Commit-ID: 7c2102e56a3f7d85b5d8f33efbd7aecc1f36fdd8 Gitweb: https://git.kernel.org/tip/7c2102e56a3f7d85b5d8f33efbd7aecc1f36fdd8 Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> AuthorDate: Mon, 18 Sep 2017 08:54:40 -0700 Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> CommitDate: Mon, 9 Oct 2017 14:24:14 -0700 sched: Make resched_cpu() unconditional 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@xxxxxxxxxxxxxx> Suggested-by: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> Acked-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx> Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Cc: stable@xxxxxxxxxxxxxxx --- kernel/sched/core.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index d17c5da..8fa7b6f 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -505,8 +505,7 @@ 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); } -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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