timers: Use proper base migration in add_timer_on()

timers: Use proper base migration in add_timer_on()

commit 22b886dd1018093920c4250dee2a9a3cb7cff7b8 upstream.

Regardless of the previous CPU a timer was on, add_timer_on()
currently simply sets timer->flags to the new CPU.  As the caller must
be seeing the timer as idle, this is locally fine, but the timer
leaving the old base while unlocked can lead to race conditions as
follows.

Let's say timer was on cpu 0.

  cpu 0					cpu 1
  -----------------------------------------------------------------------------
  del_timer(timer) succeeds
					del_timer(timer)
					  lock_timer_base(timer) locks cpu_0_base
  add_timer_on(timer, 1)
    spin_lock(&cpu_1_base->lock)
    timer->flags set to cpu_1_base
    operates on @timer			  operates on @timer

This triggered with mod_delayed_work_on() which contains
"if (del_timer()) add_timer_on()" sequence eventually leading to the
following oops.

  BUG: unable to handle kernel NULL pointer dereference at           (null)
  IP: [<ffffffff810ca6e9>] detach_if_pending+0x69/0x1a0
  ...
  Workqueue: wqthrash wqthrash_workfunc [wqthrash]
  task: ffff8800172ca680 ti: ffff8800172d0000 task.ti: ffff8800172d0000
  RIP: 0010:[<ffffffff810ca6e9>]  [<ffffffff810ca6e9>] detach_if_pending+0x69/0x1a0
  ...
  Call Trace:
   [<ffffffff810cb0b4>] del_timer+0x44/0x60
   [<ffffffff8106e836>] try_to_grab_pending+0xb6/0x160
   [<ffffffff8106e913>] mod_delayed_work_on+0x33/0x80
   [<ffffffffa0000081>] wqthrash_workfunc+0x61/0x90 [wqthrash]
   [<ffffffff8106dba8>] process_one_work+0x1e8/0x650
   [<ffffffff8106e05e>] worker_thread+0x4e/0x450
   [<ffffffff810746af>] kthread+0xef/0x110
   [<ffffffff8185980f>] ret_from_fork+0x3f/0x70

Fix it by updating add_timer_on() to perform proper migration as
__mod_timer() does.

Mike: apply tglx backport

Reported-and-tested-by: Jeff Layton <jlayton@poochiereds.net>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Chris Worley <chris.worley@primarydata.com>
Cc: bfields@fieldses.org
Cc: Michael Skralivetsky <michael.skralivetsky@primarydata.com>
Cc: Trond Myklebust <trond.myklebust@primarydata.com>
Cc: Shaohua Li <shli@fb.com>
Cc: Jeff Layton <jlayton@poochiereds.net>
Cc: kernel-team@fb.com
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/20151029103113.2f893924@tlielax.poochiereds.net
Link: http://lkml.kernel.org/r/20151104171533.GI5749@mtj.duckdns.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Mike Galbraith <mgalbraith@suse.de>
Signed-off-by: Willy Tarreau <w@1wt.eu>