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Subject[PATCH 4.4 08/96] sched: Make resched_cpu() unconditional
Date
4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

commit 7c2102e56a3f7d85b5d8f33efbd7aecc1f36fdd8 upstream.

The current implementation of synchronize_sched_expedited() incorrectly
assumes that resched_cpu() is unconditional, which it is not. This means
that synchronize_sched_expedited() can hang when resched_cpu()'s trylock
fails as follows (analysis by Neeraj Upadhyay):

o CPU1 is waiting for expedited wait to complete:

sync_rcu_exp_select_cpus
rdp->exp_dynticks_snap & 0x1 // returns 1 for CPU5
IPI sent to CPU5

synchronize_sched_expedited_wait
ret = swait_event_timeout(rsp->expedited_wq,
sync_rcu_preempt_exp_done(rnp_root),
jiffies_stall);

expmask = 0x20, CPU 5 in idle path (in cpuidle_enter())

o CPU5 handles IPI and fails to acquire rq lock.

Handles IPI
sync_sched_exp_handler
resched_cpu
returns while failing to try lock acquire rq->lock
need_resched is not set

o CPU5 calls rcu_idle_enter() and as need_resched is not set, goes to
idle (schedule() is not called).

o CPU 1 reports RCU stall.

Given that resched_cpu() is now used only by RCU, this commit fixes the
assumption by making resched_cpu() unconditional.

Reported-by: Neeraj Upadhyay <neeraju@codeaurora.org>
Suggested-by: Neeraj Upadhyay <neeraju@codeaurora.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
kernel/sched/core.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -600,8 +600,7 @@ void resched_cpu(int cpu)
struct rq *rq = cpu_rq(cpu);
unsigned long flags;

- if (!raw_spin_trylock_irqsave(&rq->lock, flags))
- return;
+ raw_spin_lock_irqsave(&rq->lock, flags);
resched_curr(rq);
raw_spin_unlock_irqrestore(&rq->lock, flags);
}

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