Inorder to support recording of tgid, the following changes are made:

* Introduce a new API (tracing_record_taskinfo) to additionally record the tgid
  along with the task's comm at the same time. This has has the benefit of not
  setting trace_cmdline_save before all the information for a task is saved.
* Add a new API tracing_record_taskinfo_sched_switch to record task information
  for 2 tasks at a time (previous and next) and use it from sched_switch probe.
* Preserve the old API (tracing_record_cmdline) and create it as a wrapper
  around the new one so that existing callers aren't affected.
* Reuse the existing sched_switch and sched_wakeup probes to record tgid
  information and add a new option 'record-tgid' to enable recording of tgid

When record-tgid option isn't enabled to being with, we take care to make sure
that there's isn't memory or runtime overhead.

Link: http://lkml.kernel.org/r/20170627020155.5139-1-joelaf@google.com

Cc: kernel-team@android.com
Cc: Ingo Molnar <mingo@redhat.com>
Tested-by: NMichael Sartain <mikesart@gmail.com>
Signed-off-by: NJoel Fernandes <joelaf@google.com>
Signed-off-by: NSteven Rostedt (VMware) <rostedt@goodmis.org>

__trace_sched_switch_state() is the last remaining PREEMPT_ACTIVE
user, move trace_sched_switch() from prepare_task_switch() to
__schedule() and propagate the @preempt argument.
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: NThomas Gleixner <tglx@linutronix.de>
Reviewed-by: NSteven Rostedt <rostedt@goodmis.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: NIngo Molnar <mingo@kernel.org>

Mathieu reported that since 317f3941 ("sched: Move the second half
of ttwu() to the remote cpu") trace_sched_wakeup() can happen out of
context of the waker.

This is a problem when you want to analyse wakeup paths because it is
now very hard to correlate the wakeup event to whoever issued the
wakeup.

OTOH trace_sched_wakeup() is issued at the point where we set
p->state = TASK_RUNNING, which is right were we hand the task off to
the scheduler, so this is an important point when looking at
scheduling behaviour, up to here its been the wakeup path everything
hereafter is due to scheduler policy.

To bridge this gap, introduce a second tracepoint: trace_sched_waking.
It is guaranteed to be called in the waker context.
Reported-by: NMathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Francis Giraldeau <francis.giraldeau@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20150609091336.GQ3644@twins.programming.kicks-ass.netSigned-off-by: NIngo Molnar <mingo@kernel.org>

The creation of tracing files and directories is for the most part
encapsulated in helper functions in trace.c. Other files do not need to
include debugfs.h or fs.h, as they may have needed to in the past.

Remove them from the files that do not need them.
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

The only code that references tracing_sched_switch_trace() and
tracing_sched_wakeup_trace() is the wakeup latency tracer. Those
two functions use to belong to the sched_switch tracer which has
long been removed. These functions were left behind because the
wakeup latency tracer used them. But since the wakeup latency tracer
is the only one to use them, they should be static functions inside
that code.
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

After the previous patch it is clear that "tracer_enabled" can never be
true, we can remove the "if (tracer_enabled)" code in probe_sched_switch()
and probe_sched_wakeup(). Plus we can obviously remove tracer_enabled,
ctx_trace, and sched_stopped as well.

Link: http://lkml.kernel.org/p/20140723193503.GA30217@redhat.comSigned-off-by: NOleg Nesterov <oleg@redhat.com>
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

tracing_{start,stop}_sched_switch_record() have no callers since
87d80de2 "tracing: Remove obsolete sched_switch tracer".

The last caller of tracing_sched_switch_assign_trace() was removed
by 30dbb20e "tracing: Remove boot tracer".

Link: http://lkml.kernel.org/p/20140723193501.GA30214@redhat.comSigned-off-by: NOleg Nesterov <oleg@redhat.com>
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

The trace event filters are still tied to event calls rather than
event files, which means you don't get what you'd expect when using
filters in the multibuffer case:

Before:

  # echo 'bytes_alloc > 8192' > /sys/kernel/debug/tracing/events/kmem/kmalloc/filter
  # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/filter
  bytes_alloc > 8192
  # mkdir /sys/kernel/debug/tracing/instances/test1
  # echo 'bytes_alloc > 2048' > /sys/kernel/debug/tracing/instances/test1/events/kmem/kmalloc/filter
  # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/filter
  bytes_alloc > 2048
  # cat /sys/kernel/debug/tracing/instances/test1/events/kmem/kmalloc/filter
  bytes_alloc > 2048

Setting the filter in tracing/instances/test1/events shouldn't affect
the same event in tracing/events as it does above.

