rcu: remove VLAs

There are two lines that were using VLAs, which are not supported by
MSVC.

1)
../lib/rcu/rte_rcu_qsbr.c:326:12: warning: variable length array used [-Wvla]
  326 |         char data[dq->esize];
      |                   ^~~~~~~~~
2)
../lib/rcu/rte_rcu_qsbr.c:389:12: warning: variable length array used [-Wvla]
  389 |         char data[dq->esize];
      |                   ^~~~~~~~~

The short-term fix is to use alloca, to allow progress with the msvc
compatibility work.
The long-term plan involves API changes and therefore can only be applied
with a new release. This long-term plan consists of introducing some
reasonable limitation on RCU DQ element size.

Signed-off-by: Andre Muezerie <[email protected]>

Author: Andre Muezerie

lib/rcu/meson.build

@@ -11,10 +11,3 @@ sources = files('rte_rcu_qsbr.c')
 headers = files('rte_rcu_qsbr.h')
 
 deps += ['ring']
-
-# FIXME: this library was enabled for mingw target (a Windows target).
-# Relying on no_wvla_cflag would trigger a build error until the VLA in rte_rcu_qsbr.c is removed.
-# Disable the warning here for now.
-if cc.has_argument('-Wvla')
-    cflags += '-Wno-vla'
-endif

lib/rcu/rte_rcu_qsbr.c

@@ -331,7 +331,7 @@ int rte_rcu_qsbr_dq_enqueue(struct rte_rcu_qsbr_dq *dq, void *e)
 		return 1;
 	}
 
-	char data[dq->esize];
+	char *data = alloca(dq->esize);
 	dq_elem = (__rte_rcu_qsbr_dq_elem_t *)data;
 	/* Start the grace period */
 	dq_elem->token = rte_rcu_qsbr_start(dq->v);
@@ -395,10 +395,10 @@ rte_rcu_qsbr_dq_reclaim(struct rte_rcu_qsbr_dq *dq, unsigned int n,
 
 	cnt = 0;
 
-	char data[dq->esize];
+	char *data = alloca(dq->esize);
 	/* Check reader threads quiescent state and reclaim resources */
 	while (cnt < n &&
-		rte_ring_dequeue_bulk_elem_start(dq->r, &data,
+		rte_ring_dequeue_bulk_elem_start(dq->r, data,
 					dq->esize, 1, available) != 0) {
 		dq_elem = (__rte_rcu_qsbr_dq_elem_t *)data;