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
