releases/4.4.103/sched-make-resched_cpu-unconditional.patch

   1 From 7c2102e56a3f7d85b5d8f33efbd7aecc1f36fdd8 Mon Sep 17 00:00:00 2001
   2 From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
   3 Date: Mon, 18 Sep 2017 08:54:40 -0700
   4 Subject: sched: Make resched_cpu() unconditional
   5
   6 From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
   7
   8 commit 7c2102e56a3f7d85b5d8f33efbd7aecc1f36fdd8 upstream.
   9
  10 The current implementation of synchronize_sched_expedited() incorrectly
  11 assumes that resched_cpu() is unconditional, which it is not.  This means
  12 that synchronize_sched_expedited() can hang when resched_cpu()'s trylock
  13 fails as follows (analysis by Neeraj Upadhyay):
  14
  15 o       CPU1 is waiting for expedited wait to complete:
  16
  17         sync_rcu_exp_select_cpus
  18              rdp->exp_dynticks_snap & 0x1   // returns 1 for CPU5
  19              IPI sent to CPU5
  20
  21         synchronize_sched_expedited_wait
  22                  ret = swait_event_timeout(rsp->expedited_wq,
  23                                            sync_rcu_preempt_exp_done(rnp_root),
  24                                            jiffies_stall);
  25
  26         expmask = 0x20, CPU 5 in idle path (in cpuidle_enter())
  27
  28 o       CPU5 handles IPI and fails to acquire rq lock.
  29
  30         Handles IPI
  31              sync_sched_exp_handler
  32                  resched_cpu
  33                      returns while failing to try lock acquire rq->lock
  34                  need_resched is not set
  35
  36 o       CPU5 calls  rcu_idle_enter() and as need_resched is not set, goes to
  37         idle (schedule() is not called).
  38
  39 o       CPU 1 reports RCU stall.
  40
  41 Given that resched_cpu() is now used only by RCU, this commit fixes the
  42 assumption by making resched_cpu() unconditional.
  43
  44 Reported-by: Neeraj Upadhyay <neeraju@codeaurora.org>
  45 Suggested-by: Neeraj Upadhyay <neeraju@codeaurora.org>
  46 Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
  47 Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
  48 Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
  49 Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
  50
  51 ---
  52  kernel/sched/core.c |    3 +--
  53  1 file changed, 1 insertion(+), 2 deletions(-)
  54
  55 --- a/kernel/sched/core.c
  56 +++ b/kernel/sched/core.c
  57 @@ -600,8 +600,7 @@ void resched_cpu(int cpu)
  58         struct rq *rq = cpu_rq(cpu);
  59         unsigned long flags;
  60
  61 -       if (!raw_spin_trylock_irqsave(&rq->lock, flags))
  62 -               return;
  63 +       raw_spin_lock_irqsave(&rq->lock, flags);
  64         resched_curr(rq);
  65         raw_spin_unlock_irqrestore(&rq->lock, flags);
  66  }