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SubjectRe: ftrace: preemptoff selftest not working
On Tue, Nov 18, 2008 at 03:47:51PM +0100, Ingo Molnar wrote:
>
> * Steven Rostedt <rostedt@goodmis.org> wrote:
>
> >
> > On Tue, 18 Nov 2008, Ingo Molnar wrote:
> > >
> > > * Steven Rostedt <rostedt@goodmis.org> wrote:
> > >
> > > > > Hence the trace buffer will be empty. The patch below makes the
> > > > > selftests working for me, since then they run in preemptible
> > > > > context. But it is ugly and I'm not proposing it for upstream ;)
> > > > >
> > > > > Just wanted to make you aware that there is a bug.
> > > >
> > > > Yep, this might be a better answer than what I put into linux-tip
> > > > (and my git repo).
> > > >
> > > > See:
> > > >
> > > > ftrace: force pass of preemptoff selftest
> > > >
> > > > The cause of the bug was the conversion of the BKL back to a
> > > > spinlock, and making it non preempt. The initcall code is called
> > > > with the BKL applied which now means it can not preempt. This breaks
> > > > the preempt tracer selftest.
> > > >
> > > > My solution was to just force a pass if this is detected. Perhaps
> > > > moving the test might be better.
> > >
> > > it would be better to just drop the BKL in that selftest. (or in all
> > > selftests - an elevated preempt count will skew a number of things)
> >
> > I have no problem with that, but does the BKL play any role for
> > being held? I have no idea why it is taken in boot up, so I'm
> > hestiant to touch it.
>
> we can drop it in selected initcalls just fine. Its only role is
> old-style init functions racing with other async contexts of
> themselves.

Something like below works fine for me...

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
---

kernel/trace/trace.c | 8 ++++++++
1 file changed, 8 insertions(+)

Index: linux-2.6/kernel/trace/trace.c
===================================================================
--- linux-2.6.orig/kernel/trace/trace.c
+++ linux-2.6/kernel/trace/trace.c
@@ -482,6 +482,13 @@ int register_tracer(struct tracer *type)
}

#ifdef CONFIG_FTRACE_STARTUP_TEST
+ /*
+ * When this gets called we hold the BKL which means that preemption
+ * is disabled. Various trace selftests however need to disable
+ * and enable preemption for successful tests. So we drop the BKL here
+ * and grab it after the tests again.
+ */
+ unlock_kernel();
if (type->selftest) {
struct tracer *saved_tracer = current_trace;
struct trace_array *tr = &global_trace;
@@ -515,6 +522,7 @@ int register_tracer(struct tracer *type)
}
printk(KERN_CONT "PASSED\n");
}
+ lock_kernel();
#endif

type->next = trace_types;

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