Re: [Intel-gfx] [PATCH] drm/i915/perf: Mark up the racy use of perf->exclusive_stream

[Lionel Landwerlin]

On 25/02/2020 15:23, Chris Wilson wrote:

Inside the general i915_oa_init_reg_state() we avoid using the
perf->mutex. However, we rely on perf->exclusive_stream being valid to
access at that point, and for that we have to control the race with
disabling perf. This relies on the disabling being a heavy barrier that
inspects all active contexts, after marking the perf->exclusive_stream
as not avaiable. This should ensure that there are no more concurrent
accesses to the perf->exclusive_stream as we destroy it.

Mark up the races around the perf->exclusive_stream so that they stand
out much more. (And hopefully we will be running kcsan to start
validating that the only races we have are carefully controlled.)

Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
Cc: Lionel Landwerlin <lionel.g.landwer...@intel.com>

Reviewed-by: Lionel Landwerlin <lionel.g.landwer...@intel.com>

---
  drivers/gpu/drm/i915/i915_perf.c | 13 +++++++------
  1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
index e34c79df6ebc..0838a12e2dc5 100644
--- a/drivers/gpu/drm/i915/i915_perf.c
+++ b/drivers/gpu/drm/i915/i915_perf.c
@@ -1405,8 +1405,10 @@ static void i915_oa_stream_destroy(struct 
i915_perf_stream *stream)
        /*
         * Unset exclusive_stream first, it will be checked while disabling
         * the metric set on gen8+.
+        *
+        * See i915_oa_init_reg_state() and lrc_configure_all_contexts()
         */
-       perf->exclusive_stream = NULL;
+       WRITE_ONCE(perf->exclusive_stream, NULL);
        perf->ops.disable_metric_set(stream);

free_oa_buffer(stream);

@@ -2847,7 +2849,7 @@ static int i915_oa_stream_init(struct i915_perf_stream 
*stream,
                goto err_oa_buf_alloc;

stream->ops = &i915_oa_stream_ops;

-       perf->exclusive_stream = stream;
+       WRITE_ONCE(perf->exclusive_stream, stream);

ret = perf->ops.enable_metric_set(stream);

        if (ret) {
@@ -2867,7 +2869,7 @@ static int i915_oa_stream_init(struct i915_perf_stream 
*stream,
        return 0;

err_enable:

-       perf->exclusive_stream = NULL;
+       WRITE_ONCE(perf->exclusive_stream, NULL);
        perf->ops.disable_metric_set(stream);

free_oa_buffer(stream);

@@ -2893,12 +2895,11 @@ void i915_oa_init_reg_state(const struct intel_context 
*ce,
  {
        struct i915_perf_stream *stream;

- /* perf.exclusive_stream serialised by lrc_configure_all_contexts() */

-
        if (engine->class != RENDER_CLASS)
                return;

- stream = engine->i915->perf.exclusive_stream;

+       /* perf.exclusive_stream serialised by lrc_configure_all_contexts() */
+       stream = READ_ONCE(engine->i915->perf.exclusive_stream);
        /*
         * For gen12, only CTX_R_PWR_CLK_STATE needs update, but the caller
         * is already doing that, so nothing to be done for gen12 here.

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