Re: Query regarding synchronize_sched_expedited and resched_cpu

[Paul E. McKenney]

On Thu, Sep 21, 2017 at 03:59:46PM +0200, Peter Zijlstra wrote:
> On Tue, Sep 19, 2017 at 08:31:26AM -0700, Paul E. McKenney wrote:
> > So I have this one queued.  Objections?
> 
> Changelog reads like its whitespace damaged.

It does, now that you mention it.  How about the updated version below?

							Thanx, Paul

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

commit c21c9b78182e35eb0e72ef4e3bba3054f26eaaea
Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Date:   Mon Sep 18 08:54:40 2017 -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>
    Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
    Acked-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index cab8c5ec128e..b2281971894c 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);
 }