Re: [PATCH v12 3/3] tracing: Centralize preemptirq tracepoints and unify their usage

[Steven Rostedt]

On Wed, 8 Aug 2018 06:00:41 -0700
"Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> 
> I suppose that an srcu_read_lock_nmi() and srcu_read_unlock_nmi() could
> be added, which would do atomic ops on sp->sda->srcu_lock_count.  Not sure
> whether this would be fast enough to be useful, but easy to provide:
> 
> int __srcu_read_lock_nmi(struct srcu_struct *sp)  /* UNTESTED. */
> {
> 	int idx;
> 
> 	idx = READ_ONCE(sp->srcu_idx) & 0x1;
> 	atomic_inc(&sp->sda->srcu_lock_count[idx]);
> 	smp_mb__after_atomic(); /* B */  /* Avoid leaking critical section. */
> 	return idx;
> }
> 
> void __srcu_read_unlock_nmi(struct srcu_struct *sp, int idx)
> {
> 	smp_mb__before_atomic(); /* C */  /* Avoid leaking critical section. */
> 	atomic_inc(&sp->sda->srcu_unlock_count[idx]);
> }
> 
> With appropriate adjustments to also allow Tiny RCU to also work.
> 
> Note that you have to use _nmi() everywhere, not just in NMI handlers.
> In fact, the NMI handlers are the one place you -don't- need to use
> _nmi(), strangely enough.
> 
> Might be worth a try -- smp_mb__{before,after}_atomic() is a no-op on
> some architectures, for example.

Note this would kill the performance that srcu gives that Joel wants.
Switching from a this_cpu_inc() to a atomic_inc() would be a huge
impact.

There's also a local_inc() if you are using per cpu pointers, that is
suppose to guarantee atomicity for single cpu operations. That's what
the ftrace ring buffer uses.

-- Steve