Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753874AbbFDPjX (ORCPT ); Thu, 4 Jun 2015 11:39:23 -0400 Received: from mail-wi0-f171.google.com ([209.85.212.171]:35420 "EHLO mail-wi0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753348AbbFDPjR (ORCPT ); Thu, 4 Jun 2015 11:39:17 -0400 From: Frederic Weisbecker To: Peter Zijlstra Cc: LKML , Frederic Weisbecker , Ingo Molnar , Fengguang Wu , Steven Rostedt Subject: [PATCH 1/3] sched: Make preempt_schedule_context() function-tracing safe Date: Thu, 4 Jun 2015 17:39:07 +0200 Message-Id: <1433432349-1021-2-git-send-email-fweisbec@gmail.com> X-Mailer: git-send-email 2.1.4 In-Reply-To: <1433432349-1021-1-git-send-email-fweisbec@gmail.com> References: <1433432349-1021-1-git-send-email-fweisbec@gmail.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2983 Lines: 75 Since function tracing disables preemption, it needs a safe preemption point to use when preemption is re-enabled without worrying about tracing recursion. Ie: to avoid tracing recursion, that preemption point can't be traced (use of notrace qualifier) and it can't call any traceable function before that preemption point disables preemption itself, which disarms the recursion. preempt_schedule() was fine until commit b30f0e3ffedfa52b1d67a302ae5860c49998e5e2 ("sched/preempt: Optimize preemption operations on __schedule() callers") because PREEMPT_ACTIVE (which has the property to disable preemption and this disarm tracing preemption recursion) was set before calling any further function. But that commit introduced the use of preempt_count_add/sub() functions to set PREEMPT_ACTIVE and because these functions are called before preemption gets a chance to be disabled, we have a tracing recursion. preempt_schedule_context() is one of the possible preemption functions used by tracing. Its special purpose is to avoid tracing recursion against context tracking. Lets enhance this function to become more generally tracing safe by disabling preemption with raw accessors, such that no function is called before preemption gets disabled and disarm the tracing recursion. This function is going to become the specific tracing-safe preemption point in further commit. Reported-by: Fengguang Wu Cc: Peter Zijlstra Cc: Ingo Molnar Cc: Fengguang Wu Cc: Steven Rostedt Signed-off-by: Frederic Weisbecker --- kernel/sched/core.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 5140db6..4c3d41f 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2960,7 +2960,13 @@ asmlinkage __visible void __sched notrace preempt_schedule_context(void) return; do { - preempt_active_enter(); + /* + * Use raw __prempt_count() ops that don't call function. + * We can't call functions before disabling preemption which + * disarm preemption tracing recursions. + */ + __preempt_count_add(PREEMPT_ACTIVE + PREEMPT_DISABLE_OFFSET); + barrier(); /* * Needs preempt disabled in case user_exit() is traced * and the tracer calls preempt_enable_notrace() causing @@ -2970,7 +2976,8 @@ asmlinkage __visible void __sched notrace preempt_schedule_context(void) __schedule(); exception_exit(prev_ctx); - preempt_active_exit(); + barrier(); + __preempt_count_sub(PREEMPT_ACTIVE + PREEMPT_DISABLE_OFFSET); } while (need_resched()); } EXPORT_SYMBOL_GPL(preempt_schedule_context); -- 2.1.4 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/