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Subject[RFC patch 14/19] bpf: Use migrate_disable() in hashtab code
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 <tglx@linutronix.de>
---
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,
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