4.4-stable review patch. If anyone has any objections, please let me know.
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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); }