From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.9 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8C2C4C19759 for ; Thu, 1 Aug 2019 18:14:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5374020644 for ; Thu, 1 Aug 2019 18:14:24 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="fO4UiudJ" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388159AbfHASOX (ORCPT ); Thu, 1 Aug 2019 14:14:23 -0400 Received: from mail-pg1-f194.google.com ([209.85.215.194]:43984 "EHLO mail-pg1-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388140AbfHASOV (ORCPT ); Thu, 1 Aug 2019 14:14:21 -0400 Received: by mail-pg1-f194.google.com with SMTP id r22so7043744pgk.10 for ; Thu, 01 Aug 2019 11:14:20 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=from:to:cc:subject:date:message-id:in-reply-to:references :mime-version:content-transfer-encoding; bh=Bi7DhQ5yFa9gdtz6q03hD/81+WCPHg2ZwPjp55qICY0=; b=fO4UiudJ9OPBgpVdBS/EVuSW3M31sA5bIBv0f7GHYLZ6KkmIIADDCqXDEx3XnUFCLs A3gewf6NuVtUkKSTx+re3DkJTvrn6JSlXBhDjVLXvFCTzKOgDCZI/6Hg/xw46wW5lXry dq5lHUiLros4HmkHn68n0nJHv1qIsk3wowRHE= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references:mime-version:content-transfer-encoding; bh=Bi7DhQ5yFa9gdtz6q03hD/81+WCPHg2ZwPjp55qICY0=; b=f3qbTTNC2mr5okupBf8is9WcaUs8i1t7PluIHndfJ99DnW0iP189ID5vBMN2FiIDwd ASmvIZAsRCg0XO/aiC9aHhBeAlXHrAr+eX5cN/nPzwye8fFH89QqcXFKgn7qwMU/O6I0 KrNmozFOcYwRgVpDBoezfPAzz+BACG8AxThTMXaQF3SWRSrfIf9AnU0jU+AvTbn+rEth UKDqo08bbGWe12vU/KK+Eb5pirBKQ1bUGRfKzMDAYE5/AwwsVASsQjZXC665yKm24+Ia HG7ACoN48KO2Krdw67yDEkXYUdHmoY+LCp7upBNSg8EBl2It2wyi6V65l+KLH/XMDfzL YnWg== X-Gm-Message-State: APjAAAUOtUVZSNF+2vMBoavHvdO0BQR0x9VrrSfjA+WZ23ZjEjI3u+B5 UockxuMQpFT6vjoOgodNFWcVHU0c X-Google-Smtp-Source: APXvYqx8IMpZeuWjlPp//DQHV1o08Drpy+nyYs9KCBr1edQSTruxWAMtgwZ97clPwroBcOKdeS3d/Q== X-Received: by 2002:a63:58c:: with SMTP id 134mr125709836pgf.106.1564683259886; Thu, 01 Aug 2019 11:14:19 -0700 (PDT) Received: from joelaf.cam.corp.google.com ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id g8sm82089165pgk.1.2019.08.01.11.14.18 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Thu, 01 Aug 2019 11:14:19 -0700 (PDT) From: "Joel Fernandes (Google)" To: linux-kernel@vger.kernel.org Cc: "Joel Fernandes (Google)" , "Paul E. McKenney" , rcu@vger.kernel.org Subject: [PATCH 1/9] Revert "rcu: Restore barrier() to rcu_read_lock() and rcu_read_unlock()" Date: Thu, 1 Aug 2019 14:14:03 -0400 Message-Id: <20190801181411.96429-2-joel@joelfernandes.org> X-Mailer: git-send-email 2.22.0.770.g0f2c4a37fd-goog In-Reply-To: <20190801181411.96429-1-joel@joelfernandes.org> References: <20190801181411.96429-1-joel@joelfernandes.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This reverts commit d6b9cd7dc8e041ee83cb1362fce59a3cdb1f2709. --- .../RCU/Design/Requirements/Requirements.html | 71 ------------------- kernel/rcu/tree_plugin.h | 11 +++ 2 files changed, 11 insertions(+), 71 deletions(-) diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html index 467251f7fef6..bdbc84f1b949 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.html +++ b/Documentation/RCU/Design/Requirements/Requirements.html @@ -2129,8 +2129,6 @@ Some of the relevant points of interest are as follows:
  • Hotplug CPU.
  • Scheduler and RCU.
  • Tracing and RCU. -
  • -Accesses to User Memory and RCU.
  • Energy Efficiency.
  • Scheduling-Clock Interrupts and RCU. @@ -2523,75 +2521,6 @@ cannot be used. The tracing folks both located the requirement and provided the needed fix, so this surprise requirement was relatively painless. -

