linux-kernel - Re: [RESEND PATCH] arm64: kgdb: fix single stepping

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Date:   Mon, 19 Sep 2016 17:29:36 -0500
From:   Jason Wessel <jason.wessel@...driver.com>
To:     AKASHI Takahiro <takahiro.akashi@...aro.org>,
        <catalin.marinas@....com>, <will.deacon@....com>,
        <broonie@...nel.org>, <yong.zhao@....com>,
        <Vijaya.Kumar@...iumnetworks.com>,
        <kgdb-bugreport@...ts.sourceforge.net>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linaro-kernel@...ts.linaro.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [RESEND PATCH] arm64: kgdb: fix single stepping

On 09/15/2016 11:32 PM, AKASHI Takahiro wrote:
> @@ -176,18 +183,14 @@ int kgdb_arch_handle_exception(int exception_vector, int signo,
>>> 		 * over and over again.
>>> 		 */
>>> 		kgdb_arch_update_addr(linux_regs, remcom_in_buffer);
>>> -		atomic_set(&kgdb_cpu_doing_single_step, -1);
>>> -		kgdb_single_step =  0;
>>
>> This is a subtle change, but I assume it is what you intended?  All the CPUs will get released into the run state when exiting the kgdb exception handler.
> You are talking about "- kgdb_single_step = 0." Right?


Correct.

> Do you think that there is any (negative) side effect of this change?


Not at all.   The kernel debugger always skids to a stop, and it is more reliable from a locking perspective if the other CPU threads are released while a single CPU is asked to single step until the next "skid" for all the other CPUs.

When you do not release the other CPUs you can end up single stepping a CPU which dead locks or never exits a lock elsewhere due to what ever it was blocking on never getting freed from another CPU.

Cheers,
Jason.