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

["Paul E. McKenney" <paulmck@xxxxxxxxxx>]

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)?

But if this means reverting the re-implementation of RCU Tasks Trace in
terms of SRCU, I will be rather annoyed with myself.  ;-)

                                                        Thanx, Paul