linux-kernel - Re: A issue about ptrace/SINGLESTEP on arm64

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Message-ID: <20171016153055.GD10500@arm.com>
Date:   Mon, 16 Oct 2017 16:30:56 +0100
From:   Will Deacon <will.deacon@....com>
To:     "chengjian (D)" <cj.chengjian@...wei.com>
Cc:     catalin.marinas@....com, oleg@...hat.com, linux@...linux.org.uk,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        "Xiexiuqi (Xie XiuQi)" <xiexiuqi@...wei.com>,
        Li Bin <huawei.libin@...wei.com>
Subject: Re: A issue about ptrace/SINGLESTEP on arm64

On Mon, Oct 16, 2017 at 12:27:17PM +0800, chengjian (D) wrote:
> Hi
> I write demo use ptrace/SINGLESTEP to count the number of instructions
> executed by the process
> The parent process fork+exec a child process, and trace(SINGLESTEP) it,
> 
> It works fine under the x86_64 architecture but has an exception under
> arm64.

My guess is that you are getting stuck in an LDXR/STXR loop, which cannot
be stepped. Can you jump the PC once the child appears to be "stuck"?

IIRC, GDB has special heuristics to step through LDXR/STXR critical
sections.

Will