On 1/19/26 3:21 PM, Leon Hwang wrote:
Switch the free-node pop paths to raw_spin_trylock*() to avoid blocking
on contended LRU locks.
If the global or per-CPU LRU lock is unavailable, refuse to refill the
local free list and return NULL instead. This allows callers to back
off safely rather than blocking or re-entering the same lock context.
This change avoids lockdep warnings and potential deadlocks caused by
re-entrant LRU lock acquisition from NMI context, as shown below:
[ 418.260323] bpf_testmod: oh no, recursing into test_1, recursion_misses 1
[ 424.982207] ================================
[ 424.982216] WARNING: inconsistent lock state
[ 424.982223] inconsistent {INITIAL USE} -> {IN-NMI} usage.
[ 424.982314] *** DEADLOCK ***
[...]
Signed-off-by: Leon Hwang <[email protected]>
---
kernel/bpf/bpf_lru_list.c | 17 ++++++++++-------
1 file changed, 10 insertions(+), 7 deletions(-)
Documentation/bpf/map_lru_hash_update.dot needs update?
diff --git a/kernel/bpf/bpf_lru_list.c b/kernel/bpf/bpf_lru_list.c
index c091f3232cc5..03d37f72731a 100644
--- a/kernel/bpf/bpf_lru_list.c
+++ b/kernel/bpf/bpf_lru_list.c
@@ -312,14 +312,15 @@ static void bpf_lru_list_push_free(struct bpf_lru_list *l,
raw_spin_unlock_irqrestore(&l->lock, flags);
}
-static void bpf_lru_list_pop_free_to_local(struct bpf_lru *lru,
+static bool bpf_lru_list_pop_free_to_local(struct bpf_lru *lru,
struct bpf_lru_locallist *loc_l)
{
struct bpf_lru_list *l = &lru->common_lru.lru_list;
struct bpf_lru_node *node, *tmp_node;
unsigned int nfree = 0;
- raw_spin_lock(&l->lock);
+ if (!raw_spin_trylock(&l->lock))
+ return false;
Could you provide some more analysis, and the effect this has on real-world
programs? Presumably they'll unexpectedly encounter a lot more frequent
-ENOMEM as an error on bpf_map_update_elem even though memory might be
available just that locks are contended?
Also, have you considered rqspinlock as a potential candidate to discover
deadlocks?