After:

  # echo 'bytes_alloc > 8192' > /sys/kernel/debug/tracing/events/kmem/kmalloc/filter
  # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/filter
  bytes_alloc > 8192
  # mkdir /sys/kernel/debug/tracing/instances/test1
  # echo 'bytes_alloc > 2048' > /sys/kernel/debug/tracing/instances/test1/events/kmem/kmalloc/filter
  # cat /sys/kernel/debug/tracing/events/kmem/kmalloc/filter
  bytes_alloc > 8192
  # cat /sys/kernel/debug/tracing/instances/test1/events/kmem/kmalloc/filter
  bytes_alloc > 2048

We'd like to just move the filter directly from ftrace_event_call to
ftrace_event_file, but there are a couple cases that don't yet have
multibuffer support and therefore have to continue using the current
event_call-based filters.  For those cases, a new USE_CALL_FILTER bit
is added to the event_call flags, whose main purpose is to keep the
old behavior for those cases until they can be updated with
multibuffer support; at that point, the USE_CALL_FILTER flag (and the
new associated call_filter_check_discard() function) can go away.

The multibuffer support also made filter_current_check_discard()
redundant, so this change removes that function as well and replaces
it with filter_check_discard() (or call_filter_check_discard() as
appropriate).

Link: http://lkml.kernel.org/r/f16e9ce4270c62f46b2e966119225e1c3cca7e60.1382620672.git.tom.zanussi@linux.intel.comSigned-off-by: NTom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

Currently, the way the latency tracers and snapshot feature works
is to have a separate trace_array called "max_tr" that holds the
snapshot buffer. For latency tracers, this snapshot buffer is used
to swap the running buffer with this buffer to save the current max
latency.

The only items needed for the max_tr is really just a copy of the buffer
itself, the per_cpu data pointers, the time_start timestamp that states
when the max latency was triggered, and the cpu that the max latency
was triggered on. All other fields in trace_array are unused by the
max_tr, making the max_tr mostly bloat.

This change removes the max_tr completely, and adds a new structure
called trace_buffer, that holds the buffer pointer, the per_cpu data
pointers, the time_start timestamp, and the cpu where the latency occurred.

The trace_array, now has two trace_buffers, one for the normal trace and
one for the max trace or snapshot. By doing this, not only do we remove
the bloat from the max_trace but the instances of traces can now use
their own snapshot feature and not have just the top level global_trace have
the snapshot feature and latency tracers for itself.
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

The global and max-tr currently use static per_cpu arrays for the CPU data
descriptors. But in order to get new allocated trace_arrays, they need to
be allocated per_cpu arrays. Instead of using the static arrays, switch
the global and max-tr to use allocated data.
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

Have the ring buffer commit function use the irq_work infrastructure to
wake up any waiters waiting on the ring buffer for new data. The irq_work
was created for such a purpose, where doing the actual wake up at the
time of adding data is too dangerous, as an event or function trace may
be in the midst of the work queue locks and cause deadlocks. The irq_work
will either delay the action to the next timer interrupt, or trigger an IPI
to itself forcing an interrupt to do the work (in a safe location).

With irq_work, all ring buffer commits can safely do wakeups, removing
the need for the ring buffer commit "nowake" variants, which were used
by events and function tracing. All commits can now safely use the
normal commit, and the "nowake" variants can be removed.

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

The functon tracing_sched_wakeup_trace() does an open coded unlock
commit and save stack. This is what the trace_nowake_buffer_unlock_commit()
is for.
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

The trace events sched_switch and sched_wakeup do the same thing
as the stand alone sched_switch tracer does. It is no longer needed.
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

This patch adds data to be passed to tracepoint callbacks.

