Re: [PATCH RFC v4 07/13] bpf, x86: Take a Tasks Trace reader in the trampoline around its call-outs

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

On Fri Sep 18, 2026 at 2:49 PM UTC, Josef Bacik wrote:
> On HAVE_RCU_TRAMPOLINE_READERS kernels Tasks RCU keeps a BPF trampoline
> image allocated only while a task using it is a Tasks Trace RCU reader
> or is executing text that rcu_tasks_trampoline_text() recognises.  The
> image itself is such text, but the C glue and the programs it calls are
> not, and only sleepable programs take rcu_read_lock_trace() today.
>
> Have the x86-64 JIT open-code rcu_read_lock_trace() and
> rcu_read_unlock_trace() in the trampoline, as ftrace_64.S does for
> ftrace_caller: one reader from just after the frame is set up to just
> before the original function is called, covering __bpf_tramp_enter()
> and the fentry and fmod_ret programs, and a second one from just after
> the original function returns to just before the final register
> restore, covering the fexit programs and __bpf_tramp_exit().  The
> original function itself runs outside both, since it may run for a long
> time and the image is pinned by im->pcref across it.  Trampolines that
> do not call the original function get a single reader around all their
> programs.  The second reader is entered before ip_after_call, so the
> ip_after_call -> ip_epilogue jump that bpf_tramp_image_put() patches in
> is inside it, and the fmod_ret early-exit branch lands after that point
> still holding the first reader, so exactly one is held on every path.
>
> The sequence uses r10 and r11, which are scratch at each emission point,
> and references current_task and rcu_tasks_trace_srcu_struct by absolute
> sign-extended address, the form the JIT already relies on for
> this_cpu_off.  Sleepable programs' own rcu_read_lock_trace() simply
> nests.  Nothing is emitted on other configurations.
>
> Suggested-by: Alexei Starovoitov <ast@kernel.org>
> Assisted-by: LLM
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>  arch/x86/net/bpf_jit_comp.c | 113 ++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 113 insertions(+)
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 2853e87797a7..c991f7ceacdf 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -14,6 +14,7 @@
>  #include <linux/memory.h>
>  #include <linux/sort.h>
>  #include <linux/execmem.h>
> +#include <linux/rcupdate_trace.h>
>  #include <asm/extable.h>
>  #include <asm/ftrace.h>
>  #include <asm/set_memory.h>
> @@ -722,6 +723,97 @@ static void emit_indirect_jump(u8 **pprog, int bpf_reg, u8 *ip)
>  	*pprog = prog;
>  }
>  
> +/*
> + * Open-coded rcu_read_lock_trace() / rcu_read_unlock_trace() for the
> + * trampoline, see CONFIG_HAVE_RCU_TRAMPOLINE_READERS and the equivalent
> + * macros in arch/x86/kernel/ftrace_64.S.  The image is not relocated, so
> + * current_task and rcu_tasks_trace_srcu_struct are referenced by absolute
> + * (sign-extended 32-bit) address, the form the JIT already relies on for
> + * this_cpu_off.  Uses r10 and r11, which are scratch at every emission
> + * point, and clobbers flags.
> + *
> + * lock:                                  unlock:
> + *   mov  r11, gs:[current_task]            mov  r11, gs:[current_task]
> + *   mov  r10d, [r11+nesting]               mov  r10d, [r11+nesting]
> + *   inc  dword ptr [r11+nesting]           sub  r10d, 1
> + *   test r10d, r10d                        jnz  2f
> + *   jnz  1f                                mov  r10, [r11+scp]
> + *   mov  r10, [&srcu.srcu_ctrp]            mov  dword ptr [r11+nesting], 0
> + *   inc  qword ptr gs:[r10+locks]          (smp_mb)
> + *   mov  [r11+scp], r10                    inc  qword ptr gs:[r10+unlocks]
> + *   (smp_mb)                               jmp  3f
> + * 1:                                     2: mov [r11+nesting], r10d
> + *                                        3:
> + */
> +static void emit_trace_rcu_reader(u8 **pprog, bool lock)
> +{
> +#ifdef CONFIG_TASKS_RCU_TRAMPOLINE_READERS
> +	const u32 nesting = offsetof(struct task_struct, trc_reader_nesting);
> +	const u32 scp = offsetof(struct task_struct, trc_reader_scp);

nesting_off ?
scp_off ?
Otherwise
EMIT(nesting, 4);
is a bit confusing.

> +	const bool mb = !IS_ENABLED(CONFIG_TASKS_TRACE_RCU_NO_MB);
> +	u8 *prog = *pprog;
> +
> +	BUILD_BUG_ON(IS_ENABLED(CONFIG_NEED_SRCU_NMI_SAFE));
> +	BUILD_BUG_ON(offsetof(struct srcu_ctr, srcu_locks) != 0);
> +	BUILD_BUG_ON(offsetof(struct srcu_ctr, srcu_unlocks) != 8);

Looks too hardcoded here.
Why not to use the same approach as with offsetof() few lines above?

Overall looks ok,
but I wonder what Paul will say that rcu_read_lock_tasks_trace()
becomes baked in into JITs and will be pretty hard to change.

We also lose rcu_tt lockdep runtime checks.
So lockdep might get confused?