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SubjectRe: [PATCH 1/3] tracepoints: Do not use call_rcu_sched() before early_initcall()
On Sun, Dec 14, 2014 at 11:53:32AM -0500, Steven Rostedt wrote:
> On Sun, 14 Dec 2014 11:41:05 -0500
> Steven Rostedt <rostedt@goodmis.org> wrote:
>
> > From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>
> >
> > In order to move enabling of trace events to just after mm_init(), the
> > tracepoint enable code can not use call_rcu_sched() because rcu isn't
> > even initialized yet. Since this can only happen before SMP is set up
> > (and even before interrupts are set up), there's no reason to use
> > call_rcu_sched() at this point.
> >
> > Instead, create a variable called tracepoint_rcu_safe that gets enabled
> > via early_initcall() and if that is not set, free the code directly
> > instead of using call_rcu_sched().
> >
> > This allows us to enable tracepoints early without issues.
> >
> > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

With the addition of read_mostly, and given that I am not going to mess
with call_rcu() this late in the 3.19 process without a blazingly good
reason:

Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Please note that you can use call_rcu() and friends as soon as rcu_init()
returns. The callbacks won't be invoked until early_initcall() time,
but they will be properly queued.

Please note also that there are places where turning a call_rcu() into
a direct function call don't work, even at times when preemption is
disabled and there is only one CPU. One example is where single-threaded
code uses call_rcu() on a list element of a list that it is traversing
within an RCU read-side critical section. A direct call to the RCU
callback could potentially destroy the pointers that the traversal was
going to use to find the next element. This means that we cannot make
call_rcu() do direct calls to the callback, as that would break quite
a bit of existing code.

Is there some definite point during boot before which you won't need to
invoke call_rcu_sched() for tracing? I am guessing "no", but have to ask.
I can probably make call_rcu_sched() work arbitrarily early, but it is a
bit uglier. And this assumes that irqs_disabled_flags(local_irq_save())
returns true during early boot. I would -hope- this would be true! ;-)

Thanx, Paul

> > ---
> > kernel/tracepoint.c | 27 ++++++++++++++++++++++++++-
> > 1 file changed, 26 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
> > index 3490407dc7b7..9b90ef0ac731 100644
> > --- a/kernel/tracepoint.c
> > +++ b/kernel/tracepoint.c
> > @@ -33,6 +33,15 @@ extern struct tracepoint * const __stop___tracepoints_ptrs[];
> > /* Set to 1 to enable tracepoint debug output */
> > static const int tracepoint_debug;
> >
> > +/*
> > + * traceoint_rcu_is_safe is set to true at early_initcall().
> > + * Tracepoints can be enabled before rcu is initialized.
> > + * When that happens, there's no reason to free via call_rcu()
> > + * because the system isn't even in SMP mode yet, and rcu isn't
> > + * initialized. Just directly free the old tracepoints instead.
> > + */
> > +static bool tracepoint_rcu_is_safe;
>
> Hmm, I probably should make this read_mostly.
>
> Although, it's not a hot path. Still, it gets set once at boot up and
> never touched again. Yeah, that deserves a read_mostly.
>
> -- Steve
>
> > +
> > #ifdef CONFIG_MODULES
> > /*
> > * Tracepoint module list mutex protects the local module list.
> > @@ -76,7 +85,16 @@ static inline void release_probes(struct tracepoint_func *old)
> > if (old) {
> > struct tp_probes *tp_probes = container_of(old,
> > struct tp_probes, probes[0]);
> > - call_rcu_sched(&tp_probes->rcu, rcu_free_old_probes);
> > + /*
> > + * Tracepoints can be enabled before RCU is initialized
> > + * at boot up. If that is the case, do not bother using
> > + * call_rcu() (because that will fail), but instead just
> > + * free directly.
> > + */
> > + if (likely(tracepoint_rcu_is_safe))
> > + call_rcu_sched(&tp_probes->rcu, rcu_free_old_probes);
> > + else
> > + rcu_free_old_probes(&tp_probes->rcu);
> > }
> > }
> >
> > @@ -518,3 +536,10 @@ void syscall_unregfunc(void)
> > }
> > }
> > #endif
> > +
> > +static __init int init_tracepoint_rcu(void)
> > +{
> > + tracepoint_rcu_is_safe = true;
> > + return 0;
> > +}
> > +early_initcall(init_tracepoint_rcu);
>



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