linux-kernel - Re: [PATCH v9 1/2] ARM: avoid tracers in soft

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Date:	Mon, 14 Apr 2014 15:37:58 -0700
From:	Sebastian Capella <sebastian.capella@...aro.org>
To:	Will Deacon <will.deacon@....com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Russell King <linux@....linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Robin Holt <holt@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Konstantin Khlebnikov <k.khlebnikov@...sung.com>,
	Steven Capper <steve.capper@...aro.org>,
	Stephen Warren <swarren@...dia.com>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v9 1/2] ARM: avoid tracers in soft_restart

Hi Will,

On 14 April 2014 03:53, Will Deacon <will.deacon@....com> wrote:
> On Tue, Mar 18, 2014 at 09:40:57PM +0000, Sebastian Capella wrote:
>> Use of tracers in local_irq_disable is causes abort loops when called
>> with irqs disabled using a temporary stack.  Replace local_irq_disable
>> with raw_local_irq_disable instead to avoid tracers.
>
> Do you have any more information about these aborts? At the time we call
> local_irq_disable, the stack is still intact, so if the issue is simply
> related to having any tracers active at the call_with_stack invocation, we'd
> be better off disabling tracing here altogether.

This is specifically for when soft_restart is called in the
hibernation path with tracers enabled.  At that point, we've already
switched to a temporary stack, and when we call local_irq_disable,
we'll see this:

In the local_irq_disable, it ends up calling trace_hardirqs_off
(CONFIG_TRACE_IRQFLAGS_SUPPORT is enabled), which calls
trace_hardirqs_off_caller which checks lockdep_recursion in the
current task, but we've switched to a temporary stack with the
call_with_stack, and get_current is returning NULL.  This
triggers a data abort, which calls trace_hardirqs_off
again and so on.

We originally had a patch which added a soft restart called
soft_restart_noirq which avoided the irq disable.

You can see the discussion here:
https://patchwork.kernel.org/patch/3677591/

Thanks!

Sebastian
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