Claude review: misc: fastrpc: Add reference counting for fastrpc_user structure

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Overall Series Review

Subject: misc: fastrpc: Add reference counting for fastrpc_user structure
Author: Anandu Krishnan E <anandu.e@oss.qualcomm.com>
Patches: 2
Reviewed: 2026-02-27T11:57:24.514716

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This single patch adds `kref`-based reference counting to `struct fastrpc_user` to prevent a use-after-free where `fastrpc_device_release()` frees the user structure while invoke contexts are still pending in a workqueue. The approach is fundamentally correct — each context takes a reference on the user in `fastrpc_context_alloc()` and releases it in `fastrpc_context_free()`, ensuring the user struct outlives all its contexts.

However, the patch has a **significant bug** in the placement of `kref_init()` in `fastrpc_device_open()`, which introduces a new race window. The reference counting logic is otherwise sound — the `fastrpc_channel_ctx` get/put accounting remains correctly balanced despite the structural changes.

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