The created functions from DECLARE_TRACE() now need a mandatory data
parameter. For example:

DECLARE_TRACE(mytracepoint, int value, value)

Will create the register function:

int register_trace_mytracepoint((void(*)(void *data, int value))probe,
                                void *data);

As the first argument, all callbacks (probes) must take a (void *data)
parameter. So a callback for the above tracepoint will look like:

void myprobe(void *data, int value)
{
}

The callback may choose to ignore the data parameter.

This change allows callbacks to register a private data pointer along
with the function probe.

	void mycallback(void *data, int value);

	register_trace_mytracepoint(mycallback, mydata);

Then the mycallback() will receive the "mydata" as the first parameter
before the args.

A more detailed example:

  DECLARE_TRACE(mytracepoint, TP_PROTO(int status), TP_ARGS(status));

  /* In the C file */

  DEFINE_TRACE(mytracepoint, TP_PROTO(int status), TP_ARGS(status));

  [...]

       trace_mytracepoint(status);

  /* In a file registering this tracepoint */

  int my_callback(void *data, int status)
  {
	struct my_struct my_data = data;
	[...]
  }

  [...]
	my_data = kmalloc(sizeof(*my_data), GFP_KERNEL);
	init_my_data(my_data);
	register_trace_mytracepoint(my_callback, my_data);

The same callback can also be registered to the same tracepoint as long
as the data registered is different. Note, the data must also be used
to unregister the callback:

	unregister_trace_mytracepoint(my_callback, my_data);

Because of the data parameter, tracepoints declared this way can not have
no args. That is:

  DECLARE_TRACE(mytracepoint, TP_PROTO(void), TP_ARGS());

will cause an error.

If no arguments are needed, a new macro can be used instead:

  DECLARE_TRACE_NOARGS(mytracepoint);

Since there are no arguments, the proto and args fields are left out.

This is part of a series to make the tracepoint footprint smaller:

   text	   data	    bss	    dec	    hex	filename
4913961	1088356	 861512	6863829	 68bbd5	vmlinux.orig
4914025	1088868	 861512	6864405	 68be15	vmlinux.class
4918492	1084612	 861512	6864616	 68bee8	vmlinux.tracepoint

Again, this patch also increases the size of the kernel, but
lays the ground work for decreasing it.

 v5: Fixed net/core/drop_monitor.c to handle these updates.

 v4: Moved the DECLARE_TRACE() DECLARE_TRACE_NOARGS out of the
     #ifdef CONFIG_TRACE_POINTS, since the two are the same in both
     cases. The __DECLARE_TRACE() is what changes.
     Thanks to Frederic Weisbecker for pointing this out.

 v3: Made all register_* functions require data to be passed and
     all callbacks to take a void * parameter as its first argument.
     This makes the calling functions comply with C standards.

     Also added more comments to the modifications of DECLARE_TRACE().

 v2: Made the DECLARE_TRACE() have the ability to pass arguments
     and added a new DECLARE_TRACE_NOARGS() for tracepoints that
     do not need any arguments.
Acked-by: NMathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: NMasami Hiramatsu <mhiramat@redhat.com>
Acked-by: NFrederic Weisbecker <fweisbec@gmail.com>
Cc: Neil Horman <nhorman@tuxdriver.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

struct rq isn't visible outside of sched.o so its near useless to
expose the pointer, also there are no users of it, so remove it.
Acked-by: NSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1272997616.1642.207.camel@laptop>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

The latency tracers (irqsoff and wakeup) can swap trace buffers
on the fly. If an event is happening and has reserved data on one of
the buffers, and the latency tracer swaps the global buffer with the
max buffer, the result is that the event may commit the data to the
wrong buffer.

This patch changes the API to the trace recording to be recieve the
buffer that was used to reserve a commit. Then this buffer can be passed
in to the commit.
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

The sched events helpers which insert the sched switch and wakeup
events into the ring buffer currently reside in trace.c
But this file is quite overloaded and the right place for these helpers
is in the sched switch tracer file.

Then move them to trace_functions.c
Signed-off-by: NFrederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>

Impact: clean up

Create a sub directory in include/trace called events to keep the
trace point headers in their own separate directory. Only headers that
declare trace points should be defined in this directory.

