Subject: [RFC patch 14/19] bpf: Use migrate_disable() in hashtab code

From: Thomas Gleixner <[email-masked]>
Date: Fri, 14 Feb 2020 14:39:31 +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 preempt_disable/enable() pairs with migrate_disable/enable()
pairs to prepare BPF to work on PREEMPT_RT enabled kernels. On a non-RT
kernel this maps to preempt_disable/enable(), i.e. no functional change.

Signed-off-by: Thomas Gleixner <[email-masked]>
---
 kernel/bpf/hashtab.c |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -698,11 +698,11 @@ static void htab_elem_free_rcu(struct rc
 	 * we're calling kfree, otherwise deadlock is possible if kprobes
 	 * are placed somewhere inside of slub
 	 */
-	preempt_disable();
+	migrate_disable();
 	__this_cpu_inc(bpf_prog_active);
 	htab_elem_free(htab, l);
 	__this_cpu_dec(bpf_prog_active);
-	preempt_enable();
+	migrate_enable();
 }
 
 static void free_htab_elem(struct bpf_htab *htab, struct htab_elem *l)
@@ -1327,7 +1327,7 @@ static int
 	}
 
 again:
-	preempt_disable();
+	migrate_disable();
 	this_cpu_inc(bpf_prog_active);
 	rcu_read_lock();
 again_nocopy:
@@ -1347,7 +1347,7 @@ static int
 		raw_spin_unlock_irqrestore(&b->lock, flags);
 		rcu_read_unlock();
 		this_cpu_dec(bpf_prog_active);
-		preempt_enable();
+		migrate_enable();
 		goto after_loop;
 	}
 
@@ -1356,7 +1356,7 @@ static int
 		raw_spin_unlock_irqrestore(&b->lock, flags);
 		rcu_read_unlock();
 		this_cpu_dec(bpf_prog_active);
-		preempt_enable();
+		migrate_enable();
 		kvfree(keys);
 		kvfree(values);
 		goto alloc;
@@ -1406,7 +1406,7 @@ static int
 
 	rcu_read_unlock();
 	this_cpu_dec(bpf_prog_active);
-	preempt_enable();
+	migrate_enable();
 	if (bucket_cnt && (copy_to_user(ukeys + total * key_size, keys,
 	    key_size * bucket_cnt) ||
 	    copy_to_user(uvalues + total * value_size, values,