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fix(metal): use-after-free in gpu::eval for primitives that synchronize mid-eval #5

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Merged
Gajesh2007 merged 1 commit into from
Aug 13, 2026
Merged

fix(metal): use-after-free in gpu::eval for primitives that synchronize mid-eval #5

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12 changes: 9 additions & 3 deletions mlx/backend/metal/eval.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -30,7 +30,6 @@ void eval(array& arr) {
auto pool = metal::new_scoped_memory_pool();
auto s = arr.primitive().stream();
auto& encoder = metal::get_command_encoder(s);
auto* command_buffer = encoder.get_command_buffer();

auto outputs = arr.outputs();
{
Expand All @@ -41,7 +40,8 @@ void eval(array& arr) {
inputs = arr.inputs();
}

debug_set_primitive_buffer_label(command_buffer, arr.primitive());
debug_set_primitive_buffer_label(
encoder.get_command_buffer(), arr.primitive());
arr.primitive().eval_gpu(arr.inputs(), outputs);
}
std::unordered_set<std::shared_ptr<array::Data>> buffers;
Expand All @@ -63,7 +63,13 @@ void eval(array& arr) {
scheduler::notify_task_completion(s);
});
} else {
command_buffer->addCompletedHandler(
// Fetch the command buffer AFTER eval_gpu: primitives that synchronize
// mid-eval (e.g. the expert-tile route's descriptor retract check)
// commit and REPLACE the encoder's buffer, so a pointer captured before
// eval_gpu would be stale here — attaching the buffer-liveness handler
// to it is a use-after-free. The current buffer holds the tail of this
// primitive's work, which is exactly what the inputs must outlive.
encoder.get_command_buffer()->addCompletedHandler(
[buffers = std::move(buffers)](MTL::CommandBuffer* cbuf) {});
}
}
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