linux-kernel - Re: [PATCH] Fix int1 recursion with unregistered breakpoints

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Message-ID: <566F371A.3090207@zytor.com>
Date:	Mon, 14 Dec 2015 13:39:38 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeff Merkey <linux.mdb@...il.com>, linux-kernel@...r.kernel.org
Cc:	Thomas Gleixner <tglx@...utronix.de>, mingo@...hat.com,
	x86@...nel.org, peterz@...radead.org, luto@...nel.org
Subject: Re: [PATCH] Fix int1 recursion with unregistered breakpoints

On 12/14/15 13:03, Jeff Merkey wrote:
> Please consider the attached patch.
> 
> I have reviewed all the code that touches this patch and have
> determined it will function and support all of the software that
> depends on this handler properly.  I have compiled and tested this
> patch with a test harness that tests the robustness of the linux
> breakpoint API and handlers in the following ways:
> 
> 1.  Setting multiple conditional breakpoints through
> arch_install_hw_breakpoint API across four processors to test the rate
> at which the interface can handle breakpoint exceptions
> 
> 2.  Setting unregistered breakpoints to test the handlers robustness
> in dealing with error handling conditions and errant or spurious
> hardware conditions and to simulate actual "lazy debug register
> switching" (which does not work BTW) with null bp handlers to test the
> robustness of the handlers.
> 
> 3.  Clearing and setting breakpoints across multiple processors then
> triggering concurrent exceptions in both interrupt and process
> contexts.
> 
> This patch improves robustness in several ways in the linux kernel:
> 
> 1.  Corrects bug in handling unregistered breakpoints.
> 
> 2.  Provides hardware check of dr7 to determine source of breakpoint
> if OS cannot ascertain the int1 source from its own state and
> variables.
> 
> 3.  Actually allows "lazy debug register switching" to function, which
> until recently has apparently never been actually seen on live
> hardware or actually tested.
> 

This is all fine and good, but you are missing one of the most important
parts of a patch: a patch description, describing in detail the problem
that it solves and why.  This description needs to be comprehensible not
just for people already initiated but for someone doing code archaeology
a decade from now.

Thanks,

	-hpa


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