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Subject: Re: [PATCH v3 0/6] Tracing vs CR2
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Vegard Nossum <vegard.nossum@oracle.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@kernel.org>,
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        Linus Torvalds <torvalds@linux-foundation.org>,
        linux_lkml_grp@oracle.com, "H. Peter Anvin" <hpa@zytor.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Juergen Gross <jgross@suse.com>,
        LKML <linux-kernel@vger.kernel.org>,
        He Zhe <zhe.he@windriver.com>,
        Joel Fernandes <joel@joelfernandes.org>
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From:   Eiichi Tsukata <devel@etsukata.com>
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Date:   Sat, 20 Jul 2019 23:23:36 +0900
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On 2019/07/20 21:49, Andy Lutomirski wrote:
> On Fri, Jul 19, 2019 at 8:59 PM Eiichi Tsukata <devel@etsukata.com> wrote:
>>
...
>>
>> ====
>>
>> debug() // dr6: 0xffff4ff0, user_mode: 1
>>   TRACE_IRQS_OFF
>>     arch_stack_user_walk()
>>       debug()  // dr6: 0xffff4ff1 == 0xffff4ff0 | 0xffff0ff1 ... (*)
>>         do_debug()
>>           WARN_ON_ONCE
>>   do_debug() // dr6: 0xffff0ff0(cleared in the above do_debug())
> 
> The dr6 register will indeed be cleared like this, but the dr6
> variable should still be 0xffff4ff0.

I should have use DR6 to mean it is a register, not variable.
"dr6" was ambiguous.

> 
>>
...
>>
>> Note: printk() in do_debug() can cause infinite loop(printk() ->
>> irq_disable() -> do_debug() -> printk() ...), so printk_deferred()
>> was preferable.
>>
> 
> Shouldn't that be fixed with my patches?  It should only be able to
> recurse two deep: do_debug() from user mode can indeed trip
> breakpoints, but the next do_debug() will clear DR7 in paranoid_entry.
> 

Sorry, I missed that. Now I confirmed your patches fixed the loop.

Thanks

Eiichi