bpf: Make sure bpf_disable_instrumentation() is safe vs preemption.

The initial implementation of migrate_disable() for mainline was a
wrapper around preempt_disable(). RT kernels substituted this with a
real migrate disable implementation.

Later on mainline gained true migrate disable support, but neither
documentation nor affected code were updated.

Remove stale comments claiming that migrate_disable() is PREEMPT_RT only.

Don't use __this_cpu_inc() in the !PREEMPT_RT path because preemption is
not disabled and the RMW operation can be preempted.

Fixes: 74d862b682f51 ("sched: Make migrate_disable/enable() independent of RT")
Signed-off-by: Sebastian Andrzej Siewior <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

include/linux/bpf.h

···
  * kprobes, tracepoints) to prevent deadlocks on map operations as any of
  * these events can happen inside a region which holds a map bucket lock
  * and can deadlock on it.
- *
- * Use the preemption safe inc/dec variants on RT because migrate disable
- * is preemptible on RT and preemption in the middle of the RMW operation
- * might lead to inconsistent state. Use the raw variants for non RT
- * kernels as migrate_disable() maps to preempt_disable() so the slightly
- * more expensive save operation can be avoided.
  */
 static inline void bpf_disable_instrumentation(void)
 {
 	migrate_disable();
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		this_cpu_inc(bpf_prog_active);
-	else
-		__this_cpu_inc(bpf_prog_active);
+	this_cpu_inc(bpf_prog_active);
 }
 
 static inline void bpf_enable_instrumentation(void)
 {
-	if (IS_ENABLED(CONFIG_PREEMPT_RT))
-		this_cpu_dec(bpf_prog_active);
-	else
-		__this_cpu_dec(bpf_prog_active);
+	this_cpu_dec(bpf_prog_active);
 	migrate_enable();
 }
 

include/linux/filter.h

···
  * This uses migrate_disable/enable() explicitly to document that the
  * invocation of a BPF program does not require reentrancy protection
  * against a BPF program which is invoked from a preempting task.
- *
- * For non RT enabled kernels migrate_disable/enable() maps to
- * preempt_disable/enable(), i.e. it disables also preemption.
  */
 static inline u32 bpf_prog_run_pin_on_cpu(const struct bpf_prog *prog,
 					  const void *ctx)