bpf: Fix NMI/tracepoint re-entry deadlock on lru locks

NMI and tracepoint BPF programs can re-enter the per-CPU or global
LRU lock that bpf_lru_pop_free()/push_free() already hold on the
same CPU, AA-deadlocking. Lockdep reports "inconsistent
{INITIAL USE} -> {IN-NMI}" on &l->lock (syzbot c69a0a2c816716f1e0d5)
and "possible recursive locking detected" on &loc_l->lock (syzbot
18b26edb69b2e19f3b33).

Prior trylock and rqspinlock based fixes (see links) were nacked
because compromised on reliability.

This patch converts every LRU lock site to rqspinlock_t and adds a
recovery path for some failure windows to avoid node leaks.

Failure recovery:

 - *_pop_free top-level: return NULL; prealloc_lru_pop() already
   treats that as no-free-element (-ENOMEM).

 - Cross-CPU steal: skip the victim's locked loc_l, try next CPU.

 - Post-steal local lock fail: publish stolen node to lockless
   per-CPU free_llist; next pop on this CPU picks it up.

 - push_free fail: mark node pending_free=1. __local_list_flush(),
   __local_list_pop_pending() reclaim the node from pending_list.
   __bpf_lru_list_shrink_inactive() reclaims the node from inactive
   list. Nodes from active list are reclaimed by __bpf_lru_list_shrink()
   or after __bpf_lru_list_rotate_active() demotes it to the inactive.

Fixes: 3a08c2fd7634 ("bpf: LRU List")
Reported-by: syzbot+c69a0a2c816716f1e0d5@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=c69a0a2c816716f1e0d5
Reported-by: syzbot+18b26edb69b2e19f3b33@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=18b26edb69b2e19f3b33
Link: https://lore.kernel.org/bpf/CAPPBnEYO4R+m+SpVc2gNj_x31R6fo1uJvj2bK2YS1P09GWT6kQ@mail.gmail.com/
Link: https://lore.kernel.org/bpf/CAPPBnEZmFA3ab8Uc=PEm0bdojZy=7T_F5_+eyZSHyZR3MBG4Vw@mail.gmail.com/
Link: https://lore.kernel.org/bpf/20251030030010.95352-1-dongml2@chinatelecom.cn/
Link: https://lore.kernel.org/bpf/20260119142120.28170-1-leon.hwang@linux.dev/
Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Link: https://lore.kernel.org/r/20260607-lru_map_spin-v3-1-bcd9332e911b@meta.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

0 files changed, 0 insertions, 0 deletions