Re: [RFC][PATCH 3/7] perf, hw_breakpoint: Fix crash in hw_breakpoint creation

[Frederic Weisbecker <fweisbec@gmail.com>]

On Thu, Oct 14, 2010 at 10:34:07PM +0200, Peter Zijlstra wrote:
> hw_breakpoint creation needs to account stuff per-task to ensure there
> is always sufficient hardware resources to back these things due to
> ptrace.
> 
> With the perf per pmu context changes the event initialization no
> longer has access to the event context, for the simple reason that we
> need to first find the pmu (result of initialization) before we can
> find the context.
> 
> This makes hw_breakpoints unhappy, because it can no longer do per
> task accounting, cure this by frobbing a task pointer in the event::hw
> bits for now...
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> ---
>  include/linux/perf_event.h |    7 +++++++
>  kernel/hw_breakpoint.c     |    8 ++++----
>  kernel/perf_event.c        |   21 ++++++++++++++++-----
>  3 files changed, 27 insertions(+), 9 deletions(-)
> 
> Index: linux-2.6/include/linux/perf_event.h
> ===================================================================
> --- linux-2.6.orig/include/linux/perf_event.h
> +++ linux-2.6/include/linux/perf_event.h
> @@ -536,6 +536,12 @@ struct hw_perf_event {
>  		struct { /* breakpoint */
>  			struct arch_hw_breakpoint	info;
>  			struct list_head		bp_list;
> +			/*
> +			 * Crufty hack to avoid the chicken and egg
> +			 * problem hw_breakpoint has with context
> +			 * creation and event initalization.
> +			 */
> +			struct task_struct		*bp_target;
>  		};
>  #endif
>  	};
> @@ -693,6 +699,7 @@ struct swevent_hlist {
>  
>  #define PERF_ATTACH_CONTEXT	0x01
>  #define PERF_ATTACH_GROUP	0x02
> +#define PERF_ATTACH_TASK	0x04



Thanks, the patch looks good.

I'm just not sure about the point of this flag...