bpf: Remove recursion prevention from rcu free callback

If an element is freed via RCU then recursion into BPF instrumentation
functions is not a concern. The element is already detached from the map
and the RCU callback does not hold any locks on which a kprobe, perf event
or tracepoint attached BPF program could deadlock.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200224145643.259118710@linutronix.de

diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index 65711a2..431cef2 100644 (file)
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -706,15 +706,7 @@ static void htab_elem_free_rcu(struct rcu_head *head)
        struct htab_elem *l = container_of(head, struct htab_elem, rcu);
        struct bpf_htab *htab = l->htab;
 
-       /* must increment bpf_prog_active to avoid kprobe+bpf triggering while
-        * we're calling kfree, otherwise deadlock is possible if kprobes
-        * are placed somewhere inside of slub
-        */
-       preempt_disable();
-       __this_cpu_inc(bpf_prog_active);
        htab_elem_free(htab, l);
-       __this_cpu_dec(bpf_prog_active);
-       preempt_enable();
 }
 
 static void free_htab_elem(struct bpf_htab *htab, struct htab_elem *l)