linux-kernel - Re: [PATCH 1/3] X86/db: Change __this_cpu_read() to this_cpu_read() in hw_breakpoint

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Date:   Mon, 13 Dec 2021 20:09:17 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 1/3] X86/db: Change __this_cpu_read() to this_cpu_read()
 in hw_breakpoint_active()

On Mon, Dec 13, 2021 at 12:22:13PM +0800, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@...ux.alibaba.com>

Lai, what you're touching is complex stuff. If your commit messages
are hard to parse then that makes reviewing those patches not a fun
experience. Not in the least.

So please try to restrain yourself and write proper English. Run your
commit message through a spellchecker at least so that there are no
funky words.

Also, read Documentation/process/submitting-patches.rst for hints on
how to write it.

The structure and the explanation is in the right direction but please
try to formulate them as understandable as possible.

> __this_cpu_read() can not be instrumented except its own debugging code
> when CONFIG_DEBUG_PREEMPT.  The debugging code will call
> __this_cpu_preempt_check().  __this_cpu_preempt_check() itself is also
> noinstr, so __this_cpu_read() can be used in noinstr.
> 
> But these is one exception when exc_debug_kernel() calls local_db_save()
> which calls hw_breakpoint_active() which calls __this_cpu_read().  If
> the data accessed by __this_cpu_preempt_check() is also watched by
> hw_breakpoints, it would cause recursive #DB.

Up until here is good.
 
> this_cpu_read() in X86 is also non instrumentable, and it doesn't access

"x86" not "X86" or any other way.

Also, read this: Documentation/process/maintainer-tip.rst

as it has more hints about commit message structure etc.

> to any extra data except the percpu cpu_dr7, and cpu_dr7 is disallowed
> to be watched in arch_build_bp_info().  So this_cpu_read() is safe to
> be used when hw_breakpoints is still active, and __this_cpu_read() here
> should be changed to this_cpu_read().
> 
> This problem can only happen when the system owner uses a kernel with
> CONFIG_DEBUG_PREEMPT enabled and deliberately use hw_breakpoints on
> the data that __this_cpu_preempt_check() accesses.  Sot it is just a
> problem with no significance.
> 
> One might suggest that, all the data accessed by noinstr functions
> should be marked in denylist for hw_breakpoints.  That would complexify

should be marked in denylist for hw_breakpoints.  That would complexify
Unknown word [denylist] in commit message, suggestions:
        ['deny list', 'deny-list', 'dentistry']

should be marked in denylist for hw_breakpoints.  That would complexify
Unknown word [complexify] in commit message, suggestions:
        ['complexity', 'complexion']

> the noinstrment framework and add hurdles to anyone that who want to

the noinstrment framework and add hurdles to anyone that who want to
Unknown word [noinstrment] in commit message, suggestions:
        ['instrument']

So you need to restrain yourself and stop inventing new English words.

> add a new noinstr function.  All we need is to suppress #DB in the IST

Who is "we"?

> interrupt entry path until safe place where #DB is disabled in hardware
> or #DB handler can handle well even it hits data accessed by noinstr
> function.  Changing __this_cpu_read() to this_cpu_read() is fit for it.

You don't need to write *what* your patch is doing - that is clear from
the diff.

I'll let Peter comment on what should actually be used.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette