Re: [PATCH 0/2] drm/amd: fix VRR race condition during IRQ handling

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On 9/8/24 09:35, Christopher Snowhill wrote:

On Mon Sep 2, 2024 at 2:40 AM PDT, tjakobi wrote:
From: Tobias Jakobi <tjakobi@xxxxxxxxxxxxxxxxxxxxx>

Hello,

this fixes a nasty race condition in the set_drr() callbacks for DCN10
and DCN35 that has existed now since quite some time, see this GitLab
issue for reference.

https://gitlab.freedesktop.org/drm/amd/-/issues/3142

The report just focuses von DCN10, but the same problem also exists in
the DCN35 code.
Does the problem not exist in the following references to funcs->set_drr?

drivers/gpu/drm/amd/display/dc/hwss/dce110/dce110_hwseq.c:      if (pipe_ctx->stream_res.tg->funcs->set_drr)
drivers/gpu/drm/amd/display/dc/hwss/dce110/dce110_hwseq.c:              pipe_ctx->stream_res.tg->funcs->set_drr(
drivers/gpu/drm/amd/display/dc/hwss/dce110/dce110_hwseq.c:              pipe_ctx[i]->stream_res.tg->funcs->set_drr(
drivers/gpu/drm/amd/display/dc/hwss/dcn20/dcn20_hwseq.c:        if (pipe_ctx->stream_res.tg->funcs->set_drr)
drivers/gpu/drm/amd/display/dc/hwss/dcn20/dcn20_hwseq.c:                pipe_ctx->stream_res.tg->funcs->set_drr(
drivers/gpu/drm/amd/display/dc/hwss/dcn20/dcn20_hwseq.c:                if (pipe_ctx->stream_res.tg->funcs->set_drr)
drivers/gpu/drm/amd/display/dc/hwss/dcn20/dcn20_hwseq.c:                        pipe_ctx->stream_res.tg->funcs->set_drr(
drivers/gpu/drm/amd/display/dc/hwss/dcn31/dcn31_hwseq.c:        if (pipe_ctx->stream_res.tg->funcs->set_drr)
drivers/gpu/drm/amd/display/dc/hwss/dcn31/dcn31_hwseq.c:                pipe_ctx->stream_res.tg->funcs->set_drr(
drivers/gpu/drm/amd/display/dc/hwss/dcn401/dcn401_hwseq.c:      if (pipe_ctx->stream_res.tg->funcs->set_drr)
drivers/gpu/drm/amd/display/dc/hwss/dcn401/dcn401_hwseq.c:              pipe_ctx->stream_res.tg->funcs->set_drr(

Maybe. But the big difference I see here, is that in this code there isn't even any kind of NULL check applied to tg. Or most of the members of *pipe_ctx. If there really is the same kind of problem here, then one would need to rewrite a bit more code to fix stuff.

E.g. in the case of  dcn31_hwseq.c, the questionable code is in dcn31_reset_back_end_for_pipe(), which is static and only called from dcn31_reset_hw_ctx_wrap(). Which is assigned to the .reset_hw_ctx_wrap callback. And this specific callback, from what I can see, is only called from dce110_reset_hw_ctx_wrap(). Which is then assigned to the .apply_ctx_to_hw callback. The callback is only called from dc_commit_state_no_check(). That one is static again, and called from dc_commit_streams().

I could trace this even further. My point is: I don't think this is called from any IRQ handler code. And given the depth and complexity of the callgraph, I have to admit, that, at least at this point, this is a bit over my head.

Sure, I could now sprinkle a bunch of x != NULL in the code, but that would be merely voodoo. And I usually try to have a theoretical basis when I apply changes to code.

Maybe if someone from the AMD display team could give some insight if there still is potentially vulnerable code in some of the instances that Christopher has posted, then I would gladly take a look.

With best wishes,
Tobias


With best wishes,
Tobias

Tobias Jakobi (2):
   drm/amd/display: Avoid race between dcn10_set_drr() and
     dc_state_destruct()
   drm/amd/display: Avoid race between dcn35_set_drr() and
     dc_state_destruct()

  .../amd/display/dc/hwss/dcn10/dcn10_hwseq.c   | 20 +++++++++++--------
  .../amd/display/dc/hwss/dcn35/dcn35_hwseq.c   | 20 +++++++++++--------
  2 files changed, 24 insertions(+), 16 deletions(-)



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