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)¤t_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 <[email protected]> >> > --- >> > 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)¤t_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 ?
