Subject: [patch V3 17/22] bpf: Use recursion prevention helpers in hashtab code

From: Thomas Gleixner <[email-masked]>
Date: Mon, 24 Feb 2020 15:01:48 +0100

The required protection is that the caller cannot be migrated to a
different CPU as these places take either a hash bucket lock or might
trigger a kprobe inside the memory allocator. Both scenarios can lead to
deadlocks. The deadlock prevention is per CPU by incrementing a per CPU
variable which temporarily blocks the invocation of BPF programs from perf
and kprobes.

Replace the open coded preempt_disable/enable() and this_cpu_inc/dec()
pairs with the new recursion prevention helpers to prepare BPF to work on
PREEMPT_RT enabled kernels. On a non-RT kernel the migrate disable/enable
in the helpers map to preempt_disable/enable(), i.e. no functional change.

Signed-off-by: Thomas Gleixner <[email-masked]>
---
V3: Use recursion prevention helpers consistently
---
 kernel/bpf/hashtab.c |   12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -1333,8 +1333,7 @@ static int
 	}
 
 again:
-	preempt_disable();
-	this_cpu_inc(bpf_prog_active);
+	bpf_disable_instrumentation();
 	rcu_read_lock();
 again_nocopy:
 	dst_key = keys;
@@ -1362,8 +1361,7 @@ static int
 		 */
 		raw_spin_unlock_irqrestore(&b->lock, flags);
 		rcu_read_unlock();
-		this_cpu_dec(bpf_prog_active);
-		preempt_enable();
+		bpf_enable_instrumentation();
 		goto after_loop;
 	}
 
@@ -1374,8 +1372,7 @@ static int
 		 */
 		raw_spin_unlock_irqrestore(&b->lock, flags);
 		rcu_read_unlock();
-		this_cpu_dec(bpf_prog_active);
-		preempt_enable();
+		bpf_enable_instrumentation();
 		kvfree(keys);
 		kvfree(values);
 		goto alloc;
@@ -1445,8 +1442,7 @@ static int
 	}
 
 	rcu_read_unlock();
-	this_cpu_dec(bpf_prog_active);
-	preempt_enable();
+	bpf_enable_instrumentation();
 	if (bucket_cnt && (copy_to_user(ukeys + total * key_size, keys,
 	    key_size * bucket_cnt) ||
 	    copy_to_user(uvalues + total * value_size, values,