rte_mcslock)
> +  then there is a possibility of large application delays.

Pinning or not doesn't matter. What matters is if the thread is 
preempted and thus is prevented from making progress for a long time.

> +  The problem is that if a thread is scheduled off the CPU while it holds
> +  a lock, then other threads will waste time spinning on the lock until
> +  the lock holder is once more rescheduled and releases the lock.
> +
> +
>   cgroup control
>   ~~~~~~~~~~~~~~
>