Switch the free-node pop paths to raw_spin_trylock*() so callers don't block on contended LRU locks. This is a narrower change than Menglong's approach [1], which aimed to eliminate the deadlock entirely.
The trylock-based approach avoids deadlocks in long-lived critical sections, while still allowing locking in short-lived ones. Although it does not completely eliminate the possibility of deadlock, it significantly reduces the likelihood in practice. LRU-related deadlocks have been observed multiple times, including: - [syzbot] [bpf?] possible deadlock in bpf_lru_push_free (2) [2] - Re: [PATCH bpf v3 0/4] bpf: Free special fields when update hash and local storage maps [3] - Raw log of CI failure [4] BTW, this series also factors out the bpf_lru_node_set_hash() helper, along with a comment describing the required ordering and locking constraints. Links: [1] https://lore.kernel.org/bpf/[email protected]/ [2] https://lore.kernel.org/bpf/[email protected]/ [3] https://lore.kernel.org/bpf/CAEf4BzbTJCUx0D=zjx6+5m5iighwlzap94hnw36zmdhaf4-...@mail.gmail.com/ [4] https://github.com/kernel-patches/bpf/actions/runs/20943173932/job/60181505085 Leon Hwang (3): bpf: Factor out bpf_lru_node_set_hash() helper bpf: Avoid deadlock using trylock when popping LRU free nodes selftests/bpf: Allow -ENOMEM on LRU map updates kernel/bpf/bpf_lru_list.c | 35 ++++++++++++++----- .../bpf/map_tests/map_percpu_stats.c | 3 +- 2 files changed, 28 insertions(+), 10 deletions(-) -- 2.52.0
