Re: [PATCH RFC v2 08/15] bpf, x86: Maintain Tasks RCU trampoline nesting in the BPF trampoline

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Fri Sep 11, 2026 at 7:08 AM PDT, Josef Bacik wrote:
> Emit an increment of current->rcu_tramp_nesting once the trampoline's
> frame is set up and a decrement before the final register restore, so
> that a task preempted while running fentry/fexit/fmod_ret/LSM programs
> or the __bpf_tramp_enter()/__bpf_tramp_exit() glue is not treated as
> Tasks-RCU quiescent.  Drop the count around the call to the original
> function: that may run arbitrarily long without sleeping and must not pin
> a Tasks RCU grace period, and the trampoline frame above it is held by
> im->pcref rather than by Tasks RCU (see bpf_tramp_image_put()).  The
> fmod_ret early-exit branch and the ip_after_call -> ip_epilogue poke both
> skip the decrement/increment pair around the original call, so the count
> stays balanced on every path.
>
> The sequence is "mov r11, gs:[current_task]; inc/dec dword [r11 + off]";
> r11 is scratch at every emission point and (u32)&current_task is a valid
> sign-extended %gs-absolute with the current per-CPU layout, the same form
> the JIT already uses for this_cpu_off.  The image is dynamically
> allocated text, so the instructions outside the bracketed region are
> covered by the irq-exit IP check.
>
> Assisted-by: LLM
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>  arch/x86/net/bpf_jit_comp.c | 43 +++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 43 insertions(+)
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 2853e87797a7..a375c1b7bd50 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -722,6 +722,31 @@ static void emit_indirect_jump(u8 **pprog, int bpf_reg, u8 *ip)
>  	*pprog = prog;
>  }
>  
> +/*
> + * Tasks RCU trampoline nesting, see rcu_tasks_trampoline_enter().
> + *
> + *   mov r11, QWORD PTR gs:[current_task]
> + *   inc/dec DWORD PTR [r11 + offsetof(struct task_struct, rcu_tramp_nesting)]
> + *
> + * r11 (AUX_REG) is scratch in the trampoline at every point this is emitted.
> + */
> +static void emit_rcu_tasks_tramp_nesting(u8 **pprog, bool enter)
> +{
> +#ifdef CONFIG_TASKS_RCU
> +	u8 *prog = *pprog;
> +
> +	/* mov r11, gs:[abs32] */
> +	EMIT2(0x65, 0x4C);
> +	EMIT3(0x8B, 0x1C, 0x25);
> +	EMIT((u32)(unsigned long)&current_task, 4);
> +	/* inc/dec dword ptr [r11 + disp32] */
> +	EMIT3(0x41, 0xFF, enter ? 0x83 : 0x8B);
> +	EMIT(offsetof(struct task_struct, rcu_tramp_nesting), 4);
> +
> +	*pprog = prog;
> +#endif

It's not a lot of overhead, but I feel it will be the death by thousand cuts.
rcu_read_lock_trace() in bpf_prog_enter_sleepable is doing the same thing...
increamenting a variable inside current.
Can they be combined? Like treat current->trc_reader_nesting > 0 as
 current->rcu_tramp_nesting > 0 ?
Or replace one with the other?
Two current->foo++ operations look redundant.

bpf trampoline is already quite heavy. I'd like to find ways to reduce
its overhead instead of adding more.