| From 7c2102e56a3f7d85b5d8f33efbd7aecc1f36fdd8 Mon Sep 17 00:00:00 2001 |
| From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> |
| Date: Mon, 18 Sep 2017 08:54:40 -0700 |
| Subject: sched: Make resched_cpu() unconditional |
| |
| 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 |
| @@ -632,8 +632,7 @@ 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); |
| } |