Re: [PATCH] linux/mem: atomically prefault hugepages in alloc_seg

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 20 May 2026 14:57:56 +0200
Michal Sieron <michal.sieron@nokia.com> wrote:

> In rare cases, when a secondary process calls rte_eal_init() it can
> cause a data race during page prefaulting in alloc_seg().
> 
> An atomic compare-exchange in a loop should eliminate the data race.
> 
> Signed-off-by: Michal Sieron <michal.sieron@nokia.com>
> ---
>  lib/eal/linux/eal_memalloc.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/eal/linux/eal_memalloc.c b/lib/eal/linux/eal_memalloc.c
> index a39bc31c7b..cb92fda2e8 100644
> --- a/lib/eal/linux/eal_memalloc.c
> +++ b/lib/eal/linux/eal_memalloc.c
> @@ -30,6 +30,7 @@
>  #include <rte_eal.h>
>  #include <rte_memory.h>
>  #include <rte_cycles.h>
> +#include <rte_atomic.h>
>  
>  #include "eal_filesystem.h"
>  #include "eal_internal_cfg.h"
> @@ -600,7 +601,9 @@ alloc_seg(struct rte_memseg *ms, void *addr, int socket_id,
>  	 * that is already there, so read the old value, and write itback.
>  	 * kernel populates the page with zeroes initially.
>  	 */
> -	*(volatile int *)addr = *(volatile int *)addr;
> +	int snapshot = *(volatile int *)addr;
> +	while (!rte_atomic_compare_exchange_strong((volatile int *)addr, &snapshot, snapshot))
> +		;
>  
>  	iova = rte_mem_virt2iova(addr);
>  	if (iova == RTE_BAD_PHYS_ADDR) {

No don't use a loop with compare_exchange_strong here.
It could get stuck.
Should just a an relaxed load be enough to get the page in?