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McKenney" , "Frederic Weisbecker" , "Neeraj Upadhyay" , "Joel Fernandes" , "Boqun Feng" , "Thomas Gleixner" , "Peter Zijlstra" , "Steven Rostedt" , "Masami Hiramatsu" , "Mark Rutland" , "Jiri Olsa" , "Alexei Starovoitov" , "Daniel Borkmann" , "Andrii Nakryiko" , , "Catalin Marinas" , "Will Deacon" , "Puranjay Mohan" , "Xu Kuohai" Cc: "Andy Lutomirski" , "Josh Triplett" , "Uladzislau Rezki" , "Mathieu Desnoyers" , "Lai Jiangshan" , "Zqiang" , "Juergen Gross" , "Luis Chamberlain" , "Ihor Solodrai" , , , , , , Subject: Re: [PATCH RFC v2 08/15] bpf, x86: Maintain Tasks RCU trampoline nesting in the BPF trampoline X-Mailer: aerc References: <20260911-b4-rcu-tasks-preempt-qs-v2-0-eaaa61ed2da4@toxicpanda.com> <20260911-b4-rcu-tasks-preempt-qs-v2-8-eaaa61ed2da4@toxicpanda.com> In-Reply-To: <20260911-b4-rcu-tasks-preempt-qs-v2-8-eaaa61ed2da4@toxicpanda.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)¤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 > --- > 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_r= eg, u8 *ip) > *pprog =3D prog; > } > =20 > +/* > + * 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_nes= ting)] > + * > + * r11 (AUX_REG) is scratch in the trampoline at every point this is emi= tted. > + */ > +static void emit_rcu_tasks_tramp_nesting(u8 **pprog, bool enter) > +{ > +#ifdef CONFIG_TASKS_RCU > + u8 *prog =3D *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 =3D prog; > +#endif It's not a lot of overhead, but I feel it will be the death by thousand cut= s. 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.