From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-ot1-f54.google.com (mail-ot1-f54.google.com [209.85.210.54]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2FD033914EB for ; Sat, 12 Sep 2026 03:28:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.210.54 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789183681; cv=none; b=aV6ApRztqpcxkxSlEZ9eLpwGY4m8FuWMLvua9+S02e3xrfw2SUd3hJO2JBKrj3yzsf72CAMEDgYH6RPY/EfEdBASKyarXEhaoITxUVh/5F4+s6CMokanupu7vJCSM4icRO/2BnelUiMbulRRGdbeYdiDxHC24auL+yCRJvQMTaQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789183681; c=relaxed/simple; bh=evOAXfBovvBe6YEwcJhGdatosPUGmIApLgiwGdgXRZ4=; h=Mime-Version:Content-Type:Date:Message-Id:From:To:Cc:Subject: References:In-Reply-To; b=I0HnoYMPk2WIeCd1G1IpsoeR+xc1T5bsZRZb4sn4eDCCwCR2FMJYhfJrBIwBctn/p8Zt+V5322ITLV1hNUm0vtRnDPqHHpNUTgLdtzrJRWbK5gWahh1T8M3dmHBhSikOtHfs+ikGOMiiGoMHU51g93uM881iuhyhOt9PmlVJt4M= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=WbAxp6GY; arc=none smtp.client-ip=209.85.210.54 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="WbAxp6GY" Received: by mail-ot1-f54.google.com with SMTP id 46e09a7af769-7f4e729368fso1719810a34.0 for ; Fri, 11 Sep 2026 20:27:59 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20251104; t=1789183679; x=1789788479; darn=vger.kernel.org; h=in-reply-to:references:subject:cc:to:from:message-id:date :content-type:content-transfer-encoding:mime-version:from:to:cc :subject:date:message-id:reply-to:content-type; bh=ud1ZpqadGMe+QubhUEhtcVCLKFg2sCSQ6NPZYlf26Tg=; b=WbAxp6GYqCCXfFThYBrGdLtLLoMXTZMpTlNWKcBiUm/ojmzaQnTyDTmr7hDWQ1tdno 9no58945yT19pjlKp9GDl76DZvF6LzrI3ezHbLg1qrhPacZA2VJHmrheC1CkQ8xBJh5U akA4C43cVuZSeuYRki3HangHKoSyaJwoQlQ8JOIhDz4ePUhk4OgMXh1b5GKWDp5aq3bR ym+yxeGoDTHX9RLUGApXQcvhZ1uEmjC7PZAs8q3GGC0H5i5wFlxSfL8ScuZ4i+vk+30H pKOcES1JikouMIn7XNGIWwm8PEWzDOks3dUfDdQCmzJEdwx7y9O2GZlmA8sRZIClTW7d Pjsg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1789183679; x=1789788479; h=in-reply-to:references:subject:cc:to:from:message-id:date :content-type:content-transfer-encoding:mime-version:x-gm-gg :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to :content-type; bh=ud1ZpqadGMe+QubhUEhtcVCLKFg2sCSQ6NPZYlf26Tg=; b=ORrRx28S9UZT+CEsh9FgkzJiF9OTJBzud7andRHWO5Wk6f8uLnhcOkuDKdk5qS4TsG LIxcB3X51HVHMqrNzE/vEqFgWlyoTaRQBcnafdNz6Isi82JKv9q5gb5t5PR49dxaKesl 8mQjBqzdUjsjpw+1HVNxIwCBjdOB7rbXb08wTlmUnNlodIDUMiK7j55kX2w+CgSaxbD+ GhVAWARVItBQC6L5atZOe0Nmm+zttcgNAHRTJtinV2TzAY/88nP4UNLXnhhmJFL9w/56 BI/vF0oQexdSsekrZ/cAY9EkYBQcFlcdBuzJJvi75COtV4KQz80aEzhdRgP1BPTzuMGR nNAg== X-Forwarded-Encrypted: i=1; AKwUvBwVHzYZCuVexPHwdG3/efEJsgg08ZbDEv2Hxe4X9y3Jl3QKoIuVKqp87kdp8sjqz+FnnGOPEop21P8afk4=@vger.kernel.org X-Gm-Message-State: AFuF++kx4AcvWB6ZDesdLfML3KF5nS2yeaCcGd/xGCvScj5tmSDpM+N0 jTcPif4Rl8xc66DiBekjYq5n21HJwE4kyVcbHgPa5rEaBoyqE8LtNw+B X-Gm-Gg: AYBFou0uSiTdlNSQt7bJGFtiOTPXx5zl0hhWYc3VT/vmo9XyeiPAWWOMphkcS/hhuCn vU82l1MZHx9/XitPe3OpJXTbm91aF/SMhh03875LMBnDpZcYoFSfIMtt35fL/nyONhy2s4hbV27 Jsix7OYZf8ou4HIG2kh30SaIILH0JZzyWuNtAUNzGl1j8I30CNBALIiKSVxPzfdiXdzdsQNBp11 S/SsuX9n/AcfisKrAPbbDIq5CoFcOK7Bu4kmFJt7iqmQwoS5o80BCzKHCpa7i6BqbF14I7jsYrq dWKoMzmxV8zmHC3RRTi8imFu/lwCGLCqvS9UimHXhwluyJWyYst16miqMpxqBwS0GcLDnSb7U1q 0WsVxHIo0pSYy3mzwz3dbjZvhirPMTvtLcw+EjLIMn12Qi0webg9wRR04lBEyjPdwQ4zTqZWY1G YMyy5rD7lAqVVAuIfx7Ebxbd79yPC8OKOplgjeotkzWL8m9+VqESqpRXt5JYvQYwyZiOaSv8k8e HBACvmQCiwZ8MfHK5UKZ2Bzb73BpuXJrb/0wXSbh/UD1W5uIz2/FW0djcuBshK+vg== X-Received: by 2002:a05:6830:2786:b0:7fa:ac4f:785 with SMTP id 46e09a7af769-80400b2042dmr6714669a34.17.1789183678934; Fri, 11 Sep 2026 20:27:58 -0700 (PDT) Received: from localhost ([2a03:2880:10ff:15::]) by smtp.gmail.com with ESMTPSA id 46e09a7af769-803f6d208f5sm4822052a34.25.2026.09.11.20.27.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 11 Sep 2026 20:27:58 -0700 (PDT) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Date: Fri, 11 Sep 2026 20:27:56 -0700 Message-Id: From: "Alexei Starovoitov" To: "Josef Bacik" , "Paul E. 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.