B4/timer nolock #10161
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B4/timer nolock #10161
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This series reworks implementation of BPF timer and workqueue APIs. The goal is to make both timers and wq non-blocking, enabling their use in NMI context. Today this code relies on a bpf_spin_lock embedded in the map element to serialize: * init of the async object, * setting/changing the callback and bpf_prog * starting/cancelling the timer/work * tearing down when the map element is deleted or the map’s user ref is dropped Current approach works, but it does not allow using the API in the contexts not allowing wait. The series apply design similar to existing bpf_task_work approach [1]: RCU and refcount to maintain lifetime guarantees and state machine to handle data races. This RFC doesn’t yet fully add NMI support for timers and workqueue work, but it takes the first step by removing the spinlock from struct bpf_async_cb. --- 1: https://lore.kernel.org/bpf/175864081800.1466288.3242104888617580131.git-patchwork-notify@kernel.org/ Signed-off-by: Mykyta Yatsenko <[email protected]> --- b4-submit-tracking --- { "series": { "revision": 1, "change-id": "20251028-timer_nolock-457f5b9daace", "prefixes": [ "OFFLIST" ] } }
Move logic for updating callback and prog owning it into a separate function: bpf_async_update_callback(). This helps to localize data race and will be reused in the next patches. Signed-off-by: Mykyta Yatsenko <[email protected]>
Move the logic that swaps the bpf_prog in struct bpf_async_cb into a dedicated helper to make follow-up patches simpler. Signed-off-by: Mykyta Yatsenko <[email protected]>
Move the timer deletion logic into a dedicated bpf_timer_delete() helper so it can be reused by later patches. Signed-off-by: Mykyta Yatsenko <[email protected]>
To manage lifetime guarantees of the struct bpf_async_cb, when no lock serializes mutations, introduce refcnt field into the struct. Implement bpf_async_tryget() and bpf_async_put() to handle the refcnt. Signed-off-by: Mykyta Yatsenko <[email protected]>
Remove lock from bpf_async_cb, refactor bpf_timer and bpf_wq kfuncs and helpers to run without it. bpf_async_cb lifetime is managed by the refcnt and RCU, so every function that uses it has to apply RCU guard. cancel_and_free() path detaches bpf_async_cb from the map value (struct bpf_async_kern) and sets the state to the terminal BPF_ASYNC_FREED atomically, concurrent readers may operate on detached bpf_async_cb safely under RCU read lock. Guarantee safe bpf_prog drop from the bpf_async_cb by handling BPF_ASYNC_FREED state in bpf_async_update_callback(). Signed-off-by: Mykyta Yatsenko <[email protected]>
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