[PATCH bpf-next] bpf: Avoid unnecessary -EBUSY from htab_lock_bucket

[Song Liu <song@kernel.org>]

htab_lock_bucket uses the following logic to avoid recursion:

1. preempt_disable();
2. check percpu counter htab->map_locked[hash] for recursion;
   2.1. if map_lock[hash] is already taken, return -BUSY;
3. raw_spin_lock_irqsave();

However, if an IRQ hits between 2 and 3, BPF programs attached to the IRQ
logic will not able to access the same hash of the hashtab and get -EBUSY.
This -EBUSY is not really necessary. Fix it by disabling IRQ before
checking map_locked:

1. preempt_disable();
2. local_irq_save();
3. check percpu counter htab->map_locked[hash] for recursion;
   3.1. if map_lock[hash] is already taken, return -BUSY;
4. raw_spin_lock().

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Song Liu <song@kernel.org>
---
 kernel/bpf/hashtab.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index a8c7e1c5abfa..347af4476662 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -155,13 +155,15 @@ static inline int htab_lock_bucket(const struct bpf_htab *htab,
 	hash = hash & min_t(u32, HASHTAB_MAP_LOCK_MASK, htab->n_buckets - 1);
 
 	preempt_disable();
+	local_irq_save(flags);
 	if (unlikely(__this_cpu_inc_return(*(htab->map_locked[hash])) != 1)) {
 		__this_cpu_dec(*(htab->map_locked[hash]));
+		local_irq_restore(flags);
 		preempt_enable();
 		return -EBUSY;
 	}
 
-	raw_spin_lock_irqsave(&b->raw_lock, flags);
+	raw_spin_lock(&b->raw_lock);
 	*pflags = flags;
 
 	return 0;
-- 
2.34.1