Re: [dpdk-dev] [PATCH] test/mcslock: remove unneeded per-lcore copy

[Olivier Matz <olivier.matz@6wind.com>]

Hi Honnappa,

On Wed, Nov 04, 2020 at 05:57:19PM +0000, Honnappa Nagarahalli wrote:
> <snip>
> 
> > 
> > Each core already comes with its local storage for mcslock (in its stack),
> > therefore there is no need to define an additional per-lcore mcslock.
> > 
> > Fixes: 32dcb9fd2a22 ("test/mcslock: add MCS queued lock unit test")
> > 
> > Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
> > ---
> >  app/test/test_mcslock.c | 16 ++++++----------
> >  1 file changed, 6 insertions(+), 10 deletions(-)
> > 
> > diff --git a/app/test/test_mcslock.c b/app/test/test_mcslock.c index
> > fbca78707d..80eaecc90a 100644
> > --- a/app/test/test_mcslock.c
> > +++ b/app/test/test_mcslock.c
> > @@ -37,10 +37,6 @@
> >   *   lock multiple times.
> >   */
> > 
> > -RTE_DEFINE_PER_LCORE(rte_mcslock_t, _ml_me); -
> > RTE_DEFINE_PER_LCORE(rte_mcslock_t, _ml_try_me); -
> > RTE_DEFINE_PER_LCORE(rte_mcslock_t, _ml_perf_me);
> > -
> >  rte_mcslock_t *p_ml;
> >  rte_mcslock_t *p_ml_try;
> >  rte_mcslock_t *p_ml_perf;
> > @@ -53,7 +49,7 @@ static int
> >  test_mcslock_per_core(__rte_unused void *arg)  {
> >  	/* Per core me node. */
> > -	rte_mcslock_t ml_me = RTE_PER_LCORE(_ml_me);
> > +	rte_mcslock_t ml_me;
> These variables are modified by other threads. IMO, it is better to keep them global (and not on the stack). From that perspective, I think we should be taking the address of the per lcore variable. For ex:
> rte_mcslock_t *ml_me = &RTE_PER_LCORE(_ml_me);

In my understanding, the only case where another thread modifies our
local variable is when the other thread releases the lock we are waiting
for.  I can't see how it could cause an issue to have the locks in the
stack. Am I missing something?

Thanks,
Olivier


> 
> > 
> >  	rte_mcslock_lock(&p_ml, &ml_me);
> >  	printf("MCS lock taken on core %u\n", rte_lcore_id()); @@ -77,7
> > +73,7 @@ load_loop_fn(void *func_param)
> >  	const unsigned int lcore = rte_lcore_id();
> > 
> >  	/**< Per core me node. */
> > -	rte_mcslock_t ml_perf_me = RTE_PER_LCORE(_ml_perf_me);
> > +	rte_mcslock_t ml_perf_me;
> > 
> >  	/* wait synchro */
> >  	while (rte_atomic32_read(&synchro) == 0) @@ -151,8 +147,8 @@
> > static int  test_mcslock_try(__rte_unused void *arg)  {
> >  	/**< Per core me node. */
> > -	rte_mcslock_t ml_me     = RTE_PER_LCORE(_ml_me);
> > -	rte_mcslock_t ml_try_me = RTE_PER_LCORE(_ml_try_me);
> > +	rte_mcslock_t ml_me;
> > +	rte_mcslock_t ml_try_me;
> > 
> >  	/* Locked ml_try in the main lcore, so it should fail
> >  	 * when trying to lock it in the worker lcore.
> > @@ -178,8 +174,8 @@ test_mcslock(void)
> >  	int i;
> > 
> >  	/* Define per core me node. */
> > -	rte_mcslock_t ml_me     = RTE_PER_LCORE(_ml_me);
> > -	rte_mcslock_t ml_try_me = RTE_PER_LCORE(_ml_try_me);
> > +	rte_mcslock_t ml_me;
> > +	rte_mcslock_t ml_try_me;
> > 
> >  	/*
> >  	 * Test mcs lock & unlock on each core
> > --
> > 2.25.1
>