Re: [PATCH RFC] tracepoint: Introduce tracepoint callbacks executing with preempt on

Paul E. McKenney
2018-04-27T16:43:09Z

On Fri, Apr 27, 2018 at 12:13:30PM -0400, Steven Rostedt wrote:
> On Fri, 27 Apr 2018 08:57:01 -0700
> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
>
> > > + if (preempt_on) { \
> > > + WARN_ON_ONCE(in_nmi()); /* no srcu from nmi */ \
> >
> > Very good on this check, thank you!
>
> I think you need to return and not call the read lock.

Works for me either way, at least assuming that the splat actually gets
printed. ;-)

> if (WARN_ON_ONCE(in_nmi()))
> return;
>
> >
> > > + idx = srcu_read_lock(&tracepoint_srcu); \
> >
> > Hmmm... Do I need to create a _notrace variant of srcu_read_lock()
> > and srcu_read_unlock()?
>
> I think so.

OK, please see the (untested) patch below. Of course,
srcu_read_lock_notrace() invokes __srcu_read_lock(), which looks as
follows:

int __srcu_read_lock(struct srcu_struct *sp)
{
int idx;

idx = READ_ONCE(sp->srcu_idx) & 0x1;
this_cpu_inc(sp->sda->srcu_lock_count[idx]);
smp_mb(); /* B */ /* Avoid leaking the critical section. */
return idx;
}

Do I also need to make a notrace version of __srcu_read_lock()?
Same question for __srcu_read_unlock(), which is similar. If so,
assuming that there is no need for a notrace variant of this_cpu_inc()
and smp_mb(), I suppose I could simply macro-ize the internals in both
cases, but perhaps you have a better approach.

Thanx, Paul

------------------------------------------------------------------------

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index 91494d7e8e41..e2e2cf05a6eb 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -195,6 +195,16 @@ static inline int srcu_read_lock(struct srcu_struct *sp) __acquires(sp)
return retval;
}

+/* Used by tracing, cannot be traced and cannot invoke lockdep. */
+static inline notrace int
+srcu_read_lock_notrace(struct srcu_struct *sp) __acquires(sp)
+{
+ int retval;
+
+ retval = __srcu_read_lock(sp);
+ return retval;
+}
+
/**
* srcu_read_unlock - unregister a old reader from an SRCU-protected structure.
* @sp: srcu_struct in which to unregister the old reader.
@@ -205,6 +215,13 @@ static inline int srcu_read_lock(struct srcu_struct *sp) __acquires(sp)
static inline void srcu_read_unlock(struct srcu_struct *sp, int idx)
__releases(sp)
{
+ __srcu_read_unlock(sp, idx);
+}
+
+/* Used by tracing, cannot be traced and cannot call lockdep. */
+static inline notrace void
+srcu_read_unlock_notrace(struct srcu_struct *sp, int idx) __releases(sp)
+{
rcu_lock_release(&(sp)->dep_map);
__srcu_read_unlock(sp, idx);
}