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Neil Horman <nhorman@tuxdriver.com>
Cc: Zhao Lei <zhaolei@cn.fujitsu.com>
Cc: Eduard - Gabriel Munteanu <eduard.munteanu@linux360.ro>
Cc: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

Unify sched_switch and sched_wakeup's action to following logic:
Do record_cmdline when start_cmdline_record() is called.
Start tracing events when the tracer is started.
Signed-off-by: NZhao Lei <zhaolei@cn.fujitsu.com>
LKML-Reference: <49D1C596.5050203@cn.fujitsu.com>
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

The wakeup tracing in sched_switch does not stop when a user
disables tracing. This is because the probe_sched_wakeup() is missing
the check to prevent the wakeup from being traced.
Signed-off-by: NZhao Lei <zhaolei@cn.fujitsu.com>
LKML-Reference: <49D1C543.3010307@cn.fujitsu.com>
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

The stopping and starting of a tracer should be light weight and
be able to be called in all contexts. The sched_switch grabbed
mutexes in the start/stop functions. This patch changes it to a
simple variable, on/off.
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>

Impact: api and pipe waiting change

Currently, the waiting used in tracing_read_pipe() is done through a
100 msecs schedule_timeout() loop which periodically check if there
are traces on the buffer.

This can cause small latencies for programs which are reading the incoming
events.

This patch makes the reader waiting for the trace_wait waitqueue except
for few tracers such as the sched and functions tracers which might be
already hold the runqueue lock while waking up the reader.

This is performed through a new callback wait_pipe() on struct tracer.
If none is implemented on a specific tracer, the default waiting for
trace_wait queue is attached.
Signed-off-by: NFrederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Impact: cleanup

Fix incorrect hint message in code and typos in comments.
Signed-off-by: NWenji Huang <wenji.huang@oracle.com>
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>

Impact: cleanup

To make it easy for ftrace plugin writers, as this was open coded in
the existing plugins
Signed-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: NFrédéric Weisbecker <fweisbec@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Impact: cleanup
Signed-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Impact: extend the wakeup tracepoint with the info whether the wakeup was real

Add the information needed to distinguish 'real' wakeups from 'false'
wakeups.
Signed-off-by: NPeter Zijlstra <peterz@infradead.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Impact: cleanup

This patch factors out common code from multiple tracers into a
tracing_reset_online_cpus() function and converts the tracers to use it.
Signed-off-by: NPekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

these warnings:

  kernel/trace/trace_sched_switch.c: In function ‘tracing_sched_register’:
  kernel/trace/trace_sched_switch.c:96: warning: passing argument 1 of ‘register_trace_sched_wakeup_new’ from incompatible pointer type
  kernel/trace/trace_sched_switch.c:112: warning: passing argument 1 of ‘unregister_trace_sched_wakeup_new’ from incompatible pointer type
  kernel/trace/trace_sched_switch.c: In function ‘tracing_sched_unregister’:
  kernel/trace/trace_sched_switch.c:121: warning: passing argument 1 of ‘unregister_trace_sched_wakeup_new’ from incompatible pointer type

Trigger because sched_wakeup_new tracepoints need the same trace
signature as sched_wakeup - which was changed recently.

Fix it.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Impact: extend the ->init() method with the ability to fail

This bring a way to know if the initialization of a tracer successed.
A tracer must return 0 on success and a traditional error (ie:
-ENOMEM) if it fails.

If a tracer fails to init, it is free to print a detailed warn. The
tracing api will not and switch to a new tracer will just return the
error from the init callback.

Note: this will be used for the return tracer.
Signed-off-by: NFrederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Impact: remove obsolete variable in trace_array structure

With the new start / stop method of ftrace, the ctrl variable
in the trace_array structure is now obsolete. Remove it.
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Impact: Remove the ctrl_update tracer method

With the new quick start/stop method of tracing, the ctrl_update
method is out of date.
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Impact: fix for sched_switch that broke dynamic ftrace startup

The commit: tracing/fastboot: use sched switch tracer from boot tracer
broke the API of the sched_switch trace. The use of the
tracing_start/stop_cmdline record is for only recording the cmdline,
NOT recording the schedule switches themselves.

Seeing that the boot tracer broke the API to do something that it
wanted, this patch adds a new interface for the API while
puting back the original interface of the old API.
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Acked-by: NFrederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Impact: boot tracer startup modified

The boot tracer calls into some of the schedule tracing private functions
that should not be exported. This patch cleans it up, and makes
way for further changes in the ftrace infrastructure.

