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Subject: Re: [PATCH v3 2.6.39-rc1-tip 14/26] 14: x86: x86 specific probe
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From: Peter Zijlstra <peterz@infradead.org>
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Date: Mon, 18 Apr 2011 18:57:56 +0200
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To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>, Steven Rostedt <rostedt@goodmis.org>, Linux-mm <linux-mm@kvack.org>, Arnaldo Carvalho de Melo <acme@infradead.org>, Linus Torvalds <torvalds@linux-foundation.org>, Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>, Ananth N Mavinakayanahalli <ananth@in.ibm.com>, Christoph Hellwig <hch@infradead.org>, Andi Kleen <andi@firstfloor.org>, Thomas Gleixner <tglx@linutronix.de>, Jonathan Corbet <corbet@lwn.net>, Oleg Nesterov <oleg@redhat.com>, Andrew Morton <akpm@linux-foundation.org>, Jim Keniston <jkenisto@linux.vnet.ibm.com>, Roland McGrath <roland@hack.frob.com>, SystemTap <systemtap@sources.redhat.com>, LKML <linux-kernel@vger.kernel.org>

On Fri, 2011-04-01 at 20:05 +0530, Srikar Dronamraju wrote:
> +void arch_uprobe_enable_sstep(struct pt_regs *regs)
> +{
> +       /*
> +        * Enable single-stepping by
> +        * - Set TF on stack
> +        * - Set TIF_SINGLESTEP: Guarantees that TF is set when
> +        *      returning to user mode.
> +        *  - Indicate that TF is set by us.
> +        */
> +       regs->flags |=3D X86_EFLAGS_TF;
> +       set_thread_flag(TIF_SINGLESTEP);
> +       set_thread_flag(TIF_FORCED_TF);
> +}
> +
> +void arch_uprobe_disable_sstep(struct pt_regs *regs)
> +{
> +       /* Disable single-stepping by clearing what we set */
> +       clear_thread_flag(TIF_SINGLESTEP);
> +       clear_thread_flag(TIF_FORCED_TF);
> +       regs->flags &=3D ~X86_EFLAGS_TF;
> +}=20

Don't you loose the single step flag if userspace was already
single-stepping when it hit your breakpoint? Also, you don't seem to
touch the blockstep settings.

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