    -Accesses to User Memory and RCU

    - -

    -The kernel needs to access user-space memory, for example, to access -data referenced by system-call parameters. -The get_user() macro does this job. - -

    -However, user-space memory might well be paged out, which means -that get_user() might well page-fault and thus block while -waiting for the resulting I/O to complete. -It would be a very bad thing for the compiler to reorder -a get_user() invocation into an RCU read-side critical -section. -For example, suppose that the source code looked like this: - -

    -
    - 1 rcu_read_lock();
- 2 p = rcu_dereference(gp);
- 3 v = p->value;
- 4 rcu_read_unlock();
- 5 get_user(user_v, user_p);
- 6 do_something_with(v, user_v);
-
    -
    - -

    -The compiler must not be permitted to transform this source code into -the following: - -

    -
    - 1 rcu_read_lock();
- 2 p = rcu_dereference(gp);
- 3 get_user(user_v, user_p); // BUG: POSSIBLE PAGE FAULT!!!
- 4 v = p->value;
- 5 rcu_read_unlock();
- 6 do_something_with(v, user_v);
-
    -
    - -

    -If the compiler did make this transformation in a -CONFIG_PREEMPT=n kernel build, and if get_user() did -page fault, the result would be a quiescent state in the middle -of an RCU read-side critical section. -This misplaced quiescent state could result in line 4 being -a use-after-free access, which could be bad for your kernel's -actuarial statistics. -Similar examples can be constructed with the call to get_user() -preceding the rcu_read_lock(). - -

    -Unfortunately, get_user() doesn't have any particular -ordering properties, and in some architectures the underlying asm -isn't even marked volatile. -And even if it was marked volatile, the above access to -p->value is not volatile, so the compiler would not have any -reason to keep those two accesses in order. - -

    -Therefore, the Linux-kernel definitions of rcu_read_lock() -and rcu_read_unlock() must act as compiler barriers, -at least for outermost instances of rcu_read_lock() and -rcu_read_unlock() within a nested set of RCU read-side critical -sections. -

    Energy Efficiency

    diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 379cb7e50a62..e1491d262892 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -288,6 +288,7 @@ void rcu_note_context_switch(bool preempt) struct rcu_data *rdp = this_cpu_ptr(&rcu_data); struct rcu_node *rnp; + barrier(); /* Avoid RCU read-side critical sections leaking down. */ trace_rcu_utilization(TPS("Start context switch")); lockdep_assert_irqs_disabled(); WARN_ON_ONCE(!preempt && t->rcu_read_lock_nesting > 0); @@ -330,6 +331,7 @@ void rcu_note_context_switch(bool preempt) if (rdp->exp_deferred_qs) rcu_report_exp_rdp(rdp); trace_rcu_utilization(TPS("End context switch")); + barrier(); /* Avoid RCU read-side critical sections leaking up. */ } EXPORT_SYMBOL_GPL(rcu_note_context_switch); @@ -813,6 +815,11 @@ static void rcu_qs(void) * dyntick-idle quiescent state visible to other CPUs, which will in * some cases serve for expedited as well as normal grace periods. * Either way, register a lightweight quiescent state. + * + * The barrier() calls are redundant in the common case when this is + * called externally, but just in case this is called from within this + * file. + * */ void rcu_all_qs(void) { @@ -827,12 +834,14 @@ void rcu_all_qs(void) return; } this_cpu_write(rcu_data.rcu_urgent_qs, false); + barrier(); /* Avoid RCU read-side critical sections leaking down. */ if (unlikely(raw_cpu_read(rcu_data.rcu_need_heavy_qs))) { local_irq_save(flags); rcu_momentary_dyntick_idle(); local_irq_restore(flags); } rcu_qs(); + barrier(); /* Avoid RCU read-side critical sections leaking up. */ preempt_enable(); } EXPORT_SYMBOL_GPL(rcu_all_qs); @@ -842,6 +851,7 @@ EXPORT_SYMBOL_GPL(rcu_all_qs); */ void rcu_note_context_switch(bool preempt) { + barrier(); /* Avoid RCU read-side critical sections leaking down. */ trace_rcu_utilization(TPS("Start context switch")); rcu_qs(); /* Load rcu_urgent_qs before other flags. */ @@ -854,6 +864,7 @@ void rcu_note_context_switch(bool preempt) rcu_tasks_qs(current); out: trace_rcu_utilization(TPS("End context switch")); + barrier(); /* Avoid RCU read-side critical sections leaking up. */ } EXPORT_SYMBOL_GPL(rcu_note_context_switch); -- 2.22.0.770.g0f2c4a37fd-goog