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Subject[patch V3 17/22] bpf: Use recursion prevention helpers 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 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 <tglx@linutronix.de>
---
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,
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