kthread: Implement park/unpark facility

To avoid the full teardown/setup of per cpu kthreads in the case of
cpu hot(un)plug, provide a facility which allows to put the kthread
into a park position and unpark it when the cpu comes online again.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Reviewed-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20120716103948.236618824@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>
kthread: Prevent unpark race which puts threads on the wrong cpu
adds smpboot.c, smpboot.h to /kernel and different smpboot.h to /include/linux

The smpboot threads rely on the park/unpark mechanism which binds per
cpu threads on a particular core. Though the functionality is racy:

CPU0	       	 	CPU1  	     	    CPU2
unpark(T)				    wake_up_process(T)
  clear(SHOULD_PARK)	T runs
			leave parkme() due to !SHOULD_PARK
  bind_to(CPU2)		BUG_ON(wrong CPU)

We cannot let the tasks move themself to the target CPU as one of
those tasks is actually the migration thread itself, which requires
that it starts running on the target cpu right away.

The solution to this problem is to prevent wakeups in park mode which
are not from unpark(). That way we can guarantee that the association
of the task to the target cpu is working correctly.

Add a new task state (TASK_PARKED) which prevents other wakeups and
use this state explicitly for the unpark wakeup.

Peter noticed: Also, since the task state is visible to userspace and
all the parked tasks are still in the PID space, its a good hint in ps
and friends that these tasks aren't really there for the moment.

The migration thread has another related issue.

CPU0	      	     	 CPU1
Bring up CPU2
create_thread(T)
park(T)
 wait_for_completion()
			 parkme()
			 complete()
sched_set_stop_task()
			 schedule(TASK_PARKED)

The sched_set_stop_task() call is issued while the task is on the
runqueue of CPU1 and that confuses the hell out of the stop_task class
on that cpu. So we need the same synchronizaion before
sched_set_stop_task().

Reported-by: Dave Jones <davej@redhat.com>
Reported-and-tested-by: Dave Hansen <dave@sr71.net>
Reported-and-tested-by: Borislav Petkov <bp@alien8.de>
Acked-by: Peter Ziljstra <peterz@infradead.org>
Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Cc: dhillf@gmail.com
Cc: Ingo Molnar <mingo@kernel.org>
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/alpine.LFD.2.02.1304091635430.21884@ionos
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>
kthread: Fix the race condition when kthread is parked

While stressing the CPU hotplug path, sometimes we hit a problem
as shown below.

[57056.416774] ------------[ cut here ]------------
[57056.489232] ksoftirqd/1 (14): undefined instruction: pc=c01931e8
[57056.489245] Code: e594a000 eb085236 e15a0000 0a000000 (e7f001f2)
[57056.489259] ------------[ cut here ]------------
[57056.492840] kernel BUG at kernel/kernel/smpboot.c:134!
[57056.513236] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP ARM
[57056.519055] Modules linked in: wlan(O) mhi(O)
[57056.523394] CPU: 0 PID: 14 Comm: ksoftirqd/1 Tainted: G        W  O 3.10.0-g3677c61-00008-g180c060 #1
[57056.532595] task: f0c8b000 ti: f0e78000 task.ti: f0e78000
[57056.537991] PC is at smpboot_thread_fn+0x124/0x218
[57056.542750] LR is at smpboot_thread_fn+0x11c/0x218
[57056.547528] pc : [<c01931e8>]    lr : [<c01931e0>]    psr: 200f0013
[57056.547528] sp : f0e79f30  ip : 00000000  fp : 00000000
[57056.558983] r10: 00000001  r9 : 00000000  r8 : f0e78000
[57056.564192] r7 : 00000001  r6 : c1195758  r5 : f0e78000  r4 : f0e5fd00
[57056.570701] r3 : 00000001  r2 : f0e79f20  r1 : 00000000  r0 : 00000000

This issue was always seen in the context of "ksoftirqd". It seems to
be happening because of a potential race condition in __kthread_parkme
where just after completing the parked completion, before the
ksoftirqd task has been scheduled again, it can go into running state.

Fix this by waiting for the task state to parked after waiting the
parked completion.

CRs-Fixed: 659674
Change-Id: If3f0e9b706eeb5d30d5a32f84378d35bb03fe794
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>
kthread: introduce to_live_kthread()

"k->vfork_done != NULL" with a barrier() after to_kthread(k) in
task_get_live_kthread(k) looks unclear, and sub-optimal because we load
->vfork_done twice.

All we need is to ensure that we do not return to_kthread(NULL).  Add a
new trivial helper which loads/checks ->vfork_done once, this also looks
more understandable.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>
kthread: kill task_get_live_kthread()

task_get_live_kthread() looks confusing and unneeded.  It does
get_task_struct() but only kthread_stop() needs this, it can be called
even if the calller doesn't have a reference when we know that this
kthread can't exit until we do kthread_stop().

kthread_park() and kthread_unpark() do not need get_task_struct(), the
callers already have the reference.  And it can not help if we can race
with the exiting kthread anyway, kthread_park() can hang forever in this
case.

Change kthread_park() and kthread_unpark() to use to_live_kthread(),
change kthread_stop() to do get_task_struct() by hand and remove
task_get_live_kthread().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>
kthread: implement probe_kthread_data()

One of the problems that arise when converting dedicated custom threadpool
to workqueue is that the shared worker pool used by workqueue anonimizes
each worker making it more difficult to identify what the worker was doing
on which target from the output of sysrq-t or debug dump from oops, BUG()
and friends.

For example, after writeback is converted to use workqueue instead of
priviate thread pool, there's no easy to tell which backing device a
writeback work item was working on at the time of task dump, which,
according to our writeback brethren, is important in tracking down issues
with a lot of mounted file systems on a lot of different devices.

This patchset implements a way for a work function to mark its execution
instance so that task dump of the worker task includes information to
indicate what the work item was doing.

An example WARN dump would look like the following.

 WARNING: at fs/fs-writeback.c:1015 bdi_writeback_workfn+0x2b4/0x3c0()
 Modules linked in:
 CPU: 0 Pid: 28 Comm: kworker/u18:0 Not tainted 3.9.0-rc1-work+ #24
 Hardware name: empty empty/S3992, BIOS 080011  10/26/2007
 Workqueue: writeback bdi_writeback_workfn (flush-8:16)
  ffffffff820a3a98 ffff88015b927cb8 ffffffff81c61855 ffff88015b927cf8
  ffffffff8108f500 0000000000000000 ffff88007a171948 ffff88007a1716b0
  ffff88015b49df00 ffff88015b8d3940 0000000000000000 ffff88015b927d08
 Call Trace:
  [<ffffffff81c61855>] dump_stack+0x19/0x1b
  [<ffffffff8108f500>] warn_slowpath_common+0x70/0xa0
  ...

This patch:

Implement probe_kthread_data() which returns kthread_data if accessible.
The function is equivalent to kthread_data() except that the specified
@task may not be a kthread or its vfork_done is already cleared rendering
struct kthread inaccessible.  In the former case, probe_kthread_data() may
return any value.  In the latter, NULL.

This will be used to safely print debug information without affecting
synchronization in the normal paths.  Workqueue debug info printing on
dump_stack() and friends will make use of it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Acked-by: Jan Kara <jack@suse.cz>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>