This patch adds a api to assign a tracer array to the schedule
context switch tracer.
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Acked-by: NFrederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Impact: change where tracing is started up and stopped

Currently, when a new tracer is selected via echo'ing a tracer name into
the current_tracer file, the startup is only done if tracing_enabled is
set to one. If tracing_enabled is changed to zero (by echo'ing 0 into
the tracing_enabled file) a full shutdown is performed.

The full startup and shutdown of a tracer can be expensive and the
user can lose out traces when echo'ing in 0 to the tracing_enabled file,
because the process takes too long. There can also be places that
the user would like to start and stop the tracer several times and
doing the full startup and shutdown of a tracer might be too expensive.

This patch performs the full startup and shutdown when a tracer is
selected. It also adds a way to do a quick start or stop of a tracer.
The quick version is just a flag that prevents the tracing from
taking place, but the overhead of the code is still there.

For example, the startup of a tracer may enable tracepoints, or enable
the function tracer.  The stop and start will just set a flag to
have the tracer ignore the calls when the tracepoint or function trace
is called.  The overhead of the tracer may still be present when
the tracer is stopped, but no tracing will occur. Setting the tracer
to the 'nop' tracer (or any other tracer) will perform the shutdown
of the tracer which will disable the tracepoint or disable the
function tracer.

The tracing_enabled file will simply start or stop tracing.

This change is all internal. The end result for the user should be the same
as before. If tracing_enabled is not set, no trace will happen.
If tracing_enabled is set, then the trace will happen. The tracing_enabled
variable is static between tracers. Enabling  tracing_enabled and
going to another tracer will keep tracing_enabled enabled. Same
is true with disabling tracing_enabled.

This patch will now provide a fast start/stop method to the users
for enabling or disabling tracing.

Note: There were two methods to the struct tracer that were never
 used: The methods start and stop. These were to be used as a hook
 to the reading of the trace output, but ended up not being
 necessary. These two methods are now used to enable the start
 and stop of each tracer, in case the tracer needs to do more than
 just not write into the buffer. For example, the irqsoff tracer
 must stop recording max latencies when tracing is stopped.
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Impact: fix boot tracer + sched tracer coupling bug

Fix a bug that made the sched_switch tracer unable to run
if set as the current_tracer after the boot tracer.
Signed-off-by: NFrederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Impact: cleanup

This patch applies some corrections suggested by Steven Rostedt.

Change the type of shed_ref into int since it is used
into a Mutex, we don't need it anymore as an atomic
variable in the sched_switch tracer.
Also change the name of the register mutex.
Signed-off-by: NFrederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Impact: enhance boot trace output with scheduling events

Use the sched_switch tracer from the boot tracer.

We also can trace schedule events inside the initcalls.
Sched tracing is disabled after the initcall has finished and
then reenabled before the next one is started.
Signed-off-by: NFrederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Impact: cleanup

When init_sched_switch_trace() is called, it has no reason to start
the sched tracer if the sched_ref is not zero.

_ If this is non-zero, the tracer is already used, but we can register it
to the tracing engine. There is already a security which avoid the tracer
probes not to be resgistered twice.

_ If this is zero, this block will not be used.
Signed-off-by: NFrederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Impact: fix race condition in sched_switch tracer

This patch fixes a race condition in the sched_switch tracer. If
several tasks (IE: concurrent initcalls) are playing with
tracing_start_cmdline_record() and tracing_stop_cmdline_record(), the
following situation could happen:

_ Task A and B are using the same tracepoint probe. Task A holds it.
  Task B is sleeping and doesn't hold it.

_ Task A frees the sched tracer, then sched_ref is decremented to 0.

_ Task A is preempted and hadn't yet unregistered its tracepoint
  probe, then B runs.

_ B increments sched_ref, sees it's 1 and then guess it has to
  register its probe. But it has not been freed by task A.

_ A lot of bad things can happen after that...
Signed-off-by: NFrederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Now that the ring buffer is reentrant, some of the ftrace tracers
(sched_swich, debugging traces) can also be reentrant.

Note: Never make the function tracer reentrant, that can cause
  recursion problems all over the kernel. The function tracer
  must disable reentrancy.
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>