linux-kernel - Re: [PATCH RESEND] arm64: arch_timer: fix the infinite recursion when enable ftrace and erratum workaround

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Date:   Mon, 10 Jul 2017 14:54:27 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Mark Rutland <mark.rutland@....com>,
        Ding Tianhong <dingtianhong@...wei.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        LinuxArm <linuxarm@...wei.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RESEND] arm64: arch_timer: fix the infinite recursion when
 enable ftrace and erratum workaround

On 10/07/17 12:22, Mark Rutland wrote:
> On Sun, Jul 09, 2017 at 04:30:54PM +0800, Ding Tianhong wrote:
>> When enable preempt and debug ftrace, and perform the following steps, the
>> system will hang:
>> mount -t debugfs nodev /sys/kernel/debug/
>> cd /sys/kernel/debug/tracing/
>> echo function_graph > current_tracer
>>
>> This is because tracing the preempt_disable/enable calls would cause
>> trace_clock() which would get local timer to go into infinite recursion
>> when enable the arch timer erratum workaround for some chips, so Prevent
>> tracing of preempt_disable/enable() in arch_timer_reg_read_stable().
>>
>> This problem is similar to the fixed by upstream commit 96b3d28bf4
>> ("sched/clock: Prevent tracing recursion in sched_clock_cpu()").
> 
> As I mentioned before, the patch itself looks fine to me, but the commit
> message is somewhat difficult to read.
> 
> Can we please change this to:
> 
>   arm64: arch_timer: avoid infinite recursion when ftrace is enabled
> 
>   On platforms with an arch timer erratum workaround, it's possible for
>   arch_timer_reg_read_stable() to recurse into itself when certain
>   tracing options are enabled, leading to stack overflows and related
>   problems.
>  
>   For example, when PREEMPT_TRACER and FUNCTION_GRAPH_TRACER are
>   selected, it's possible to trigger this with:
>   
>   $ mount -t debugfs nodev /sys/kernel/debug/
>   $ echo function_graph > /sys/kernel/debug/tracing/current_tracer
> 
>   The problem is that in such cases, preempt_disable() instrumentation
>   attempts to acquire a timestamp via trace_clock(), resulting in a call
>   back to arch_timer_reg_read_stable(), and hence recursion.
> 
>   This patch changes arch_timer_reg_read_stable() to use
>   preempt_{disable,enable}_notrace(), which avoids this.
> 
> With that commit message:
> 
> Acked-by: Mark Rutland <mark.rutland@....com>
> 
> Daniel, Thomas, would you be happy to fold that in when picking this? Or
> would you prefer that I fix this up and resend?

With the proposed changes:

Acked-by: Marc Zyngier <marc.zyngier@....com>

	M.
-- 
Jazz is not dead. It just smells funny...