On Wed, Jul 10, 2019 at 11:25:49AM -0400, Steven Rostedt wrote:
> I was doing a bit of an audit on trace events and found this:
>
> # cat /debug/tracing/events/i915/intel_disable_plane/format
> name: intel_disable_plane
> ID: 1358
> format:
> field:unsigned short common_type; offset:0; size:2; signed:0;
> field:unsigned char common_flags; offset:2; size:1; signed:0;
> field:unsigned char common_preempt_count; offset:3; size:1; signed:0;
> field:int common_pid; offset:4; size:4; signed:1;
>
> field:enum pipe pipe; offset:8; size:4; signed:1;
> field:const char * name; offset:16; size:8; signed:0;
> field:u32 frame; offset:24; size:4; signed:0;
> field:u32 scanline; offset:28; size:4; signed:0;
>
> print fmt: "pipe %c, plane %s, frame=%u, scanline=%u", ((REC->pipe) + 'A'), REC->name, REC->frame, REC->scanline
>
>
> Same goes for intel_update_plane.
>
>
> The problem here is:
>
> field:const char * name; offset:16; size:8; signed:0;
>
> print fmt: "pipe %c, plane %s, frame=%u, scanline=%u", ((REC->pipe) + 'A'), REC->name, REC->frame, REC->scanline
>
>
> Where the TRACE_EVENT() macro has:
>
> TP_fast_assign(
> __entry->pipe = crtc->pipe;
> __entry->name = plane->name;
> __entry->frame = crtc->base.dev->driver->get_vblank_counter(crtc->base.dev,
> crtc->pipe);
> __entry->scanline = intel_get_crtc_scanline(crtc);
> ),
>
> TP_printk("pipe %c, plane %s, frame=%u, scanline=%u",
> pipe_name(__entry->pipe), __entry->name,
> __entry->frame, __entry->scanline)
>
>
> The issue here is that you record a pointer address to "plane->name"
> and then sometime in the distant future access that same address.
> There's usually no guarantee that the contents at that address will
> exist when the buffer is read.
The only way those can disappear is if the device goes away. But I have
no problem going with your patch. Want to provide a proper commit message
for it?
>
> The proper way to record strings, is to record the string into the ring
> buffer itself, and not rely on it existing hours or days later.
>
> I recommend the following patch:
>
> -- Steve
>
> diff --git a/drivers/gpu/drm/i915/i915_trace.h b/drivers/gpu/drm/i915/i915_trace.h
> index 12893304c8f8..d41d914a16ca 100644
> --- a/drivers/gpu/drm/i915/i915_trace.h
> +++ b/drivers/gpu/drm/i915/i915_trace.h
> @@ -298,16 +298,16 @@ TRACE_EVENT(intel_update_plane,
>
> TP_STRUCT__entry(
> __field(enum pipe, pipe)
> - __field(const char *, name)
> __field(u32, frame)
> __field(u32, scanline)
> __array(int, src, 4)
> __array(int, dst, 4)
> + __string(name, plane->name)
> ),
>
> TP_fast_assign(
> + __assign_str(name, plane->name);
> __entry->pipe = crtc->pipe;
> - __entry->name = plane->name;
> __entry->frame = crtc->base.dev->driver->get_vblank_counter(crtc->base.dev,
> crtc->pipe);
> __entry->scanline = intel_get_crtc_scanline(crtc);
> @@ -316,7 +316,7 @@ TRACE_EVENT(intel_update_plane,
> ),
>
> TP_printk("pipe %c, plane %s, frame=%u, scanline=%u, " DRM_RECT_FP_FMT " -> " DRM_RECT_FMT,
> - pipe_name(__entry->pipe), __entry->name,
> + pipe_name(__entry->pipe), __get_str(name),
> __entry->frame, __entry->scanline,
> DRM_RECT_FP_ARG((const struct drm_rect *)__entry->src),
> DRM_RECT_ARG((const struct drm_rect *)__entry->dst))
> @@ -328,21 +328,21 @@ TRACE_EVENT(intel_disable_plane,
>
> TP_STRUCT__entry(
> __field(enum pipe, pipe)
> - __field(const char *, name)
> __field(u32, frame)
> __field(u32, scanline)
> + __string(name, plane->name)
> ),
>
> TP_fast_assign(
> + __assign_str(name, plane->name);
> __entry->pipe = crtc->pipe;
> - __entry->name = plane->name;
> __entry->frame = crtc->base.dev->driver->get_vblank_counter(crtc->base.dev,
> crtc->pipe);
> __entry->scanline = intel_get_crtc_scanline(crtc);
> ),
>
> TP_printk("pipe %c, plane %s, frame=%u, scanline=%u",
> - pipe_name(__entry->pipe), __entry->name,
> + pipe_name(__entry->pipe), __get_str(name),
> __entry->frame, __entry->scanline)
> );
>
--
Ville Syrjälä
Intel
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