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

["Alexei Starovoitov" <alexei.starovoitov@xxxxxxxxx>]

On Fri Sep 11, 2026 at 10:10 PM PDT, Paul E. McKenney wrote:
> On Fri, Sep 11, 2026 at 08:27:56PM -0700, Alexei Starovoitov wrote:
>> 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@xxxxxxxxxxxxxx>
>> > ---
>> >  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.
>
> Replace rcu_read_lock_trace() with Josef's rcu_tasks_trampoline_enter)?

If necessary...
what I don't understand why we need another rcu_tasks_trampoline_enter-like 
counter.
Can existing rcu_read_lock_trace() current be used ?
It's already doing current->trc_reader_nesting++
so use that as a signal ?