Messages in this thread | | | Date | Thu, 6 Apr 2017 13:20:03 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 1/4] ftrace: Add use of synchronize_rcu_tasks() with dynamic trampolines |
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On Thu, Apr 06, 2017 at 02:13:29PM -0400, Steven Rostedt wrote: > On Thu, 6 Apr 2017 11:05:53 -0700 > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote: > > > On Thu, Apr 06, 2017 at 12:42:38PM -0400, Steven Rostedt wrote: > > > From: "Steven Rostedt (VMware)" <rostedt@goodmis.org> > > > > > > The function tracer needs to be more careful than other subsystems when it > > > comes to freeing data. Especially if that data is actually executable code. > > > When a single function is traced, a trampoline can be dynamically allocated > > > which is called to jump to the function trace callback. When the callback is > > > no longer needed, the dynamic allocated trampoline needs to be freed. This > > > is where the issues arise. The dynamically allocated trampoline must not be > > > used again. As function tracing can trace all subsystems, including > > > subsystems that are used to serialize aspects of freeing (namely RCU), it > > > must take extra care when doing the freeing. > > > > > > Before synchronize_rcu_tasks() was around, there was no way for the function > > > tracer to know that nothing was using the dynamically allocated trampoline > > > when CONFIG_PREEMPT was enabled. That's because a task could be indefinitely > > > preempted while sitting on the trampoline. Now with synchronize_rcu_tasks(), > > > it will wait till all tasks have either voluntarily scheduled (not on the > > > trampoline) or goes into userspace (not on the trampoline). Then it is safe > > > to free the trampoline even with CONFIG_PREEMPT set. > > > > > > Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> > > > > One question below. Other than that: > > > > Acked-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> > > Thanks! > > > > > > Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org> > > > --- > > > kernel/trace/Kconfig | 3 ++- > > > kernel/trace/ftrace.c | 42 ++++++++++++++++++------------------------ > > > 2 files changed, 20 insertions(+), 25 deletions(-) > > > > > > diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig > > > index d4a06e714645..67b463b4f169 100644 > > > --- a/kernel/trace/Kconfig > > > +++ b/kernel/trace/Kconfig > > > @@ -134,7 +134,8 @@ config FUNCTION_TRACER > > > select KALLSYMS > > > select GENERIC_TRACER > > > select CONTEXT_SWITCH_TRACER > > > - select GLOB > > > + select GLOB > > > > Does GLOB really want to be selected in production environments? > > I could understand "select GLOB if WE_ARE_TESTING" or some such. > > Note, this patch just fixes the whitespace issue for the "select GLOB". > All the selects had a tab in front of them, where as GLOB had all > spaces. > > As for your question, FUNCTION_TRACER depends on glob. It's what is > used for the function filters. You can do: > > echo '*sync*rcu*' > /sys/kernel/tracing/set_ftrace_filter > > And get: > > synchronize_rcu_tasks > synchronize_srcu > synchronize_srcu_expedited > sync_rcu_exp_select_cpus > sync_rcu_exp_handler > synchronize_rcu_bh.part.65 > synchronize_rcu_expedited > synchronize_rcu.part.67 > synchronize_rcu > synchronize_rcu_bh > > selected. > > So yes, production environments do want it selected.
Now that you mention it, that does seem like it might be important. ;-)
Ah, I was mistakenly looking at GLOB_SELFTEST instead of just plain GLOB -- sorry for the noise!
Thanx, Paul
> -- Steve > > > > > > > + select TASKS_RCU if PREEMPT > > > help > > > Enable the kernel to trace every kernel function. This is done > > > by using a compiler feature to insert a small, 5-byte No-Operation >
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