Re: [PATCH] bpf: force inc_active()/dec_active() to be inline functions

[Yafang Shao]

On Sat, Jul 22, 2023 at 3:48 PM Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
>
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> Splitting these out into separate helper functions means that we
> actually pass an uninitialized variable into another function call
> if dec_active() happens to not be inlined, and CONFIG_PREEMPT_RT
> is disabled:

Do you mean that the compiler can remove the flags automatically when
dec_active() is inlined, but can't remove it automatically when
dec_active() is not inlined ?
If so, why can't we improve the compiler ?

If we have to change the kernel, what about the change below?

index 51d6389..17191ee 100644
--- a/kernel/bpf/memalloc.c
+++ b/kernel/bpf/memalloc.c
@@ -165,15 +165,17 @@ static struct mem_cgroup *get_memcg(const struct
bpf_mem_cache *c)
 #endif
 }

-static void inc_active(struct bpf_mem_cache *c, unsigned long *flags)
+static unsigned long inc_active(struct bpf_mem_cache *c)
 {
+       unsigned long flags = 0;
+
        if (IS_ENABLED(CONFIG_PREEMPT_RT))
                /* In RT irq_work runs in per-cpu kthread, so disable
                 * interrupts to avoid preemption and interrupts and
                 * reduce the chance of bpf prog executing on this cpu
                 * when active counter is busy.
                 */
-               local_irq_save(*flags);
+               local_irq_save(flags);
        /* alloc_bulk runs from irq_work which will not preempt a bpf
         * program that does unit_alloc/unit_free since IRQs are
         * disabled there. There is no race to increment 'active'
@@ -181,6 +183,7 @@ static void inc_active(struct bpf_mem_cache *c,
unsigned long *flags)
         * bpf prog preempted this loop.
         */
        WARN_ON_ONCE(local_inc_return(&c->active) != 1);
+       return flags;
 }


>
> kernel/bpf/memalloc.c: In function 'add_obj_to_free_list':
> kernel/bpf/memalloc.c:200:9: error: 'flags' is used uninitialized [-Werror=uninitialized]
>   200 |         dec_active(c, flags);
>
> Mark the two functions as __always_inline so gcc can see through
> this regardless of optimizations and other options that may
> prevent it otherwise.
>
> Fixes: 18e027b1c7c6d ("bpf: Factor out inc/dec of active flag into helpers.")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>  kernel/bpf/memalloc.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/bpf/memalloc.c b/kernel/bpf/memalloc.c
> index 51d6389e5152e..23906325298da 100644
> --- a/kernel/bpf/memalloc.c
> +++ b/kernel/bpf/memalloc.c
> @@ -165,7 +165,7 @@ static struct mem_cgroup *get_memcg(const struct bpf_mem_cache *c)
>  #endif
>  }
>
> -static void inc_active(struct bpf_mem_cache *c, unsigned long *flags)
> +static __always_inline void inc_active(struct bpf_mem_cache *c, unsigned long *flags)
>  {
>         if (IS_ENABLED(CONFIG_PREEMPT_RT))
>                 /* In RT irq_work runs in per-cpu kthread, so disable
> @@ -183,7 +183,7 @@ static void inc_active(struct bpf_mem_cache *c, unsigned long *flags)
>         WARN_ON_ONCE(local_inc_return(&c->active) != 1);
>  }
>
> -static void dec_active(struct bpf_mem_cache *c, unsigned long flags)
> +static __always_inline void dec_active(struct bpf_mem_cache *c, unsigned long flags)
>  {
>         local_dec(&c->active);
>         if (IS_ENABLED(CONFIG_PREEMPT_RT))
> --
> 2.39.2
>
>


-- 
Regards
Yafang