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SubjectRe: [PATCH] sample/hw_breakpoint: avoid sample hw_breakpoint recursion for arm/arm64
On Mon, Sep 23, 2019 at 04:09:35PM +0800, wangxu wrote:
> From: Wang Xu <wangxu72@huawei.com>
>
> For x86/ppc, hw_breakpoint is triggered after the instruction is
> executed.
>
> For arm/arm64, which is triggered before the instruction executed.
> Arm/arm64 skips the instruction by using single step. But it only
> supports default overflow_handler.

Where is the recusion.. ?

> This patch provides a chance to avoid sample hw_breakpoint recursion
> for arm/arm64 by adding 'struct perf_event_attr.bp_step'.

This patch also lacks justification for why this needs to come with ABI
changes. There is also a distinct lack of comments.

> Signed-off-by: Wang Xu <wangxu72@huawei.com>
> ---
> include/linux/perf_event.h | 3 +++
> include/uapi/linux/perf_event.h | 3 ++-
> samples/hw_breakpoint/data_breakpoint.c | 1 +
> 3 files changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 61448c1..f270eb7 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -1024,6 +1024,9 @@ extern int perf_event_output(struct perf_event *event,
> return true;
> if (unlikely(event->overflow_handler == perf_event_output_backward))
> return true;
> + /* avoid sample hw_breakpoint recursion */
> + if (unlikely(event->attr.bp_step))
> + return true;

This is just _wrong_.. it says that every event with bp_step set always
is a 'default overflow handler', irrespective of what the overflow
handler actually is.

> return false;
> }
>

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